Specification of the Exim Mail Transfer Agent

Exim Maintainers

Copyright (c) 2023 The Exim Maintainers

Revision 4.97  04 Nov 2023 EM

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TABLE OF CONTENTS

1. Introduction
    1.1. Exim documentation
    1.2. FTP site and websites
    1.3. Mailing lists
    1.4. Bug reports
    1.5. Where to find the Exim distribution
    1.6. Limitations
    1.7. Runtime configuration
    1.8. Calling interface
    1.9. Terminology
2. Incorporated code
3. How Exim receives and delivers mail
    3.1. Overall philosophy
    3.2. Policy control
    3.3. User filters
    3.4. Message identification
    3.5. Receiving mail
    3.6. Handling an incoming message
    3.7. Life of a message
    3.8. Processing an address for delivery
    3.9. Processing an address for verification
    3.10. Running an individual router
    3.11. Duplicate addresses
    3.12. Router preconditions
    3.13. Delivery in detail
    3.14. Retry mechanism
        3.14.1. Temporary delivery failure
        3.14.2. Permanent delivery failure
        3.14.3. Failures to deliver bounce messages
4. Building and installing Exim
    4.1. Unpacking
    4.2. Multiple machine architectures and operating systems
    4.3. PCRE2 library
    4.4. DBM libraries
    4.5. Pre-building configuration
    4.6. Support for iconv()
    4.7. Including TLS/SSL encryption support
    4.8. Use of tcpwrappers
    4.9. Including support for IPv6
    4.10. Dynamically loaded lookup module support
    4.11. The building process
    4.12. Output from "make"
    4.13. Overriding build-time options for Exim
    4.14. OS-specific header files
    4.15. Overriding build-time options for the monitor
    4.16. Installing Exim binaries and scripts
    4.17. Installing info documentation
    4.18. Setting up the spool directory
    4.19. Testing
    4.20. Replacing another MTA with Exim
    4.21. Running the daemon
    4.22. Upgrading Exim
    4.23. Stopping the Exim daemon on Solaris
5. The Exim command line
    5.1. Setting options by program name
    5.2. Trusted and admin users
    5.3. Command line options
6. The Exim runtime configuration file
    6.1. Using a different configuration file
    6.2. Configuration file format
    6.3. File inclusions in the configuration file
    6.4. Macros in the configuration file
    6.5. Macro substitution
    6.6. Redefining macros
    6.7. Overriding macro values
    6.8. Example of macro usage
    6.9. Builtin macros
    6.10. Conditional skips in the configuration file
    6.11. Common option syntax
    6.12. Boolean options
    6.13. Integer values
    6.14. Octal integer values
    6.15. Fixed point numbers
    6.16. Time intervals
    6.17. String values
    6.18. Expanded strings
    6.19. User and group names
    6.20. List construction
    6.21. Changing list separators
    6.22. Empty items in lists
    6.23. Format of driver configurations
7. The default configuration file
    7.1. Macros
    7.2. Main configuration settings
    7.3. ACL configuration
    7.4. Router configuration
    7.5. Transport configuration
    7.6. Default retry rule
    7.7. Rewriting configuration
    7.8. Authenticators configuration
8. Regular expressions
9. File and database lookups
    9.1. Examples of different lookup syntax
    9.2. Lookup types
    9.3. Single-key lookup types
        9.3.1. cdb
        9.3.2. dbm
        9.3.3. dbmjz
        9.3.4. dbmnz
        9.3.5. dsearch
        9.3.6. iplsearch
        9.3.7. json
        9.3.8. lmdb
        9.3.9. lsearch
        9.3.10. nis
        9.3.11. (n)wildlsearch
        9.3.12. spf
    9.4. Query-style lookup types
        9.4.1. dnsdb
        9.4.2. ibase
        9.4.3. ldap
        9.4.4. mysql
        9.4.5. nisplus
        9.4.6. oracle
        9.4.7. passwd
        9.4.8. pgsql
        9.4.9. redis
        9.4.10. sqlite
        9.4.11. testdb
        9.4.12. whoson
    9.5. Temporary errors in lookups
    9.6. Default values in single-key lookups
    9.7. Partial matching in single-key lookups
    9.8. Lookup caching
    9.9. Quoting lookup data
    9.10. More about dnsdb
        9.10.1. Dnsdb lookup modifiers
        9.10.2. Pseudo dnsdb record types
        9.10.3. Multiple dnsdb lookups
    9.11. More about LDAP
        9.11.1. Format of LDAP queries
        9.11.2. LDAP quoting
        9.11.3. LDAP connections
        9.11.4. LDAP authentication and control information
        9.11.5. Format of data returned by LDAP
    9.12. More about NIS+
    9.13. SQL lookups
        9.13.1. More about MySQL, PostgreSQL, Oracle, InterBase, and Redis
        9.13.2. Specifying the server in the query
        9.13.3. Special MySQL features
        9.13.4. Special PostgreSQL features
        9.13.5. More about SQLite
        9.13.6. More about Redis
10. Domain, host, address, and local part lists
    10.1. Results of list checking
    10.2. Expansion of lists
        10.2.1. Negated items in lists
        10.2.2. File names in lists
        10.2.3. An lsearch file is not an out-of-line list
        10.2.4. Named lists
        10.2.5. Named lists compared with macros
        10.2.6. Named list caching
    10.3. Domain lists
    10.4. Host lists
        10.4.1. Special host list patterns
        10.4.2. Host list patterns that match by IP address
        10.4.3. Host list patterns for single-key lookups by host address
        10.4.4. Host list patterns that match by host name
        10.4.5. Behaviour when an IP address or name cannot be found
        10.4.6. Mixing wildcarded host names and addresses in host lists
        10.4.7. Temporary DNS errors when looking up host information
        10.4.8. Host list patterns for single-key lookups by host name
        10.4.9. Host list patterns for query-style lookups
    10.5. Address lists
        10.5.1. Case of letters in address lists
    10.6. Local part lists
11. String expansions
    11.1. Literal text in expanded strings
    11.2. Character escape sequences in expanded strings
    11.3. Testing string expansions
    11.4. Forced expansion failure
    11.5. Expansion items
    11.6. Expansion operators
    11.7. Expansion conditions
    11.8. Combining expansion conditions
    11.9. Expansion variables
12. Embedded Perl
    12.1. Setting up so Perl can be used
    12.2. Calling Perl subroutines
    12.3. Calling Exim functions from Perl
    12.4. Use of standard output and error by Perl
13. Starting the daemon and the use of network interfaces
    13.1. Starting a listening daemon
    13.2. Special IP listening addresses
    13.3. Overriding local_interfaces and daemon_smtp_ports
    13.4. Support for the submissions (aka SSMTP or SMTPS) protocol
    13.5. IPv6 address scopes
    13.6. Disabling IPv6
    13.7. Examples of starting a listening daemon
    13.8. Recognizing the local host
    13.9. Delivering to a remote host
14. Main configuration
    14.1. Miscellaneous
    14.2. Exim parameters
    14.3. Privilege controls
    14.4. Logging
    14.5. Frozen messages
    14.6. Data lookups
    14.7. Message ids
    14.8. Embedded Perl Startup
    14.9. Daemon
    14.10. Resource control
    14.11. Policy controls
    14.12. Callout cache
    14.13. TLS
    14.14. Local user handling
    14.15. All incoming messages (SMTP and non-SMTP)
    14.16. Non-SMTP incoming messages
    14.17. Incoming SMTP messages
    14.18. SMTP extensions
    14.19. Processing messages
    14.20. System filter
    14.21. Routing and delivery
    14.22. Bounce and warning messages
    14.23. Alphabetical list of main options
15. Generic options for routers
16. The accept router
17. The dnslookup router
    17.1. Problems with DNS lookups
    17.2. Declining addresses by dnslookup
    17.3. Private options for dnslookup
    17.4. Effect of qualify_single and search_parents
18. The ipliteral router
19. The iplookup router
20. The manualroute router
    20.1. Private options for manualroute
    20.2. Routing rules in route_list
    20.3. Routing rules in route_data
    20.4. Format of the list of hosts
    20.5. Format of one host item
    20.6. How the list of hosts is used
    20.7. How the options are used
    20.8. Manualroute examples
21. The queryprogram router
22. The redirect router
    22.1. Redirection data
    22.2. Forward files and address verification
    22.3. Interpreting redirection data
    22.4. Items in a non-filter redirection list
    22.5. Redirecting to a local mailbox
    22.6. Special items in redirection lists
    22.7. Duplicate addresses
    22.8. Repeated redirection expansion
    22.9. Errors in redirection lists
    22.10. Private options for the redirect router
23. Environment for running local transports
    23.1. Concurrent deliveries
    23.2. Uids and gids
    23.3. Current and home directories
    23.4. Expansion variables derived from the address
24. Generic options for transports
25. Address batching in local transports
26. The appendfile transport
    26.1. The file and directory options
    26.2. Private options for appendfile
    26.3. Operational details for appending
    26.4. Operational details for delivery to a new file
    26.5. Maildir delivery
    26.6. Using tags to record message sizes
    26.7. Using a maildirsize file
    26.8. Mailstore delivery
    26.9. Non-special new file delivery
27. The autoreply transport
    27.1. Private options for autoreply
28. The lmtp transport
29. The pipe transport
    29.1. Concurrent delivery
    29.2. Returned status and data
    29.3. How the command is run
    29.4. Environment variables
    29.5. Private options for pipe
    29.6. Using an external local delivery agent
30. The smtp transport
    30.1. Multiple messages on a single connection
    30.2. Use of the $host and $host_address variables
    30.3. Use of $tls_cipher and $tls_peerdn
    30.4. Private options for smtp
    30.5. How the limits for the number of hosts to try are used
31. Address rewriting
    31.1. Explicitly configured address rewriting
    31.2. When does rewriting happen?
    31.3. Testing the rewriting rules that apply on input
    31.4. Rewriting rules
    31.5. Rewriting patterns
    31.6. Rewriting replacements
        31.6.1. Rewriting flags
        31.6.2. Flags specifying which headers and envelope addresses to
        rewrite
        31.6.3. The SMTP-time rewriting flag
        31.6.4. Flags controlling the rewriting process
    31.7. Rewriting examples
32. Retry configuration
    32.1. Changing retry rules
    32.2. Format of retry rules
    32.3. Choosing which retry rule to use for address errors
    32.4. Choosing which retry rule to use for host and message errors
    32.5. Retry rules for specific errors
    32.6. Retry rules for specified senders
    32.7. Retry parameters
    32.8. Retry rule examples
    32.9. Timeout of retry data
    32.10. Long-term failures
    32.11. Deliveries that work intermittently
33. SMTP authentication
    33.1. Generic options for authenticators
    33.2. The AUTH parameter on MAIL commands
    33.3. Authentication on an Exim server
    33.4. Testing server authentication
    33.5. Authentication by an Exim client
34. The plaintext authenticator
    34.1. Avoiding cleartext use
    34.2. Plaintext server options
    34.3. Using plaintext in a server
    34.4. The PLAIN authentication mechanism
    34.5. The LOGIN authentication mechanism
    34.6. Support for different kinds of authentication
    34.7. Using plaintext in a client
35. The cram_md5 authenticator
    35.1. Using cram_md5 as a server
    35.2. Using cram_md5 as a client
36. The cyrus_sasl authenticator
    36.1. Using cyrus_sasl as a server
37. The dovecot authenticator
38. The gsasl authenticator
    38.1. gsasl auth variables
39. The heimdal_gssapi authenticator
    39.1. heimdal_gssapi auth variables
40. The spa authenticator
    40.1. Using spa as a server
    40.2. Using spa as a client
41. The external authenticator
    41.1. External options
    41.2. Using external in a server
    41.3. Using external in a client
42. The tls authenticator
43. Encrypted SMTP connections using TLS/SSL
    43.1. Support for the "submissions" (aka "ssmtp" and "smtps") protocol
    43.2. OpenSSL vs GnuTLS
    43.3. GnuTLS parameter computation
    43.4. Requiring specific ciphers in OpenSSL
    43.5. Requiring specific ciphers or other parameters in GnuTLS
    43.6. Configuring an Exim server to use TLS
        43.6.1. Requesting and verifying client certificates
        43.6.2. Caching of static server configuration items
    43.7. Configuring an Exim client to use TLS
        43.7.1. Caching of static client configuration items
    43.8. Use of TLS Server Name Indication
        43.8.1. ALPN
    43.9. Multiple messages on the same encrypted TCP/IP connection
    43.10. Certificates and all that
        43.10.1. Certificate chains
        43.10.2. Self-signed certificates
        43.10.3. Revoked certificates
    43.11. TLS Resumption
    43.12. DANE
        43.12.1. DNS records
        43.12.2. Interaction with OCSP
        43.12.3. Client configuration
        43.12.4. Observability
        43.12.5. General
44. Access control lists
    44.1. Testing ACLs
    44.2. Specifying when ACLs are used
        44.2.1. The non-SMTP ACLs
        44.2.2. The SMTP connect ACL
        44.2.3. The EHLO/HELO ACL
        44.2.4. The DATA ACLs
        44.2.5. The SMTP DKIM ACL
        44.2.6. The SMTP MIME ACL
        44.2.7. The SMTP PRDR ACL
        44.2.8. The QUIT ACL
        44.2.9. The not-QUIT ACL
    44.3. Finding an ACL to use
    44.4. ACL return codes
    44.5. Unset ACL options
    44.6. Data for message ACLs
    44.7. Data for non-message ACLs
    44.8. Format of an ACL
    44.9. ACL verbs
    44.10. ACL variables
    44.11. Condition and modifier processing
    44.12. ACL modifiers
    44.13. Use of the control modifier
    44.14. Summary of message fixup control
    44.15. Adding header lines in ACLs
    44.16. Removing header lines in ACLs
    44.17. ACL conditions
    44.18. Using DNS lists
        44.18.1. Specifying the IP address for a DNS list lookup
        44.18.2. DNS lists keyed on domain names
        44.18.3. Multiple explicit keys for a DNS list
        44.18.4. Data returned by DNS lists
        44.18.5. Variables set from DNS lists
        44.18.6. Additional matching conditions for DNS lists
        44.18.7. Negated DNS matching conditions
        44.18.8. Handling multiple DNS records from a DNS list
        44.18.9. Detailed information from merged DNS lists
        44.18.10. DNS lists and IPv6
    44.19. Previously seen user and hosts
    44.20. Rate limiting incoming messages
        44.20.1. Ratelimit options for what is being measured
        44.20.2. Ratelimit update modes
        44.20.3. Ratelimit options for handling fast clients
        44.20.4. Limiting the rate of different events
        44.20.5. Using rate limiting
    44.21. Address verification
    44.22. Callout verification
        44.22.1. Additional parameters for callouts
        44.22.2. Callout caching
    44.23. Quota caching
    44.24. Sender address verification reporting
    44.25. Redirection while verifying
    44.26. Client SMTP authorization (CSA)
    44.27. Bounce address tag validation
    44.28. Using an ACL to control relaying
    44.29. Checking a relay configuration
45. Content scanning at ACL time
    45.1. Scanning for viruses
    45.2. Scanning with SpamAssassin and Rspamd
    45.3. Calling SpamAssassin from an Exim ACL
    45.4. Scanning MIME parts
    45.5. Scanning with regular expressions
46. Adding a local scan function to Exim
    46.1. Building Exim to use a local scan function
    46.2. API for local_scan()
    46.3. Configuration options for local_scan()
    46.4. Available Exim variables
    46.5. Structure of header lines
    46.6. Structure of recipient items
    46.7. Available Exim functions
    46.8. More about Exim's memory handling
47. System-wide message filtering
    47.1. Specifying a system filter
    47.2. Testing a system filter
    47.3. Contents of a system filter
    47.4. Additional variable for system filters
    47.5. Defer, freeze, and fail commands for system filters
    47.6. Adding and removing headers in a system filter
    47.7. Setting an errors address in a system filter
    47.8. Per-address filtering
48. Message processing
    48.1. Submission mode for non-local messages
    48.2. Line endings
    48.3. Unqualified addresses
    48.4. The UUCP From line
    48.5. Header lines
        48.5.1. Resent- header lines
        48.5.2. Auto-Submitted:
        48.5.3. Bcc:
        48.5.4. Date:
        48.5.5. Delivery-date:
        48.5.6. Envelope-to:
        48.5.7. From:
        48.5.8. Message-ID:
        48.5.9. Received:
        48.5.10. References:
        48.5.11. Return-path:
        48.5.12. Sender:
    48.6. Adding and removing header lines in routers and transports
    48.7. Constructed addresses
    48.8. Case of local parts
    48.9. Dots in local parts
    48.10. Rewriting addresses
49. SMTP processing
    49.1. Outgoing SMTP and LMTP over TCP/IP
        49.1.1. Errors in outgoing SMTP
    49.2. Incoming SMTP messages over TCP/IP
        49.2.1. Unrecognized SMTP commands
        49.2.2. Syntax and protocol errors in SMTP commands
        49.2.3. Use of non-mail SMTP commands
        49.2.4. The VRFY and EXPN commands
        49.2.5. The ETRN command
    49.3. Incoming local SMTP
    49.4. Outgoing batched SMTP
    49.5. Incoming batched SMTP
50. Customizing bounce and warning messages
    50.1. Customizing bounce messages
    50.2. Customizing warning messages
51. Some common configuration settings
    51.1. Sending mail to a smart host
    51.2. Using Exim to handle mailing lists
    51.3. Syntax errors in mailing lists
    51.4. Re-expansion of mailing lists
    51.5. Closed mailing lists
    51.6. Variable Envelope Return Paths (VERP)
    51.7. Virtual domains
    51.8. Multiple user mailboxes
    51.9. Simplified vacation processing
    51.10. Taking copies of mail
    51.11. Intermittently connected hosts
    51.12. Exim on the upstream server host
    51.13. Exim on the intermittently connected client host
52. Using Exim as a non-queueing client
53. Log files
    53.1. Where the logs are written
    53.2. Logging to local files that are periodically "cycled"
    53.3. Datestamped log files
    53.4. Logging to syslog
    53.5. Log line flags
    53.6. Logging message reception
    53.7. Logging deliveries
    53.8. Discarded deliveries
    53.9. Deferred deliveries
    53.10. Delivery failures
    53.11. Fake deliveries
    53.12. Completion
    53.13. Summary of Fields in Log Lines
    53.14. Other log entries
    53.15. Reducing or increasing what is logged
    53.16. Message log
54. Exim utilities
    54.1. Finding out what Exim processes are doing (exiwhat)
    54.2. Selective queue listing (exiqgrep)
    54.3. Summarizing the queue (exiqsumm)
    54.4. Extracting specific information from the log (exigrep)
    54.5. Selecting messages by various criteria (exipick)
    54.6. Cycling log files (exicyclog)
    54.7. Mail statistics (eximstats)
    54.8. Checking access policy (exim_checkaccess)
    54.9. Making DBM files (exim_dbmbuild)
    54.10. Finding individual retry times (exinext)
    54.11. Hints database maintenance
    54.12. exim_dumpdb
    54.13. exim_tidydb
    54.14. exim_fixdb
    54.15. Mailbox maintenance (exim_lock)
    54.16. Message Ids for humans (exim_msgdate)
55. The Exim monitor
    55.1. Running the monitor
    55.2. The stripcharts
    55.3. Main action buttons
    55.4. The log display
    55.5. The queue display
    55.6. The queue menu
56. Security considerations
    56.1. Building a more "hardened" Exim
    56.2. Root privilege
    56.3. Running Exim without privilege
    56.4. Delivering to local files
    56.5. Running local commands
    56.6. Trust in configuration data
    56.7. IPv4 source routing
    56.8. The VRFY, EXPN, and ETRN commands in SMTP
    56.9. Privileged users
    56.10. Spool files
    56.11. Use of argv[0]
    56.12. Use of %f formatting
    56.13. Embedded Exim path
    56.14. Dynamic module directory
    56.15. Use of sprintf()
    56.16. Use of debug_printf() and log_write()
    56.17. Use of strcat() and strcpy()
57. Format of spool files
    57.1. Format of the -H file
    57.2. Format of the -D file
58. DKIM, SPF, SRS and DMARC
    58.1. DKIM (DomainKeys Identified Mail)
        58.1.1. Signing outgoing messages
        58.1.2. Verifying DKIM signatures in incoming mail
    58.2. SPF (Sender Policy Framework)
        58.2.1. SRS (Sender Rewriting Scheme)
    58.3. DMARC
        58.3.1. Configuration
        58.3.2. Controls
        58.3.3. ACL
        58.3.4. Logging
        58.3.5. Example
59. Proxies
    59.1. Inbound proxies
    59.2. Outbound proxies
    59.3. Logging
60. Internationalisation
    60.1. MTA operations
    60.2. MDA operations
61. Events
62. Adding new drivers or lookup types



===============================================================================
1. INTRODUCTION

Exim is a mail transfer agent (MTA) for hosts that are running Unix or
Unix-like operating systems. It was designed on the assumption that it would be
run on hosts that are permanently connected to the Internet. However, it can be
used on intermittently connected hosts with suitable configuration adjustments.

Configuration files currently exist for the following operating systems: AIX,
BSD/OS (aka BSDI), Darwin (Mac OS X), DGUX, Dragonfly, FreeBSD, GNU/Hurd, GNU/
Linux, HI-OSF (Hitachi), HI-UX, HP-UX, IRIX, MIPS RISCOS, NetBSD, OpenBSD,
OpenUNIX, QNX, SCO, SCO SVR4.2 (aka UNIX-SV), Solaris (aka SunOS5), SunOS4,
Tru64-Unix (formerly Digital UNIX, formerly DEC-OSF1), Ultrix, and UnixWare.
Some of these operating systems are no longer current and cannot easily be
tested, so the configuration files may no longer work in practice.

There are also configuration files for compiling Exim in the Cygwin environment
that can be installed on systems running Windows. However, this document does
not contain any information about running Exim in the Cygwin environment.

The terms and conditions for the use and distribution of Exim are contained in
the file NOTICE. Exim is distributed under the terms of the GNU General Public
Licence, a copy of which may be found in the file LICENCE.

The use, supply, or promotion of Exim for the purpose of sending bulk,
unsolicited electronic mail is incompatible with the basic aims of Exim, which
revolve around the free provision of a service that enhances the quality of
personal communications. The author of Exim regards indiscriminate mass-mailing
as an antisocial, irresponsible abuse of the Internet.

Exim owes a great deal to Smail 3 and its author, Ron Karr. Without the
experience of running and working on the Smail 3 code, I could never have
contemplated starting to write a new MTA. Many of the ideas and user interfaces
were originally taken from Smail 3, though the actual code of Exim is entirely
new, and has developed far beyond the initial concept.

Many people, both in Cambridge and around the world, have contributed to the
development and the testing of Exim, and to porting it to various operating
systems. I am grateful to them all. The distribution now contains a file called
ACKNOWLEDGMENTS, in which I have started recording the names of contributors.


1.1 Exim documentation
----------------------

This edition of the Exim specification applies to version 4.97 of Exim.
Substantive changes from the 4.96 edition are marked in some renditions of this
document; this paragraph is so marked if the rendition is capable of showing a
change indicator.

This document is very much a reference manual; it is not a tutorial. The reader
is expected to have some familiarity with the SMTP mail transfer protocol and
with general Unix system administration. Although there are some discussions
and examples in places, the information is mostly organized in a way that makes
it easy to look up, rather than in a natural order for sequential reading.
Furthermore, this manual aims to cover every aspect of Exim in detail,
including a number of rarely-used, special-purpose features that are unlikely
to be of very wide interest.

An "easier" discussion of Exim which provides more in-depth explanatory,
introductory, and tutorial material can be found in a book entitled The Exim
SMTP Mail Server (second edition, 2007), published by UIT Cambridge (https://
www.uit.co.uk/exim-book/).

The book also contains a chapter that gives a general introduction to SMTP and
Internet mail. Inevitably, however, the book is unlikely to be fully up-to-date
with the latest release of Exim. (Note that the earlier book about Exim,
published by O'Reilly, covers Exim 3, and many things have changed in Exim 4.)

If you are using a Debian distribution of Exim, you will find information about
Debian-specific features in the file /usr/share/doc/exim4-base/README.Debian.
The command man update-exim.conf is another source of Debian-specific
information.

As Exim develops, there may be features in newer versions that have not yet
made it into this document, which is updated only when the most significant
digit of the fractional part of the version number changes. Specifications of
new features that are not yet in this manual are placed in the file doc/
NewStuff in the Exim distribution.

Some features may be classified as "experimental". These may change
incompatibly while they are developing, or even be withdrawn. For this reason,
they are not documented in this manual. Information about experimental features
can be found in the file doc/experimental.txt.

All changes to Exim (whether new features, bug fixes, or other kinds of change)
are noted briefly in the file called doc/ChangeLog.

This specification itself is available as an ASCII file in doc/spec.txt so that
it can easily be searched with a text editor. Other files in the doc directory
are:

OptionLists.txt  list of all options in alphabetical order
dbm.discuss.txt  discussion about DBM libraries
exim.8           a man page of Exim's command line options
experimental.txt documentation of experimental features
filter.txt       specification of the filter language
Exim3.upgrade    upgrade notes from release 2 to release 3
Exim4.upgrade    upgrade notes from release 3 to release 4
openssl.txt      installing a current OpenSSL release

The main specification and the specification of the filtering language are also
available in other formats (HTML, PostScript, PDF, and Texinfo). Section 1.5
below tells you how to get hold of these.


1.2 FTP site and websites
-------------------------

The primary site for Exim source distributions is the exim.org FTP site,
available over HTTPS, HTTP and FTP. These services, and the exim.org website,
are hosted at the University of Cambridge.

As well as Exim distribution tar files, the Exim website contains a number of
differently formatted versions of the documentation. A recent addition to the
online information is the Exim wiki (https://wiki.exim.org), which contains
what used to be a separate FAQ, as well as various other examples, tips, and
know-how that have been contributed by Exim users. The wiki site should always
redirect to the correct place, which is currently provided by GitHub, and is
open to editing by anyone with a GitHub account.

An Exim Bugzilla exists at https://bugs.exim.org. You can use this to report
bugs, and also to add items to the wish list. Please search first to check that
you are not duplicating a previous entry. Please do not ask for configuration
help in the bug-tracker.


1.3 Mailing lists
-----------------

The following Exim mailing lists exist:

exim-announce@lists.exim.org Moderated, low volume announcements list
exim-users@lists.exim.org    General discussion list
exim-users-de@lists.exim.org General discussion list in German language
exim-dev@lists.exim.org      Discussion of bugs, enhancements, etc.
exim-cvs@lists.exim.org      Automated commit messages from the VCS

You can subscribe to these lists, change your existing subscriptions, and view
or search the archives via the mailing lists link on the Exim home page. If you
are using a Debian distribution of Exim, you may wish to subscribe to the
Debian-specific mailing list pkg-exim4-users@lists.alioth.debian.org via this
web page:

https://alioth-lists.debian.net/cgi-bin/mailman/listinfo/pkg-exim4-users

Please ask Debian-specific questions on that list and not on the general Exim
lists.


1.4 Bug reports
---------------

Reports of obvious bugs can be emailed to bugs@exim.org or reported via the
Bugzilla (https://bugs.exim.org). However, if you are unsure whether some
behaviour is a bug or not, the best thing to do is to post a message to the 
exim-dev mailing list and have it discussed.


1.5 Where to find the Exim distribution
---------------------------------------

The master distribution site for the Exim distribution is

https://downloads.exim.org/

The service is available over HTTPS, HTTP and FTP. We encourage people to
migrate to HTTPS.

The content served at https://downloads.exim.org/ is identical to the content
served at https://ftp.exim.org/pub/exim and ftp://ftp.exim.org/pub/exim.

If accessing via a hostname containing ftp, then the file references that
follow are relative to the exim directories at these sites. If accessing via
the hostname downloads then the subdirectories described here are top-level
directories.

There are now quite a number of independent mirror sites around the world.
Those that I know about are listed in the file called Mirrors.

Within the top exim directory there are subdirectories called exim3 (for
previous Exim 3 distributions), exim4 (for the latest Exim 4 distributions),
and Testing for testing versions. In the exim4 subdirectory, the current
release can always be found in files called

exim-n.nn.tar.xz
exim-n.nn.tar.gz
exim-n.nn.tar.bz2

where n.nn is the highest such version number in the directory. The three files
contain identical data; the only difference is the type of compression. The .xz
file is usually the smallest, while the .gz file is the most portable to old
systems.

The distributions will be PGP signed by an individual key of the Release
Coordinator. This key will have a uid containing an email address in the 
exim.org domain and will have signatures from other people, including other
Exim maintainers. We expect that the key will be in the "strong set" of PGP
keys. There should be a trust path to that key from the Exim Maintainer's PGP
keys, a version of which can be found in the release directory in the file
Exim-Maintainers-Keyring.asc. All keys used will be available in public
keyserver pools, such as pool.sks-keyservers.net.

At the time of the last update, releases were being made by Jeremy Harris and
signed with key 0xBCE58C8CE41F32DF. Other recent keys used for signing are
those of Heiko Schlittermann, 0x26101B62F69376CE, and of Phil Pennock, 
0x4D1E900E14C1CC04.

The signatures for the tar bundles are in:

exim-n.nn.tar.xz.asc
exim-n.nn.tar.gz.asc
exim-n.nn.tar.bz2.asc

For each released version, the log of changes is made available in a separate
file in the directory ChangeLogs so that it is possible to find out what has
changed without having to download the entire distribution.

The main distribution contains ASCII versions of this specification and other
documentation; other formats of the documents are available in separate files
inside the exim4 directory of the FTP site:

exim-html-n.nn.tar.gz
exim-pdf-n.nn.tar.gz
exim-postscript-n.nn.tar.gz
exim-texinfo-n.nn.tar.gz

These tar files contain only the doc directory, not the complete distribution,
and are also available in .bz2 and .xz forms.


1.6 Limitations
---------------

  * Exim is designed for use as an Internet MTA, and therefore handles
    addresses in RFC 2822 domain format only. It cannot handle UUCP "bang
    paths", though simple two-component bang paths can be converted by a
    straightforward rewriting configuration. This restriction does not prevent
    Exim from being interfaced to UUCP as a transport mechanism, provided that
    domain addresses are used.

  * Exim insists that every address it handles has a domain attached. For
    incoming local messages, domainless addresses are automatically qualified
    with a configured domain value. Configuration options specify from which
    remote systems unqualified addresses are acceptable. These are then
    qualified on arrival.

  * The only external transport mechanisms that are currently implemented are
    SMTP and LMTP over a TCP/IP network (including support for IPv6). However,
    a pipe transport is available, and there are facilities for writing
    messages to files and pipes, optionally in batched SMTP format; these
    facilities can be used to send messages to other transport mechanisms such
    as UUCP, provided they can handle domain-style addresses. Batched SMTP
    input is also catered for.

  * Exim is not designed for storing mail for dial-in hosts. When the volumes
    of such mail are large, it is better to get the messages "delivered" into
    files (that is, off Exim's queue) and subsequently passed on to the dial-in
    hosts by other means.

  * Although Exim does have basic facilities for scanning incoming messages,
    these are not comprehensive enough to do full virus or spam scanning. Such
    operations are best carried out using additional specialized software
    packages. If you compile Exim with the content-scanning extension,
    straightforward interfaces to a number of common scanners are provided.


1.7 Runtime configuration
-------------------------

Exim's runtime configuration is held in a single text file that is divided into
a number of sections. The entries in this file consist of keywords and values,
in the style of Smail 3 configuration files. A default configuration file which
is suitable for simple online installations is provided in the distribution,
and is described in chapter 7 below.


1.8 Calling interface
---------------------

Like many MTAs, Exim has adopted the Sendmail command line interface so that it
can be a straight replacement for /usr/lib/sendmail or /usr/sbin/sendmail when
sending mail, but you do not need to know anything about Sendmail in order to
run Exim. For actions other than sending messages, Sendmail-compatible options
also exist, but those that produce output (for example, -bp, which lists the
messages in the queue) do so in Exim's own format. There are also some
additional options that are compatible with Smail 3, and some further options
that are new to Exim. Chapter 5 documents all Exim's command line options. This
information is automatically made into the man page that forms part of the Exim
distribution.

Control of messages in the queue can be done via certain privileged command
line options. There is also an optional monitor program called eximon, which
displays current information in an X window, and which contains a menu
interface to Exim's command line administration options.


1.9 Terminology
---------------

The body of a message is the actual data that the sender wants to transmit. It
is the last part of a message and is separated from the header (see below) by a
blank line.

When a message cannot be delivered, it is normally returned to the sender in a
delivery failure message or a "non-delivery report" (NDR). The term bounce is
commonly used for this action, and the error reports are often called bounce
messages. This is a convenient shorthand for "delivery failure error report".
Such messages have an empty sender address in the message's envelope (see
below) to ensure that they cannot themselves give rise to further bounce
messages.

The term default appears frequently in this manual. It is used to qualify a
value which is used in the absence of any setting in the configuration. It may
also qualify an action which is taken unless a configuration setting specifies
otherwise.

The term defer is used when the delivery of a message to a specific destination
cannot immediately take place for some reason (a remote host may be down, or a
user's local mailbox may be full). Such deliveries are deferred until a later
time.

The word domain is sometimes used to mean all but the first component of a
host's name. It is not used in that sense here, where it normally refers to the
part of an email address following the @ sign.

A message in transit has an associated envelope, as well as a header and a
body. The envelope contains a sender address (to which bounce messages should
be delivered), and any number of recipient addresses. References to the sender
or the recipients of a message usually mean the addresses in the envelope. An
MTA uses these addresses for delivery, and for returning bounce messages, not
the addresses that appear in the header lines.

The header of a message is the first part of a message's text, consisting of a
number of lines, each of which has a name such as From:, To:, Subject:, etc.
Long header lines can be split over several text lines by indenting the
continuations. The header is separated from the body by a blank line.

The term local part, which is taken from RFC 2822, is used to refer to the part
of an email address that precedes the @ sign. The part that follows the @ sign
is called the domain or mail domain.

The terms local delivery and remote delivery are used to distinguish delivery
to a file or a pipe on the local host from delivery by SMTP over TCP/IP to
another host. As far as Exim is concerned, all hosts other than the host it is
running on are remote.

Return path is another name that is used for the sender address in a message's
envelope.

The term queue is used to refer to the set of messages awaiting delivery
because this term is in widespread use in the context of MTAs. However, in
Exim's case, the reality is more like a pool than a queue, because there is
normally no ordering of waiting messages.

The term queue runner is used to describe a process that scans the queue and
attempts to deliver those messages whose retry times have come. This term is
used by other MTAs and also relates to the command runq, but in Exim the
waiting messages are normally processed in an unpredictable order.

The term spool directory is used for a directory in which Exim keeps the
messages in its queue - that is, those that it is in the process of delivering.
This should not be confused with the directory in which local mailboxes are
stored, which is called a "spool directory" by some people. In the Exim
documentation, "spool" is always used in the first sense.



===============================================================================
2. INCORPORATED CODE

A number of pieces of external code are included in the Exim distribution.

  * Regular expressions are supported in the main Exim program and in the Exim
    monitor using the freely-distributable PCRE2 library, copyright (c)
    University of Cambridge. The source to PCRE2 is not longer shipped with
    Exim, so you will need to use the version of PCRE2 shipped with your
    system, or obtain and install the full version of the library from https://
    github.com/PhilipHazel/pcre2/releases.

  * Support for the cdb (Constant DataBase) lookup method is provided by code
    contributed by Nigel Metheringham of (at the time he contributed it) Planet
    Online Ltd. The implementation is completely contained within the code of
    Exim. It does not link against an external cdb library. The code contains
    the following statements:

        Copyright (c) 1998 Nigel Metheringham, Planet Online Ltd

        This program is free software; you can redistribute it and/or modify it
        under the terms of the GNU General Public License as published by the
        Free Software Foundation; either version 2 of the License, or (at your
        option) any later version. This code implements Dan Bernstein's
        Constant DataBase (cdb) spec. Information, the spec and sample code for
        cdb can be obtained from https://cr.yp.to/cdb.html. This implementation
        borrows some code from Dan Bernstein's implementation (which has no
        license restrictions applied to it).

  * Client support for Microsoft's Secure Password Authentication is provided
    by code contributed by Marc Prud'hommeaux. Server support was contributed
    by Tom Kistner. This includes code taken from the Samba project, which is
    released under the Gnu GPL.

  * Support for calling the Cyrus pwcheck and saslauthd daemons is provided by
    code taken from the Cyrus-SASL library and adapted by Alexander S.
    Sabourenkov. The permission notice appears below, in accordance with the
    conditions expressed therein.

        Copyright (c) 2001 Carnegie Mellon University. All rights reserved.

        Redistribution and use in source and binary forms, with or without
        modification, are permitted provided that the following conditions are
        met:

         1. Redistributions of source code must retain the above copyright
            notice, this list of conditions and the following disclaimer.

         2. Redistributions in binary form must reproduce the above copyright
            notice, this list of conditions and the following disclaimer in the
            documentation and/or other materials provided with the
            distribution.

         3. The name "Carnegie Mellon University" must not be used to endorse
            or promote products derived from this software without prior
            written permission. For permission or any other legal details,
            please contact

                          Office of Technology Transfer
                          Carnegie Mellon University
                          5000 Forbes Avenue
                          Pittsburgh, PA  15213-3890
                          (412) 268-4387, fax: (412) 268-7395
                          tech-transfer@andrew.cmu.edu

         4. Redistributions of any form whatsoever must retain the following
            acknowledgment:

            "This product includes software developed by Computing Services at
            Carnegie Mellon University (https://www.cmu.edu/computing/."

            CARNEGIE MELLON UNIVERSITY DISCLAIMS ALL WARRANTIES WITH REGARD TO
            THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
            AND FITNESS, IN NO EVENT SHALL CARNEGIE MELLON UNIVERSITY BE LIABLE
            FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
            WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN
            AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
            OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
            SOFTWARE.

  * The Exim Monitor program, which is an X-Window application, includes
    modified versions of the Athena StripChart and TextPop widgets. This code
    is copyright by DEC and MIT, and their permission notice appears below, in
    accordance with the conditions expressed therein.

        Copyright 1987, 1988 by Digital Equipment Corporation, Maynard,
        Massachusetts, and the Massachusetts Institute of Technology,
        Cambridge, Massachusetts.

        All Rights Reserved

        Permission to use, copy, modify, and distribute this software and its
        documentation for any purpose and without fee is hereby granted,
        provided that the above copyright notice appear in all copies and that
        both that copyright notice and this permission notice appear in
        supporting documentation, and that the names of Digital or MIT not be
        used in advertising or publicity pertaining to distribution of the
        software without specific, written prior permission.

        DIGITAL DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
        INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
        EVENT SHALL DIGITAL BE LIABLE FOR ANY SPECIAL, INDIRECT OR
        CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF
        USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
        OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
        PERFORMANCE OF THIS SOFTWARE.

  * The DMARC implementation uses the OpenDMARC library which is Copyrighted by
    The Trusted Domain Project. Portions of Exim source which use OpenDMARC
    derived code are indicated in the respective source files. The full
    OpenDMARC license is provided in the LICENSE.opendmarc file contained in
    the distributed source code.

  * Many people have contributed code fragments, some large, some small, that
    were not covered by any specific license requirements. It is assumed that
    the contributors are happy to see their code incorporated into Exim under
    the GPL.



===============================================================================
3. HOW EXIM RECEIVES AND DELIVERS MAIL


3.1 Overall philosophy
----------------------

Exim is designed to work efficiently on systems that are permanently connected
to the Internet and are handling a general mix of mail. In such circumstances,
most messages can be delivered immediately. Consequently, Exim does not
maintain independent queues of messages for specific domains or hosts, though
it does try to send several messages in a single SMTP connection after a host
has been down, and it also maintains per-host retry information.


3.2 Policy control
------------------

Policy controls are now an important feature of MTAs that are connected to the
Internet. Perhaps their most important job is to stop MTAs from being abused as
"open relays" by misguided individuals who send out vast amounts of unsolicited
junk and want to disguise its source. Exim provides flexible facilities for
specifying policy controls on incoming mail:

  * Exim 4 (unlike previous versions of Exim) implements policy controls on
    incoming mail by means of Access Control Lists (ACLs). Each list is a
    series of statements that may either grant or deny access. ACLs can be used
    at several places in the SMTP dialogue while receiving a message from a
    remote host. However, the most common places are after each RCPT command,
    and at the very end of the message. The sysadmin can specify conditions for
    accepting or rejecting individual recipients or the entire message,
    respectively, at these two points (see chapter 44). Denial of access
    results in an SMTP error code.

  * An ACL is also available for locally generated, non-SMTP messages. In this
    case, the only available actions are to accept or deny the entire message.

  * When Exim is compiled with the content-scanning extension, facilities are
    provided in the ACL mechanism for passing the message to external virus and
    /or spam scanning software. The result of such a scan is passed back to the
    ACL, which can then use it to decide what to do with the message.

  * When a message has been received, either from a remote host or from the
    local host, but before the final acknowledgment has been sent, a locally
    supplied C function called local_scan() can be run to inspect the message
    and decide whether to accept it or not (see chapter 46). If the message is
    accepted, the list of recipients can be modified by the function.

  * Using the local_scan() mechanism is another way of calling external scanner
    software. The SA-Exim add-on package works this way. It does not require
    Exim to be compiled with the content-scanning extension.

  * After a message has been accepted, a further checking mechanism is
    available in the form of the system filter (see chapter 47). This runs at
    the start of every delivery process.


3.3 User filters
----------------

In a conventional Exim configuration, users are able to run private filters by
setting up appropriate .forward files in their home directories. See chapter 22
(about the redirect router) for the configuration needed to support this, and
the separate document entitled Exim's interfaces to mail filtering for user
details. Two different kinds of filtering are available:

  * Sieve filters are written in the standard filtering language that is
    defined by RFC 3028.

  * Exim filters are written in a syntax that is unique to Exim, but which is
    more powerful than Sieve, which it pre-dates.

User filters are run as part of the routing process, described below.


3.4 Message identification
--------------------------

Every message handled by Exim is given a message id which is 23 characters
long. It is divided into three parts, separated by hyphens, for example
"16VDhn-000000001bo-D342". Each part is a sequence of letters and digits,
normally encoding numbers in base 62. However, in the Darwin operating system
(Mac OS X) and when Exim is compiled to run under Cygwin, base 36 (avoiding the
use of lower case letters) is used instead, because the message id is used to
construct filenames, and the names of files in those systems are not always
case-sensitive.

The detail of the contents of the message id have changed as Exim has evolved.
Earlier versions relied on the operating system not re-using a process id (pid)
within one second. On modern operating systems, this assumption can no longer
be made, so the algorithm had to be changed. To retain backward compatibility,
the format of the message id was retained, which is why the following rules are
somewhat eccentric:

  * The first six characters of the message id are the time at which the
    message started to be received, to a granularity of one second. That is,
    this field contains the number of seconds since the start of the epoch (the
    normal Unix way of representing the date and time of day).

  * After the first hyphen, the next

    eleven

    characters are the id of the process that received the message.

  * There are two different possibilities for the final four characters:

     1. If localhost_number is not set, this value is the fractional part of
        the time of reception, normally in units of microseconds. but for
        systems that must use base 36 instead of base 62 (because of
        case-insensitive file systems), the units are 2 us.

     2. If localhost_number is set, it is multiplied by 500000 (250000) and
        added to the fractional part of the time, which in this case is in
        units of 2 us (4 us).

After a message has been received, Exim waits for the clock to tick at the
appropriate resolution before proceeding, so that if another message is
received by the same process, or by another process with the same (re-used)
pid, it is guaranteed that the time will be different. In most cases, the clock
will already have ticked while the message was being received.

The exim_msgdate utility (see section 54.16) can be used to display the date,
and optionally the process id, of an Exim Message ID.


3.5 Receiving mail
------------------

The only way Exim can receive mail from another host is using SMTP over TCP/IP,
in which case the sender and recipient addresses are transferred using SMTP
commands. However, from a locally running process (such as a user's MUA), there
are several possibilities:

  * If the process runs Exim with the -bm option, the message is read
    non-interactively (usually via a pipe), with the recipients taken from the
    command line, or from the body of the message if -t is also used.

  * If the process runs Exim with the -bS option, the message is also read
    non-interactively, but in this case the recipients are listed at the start
    of the message in a series of SMTP RCPT commands, terminated by a DATA
    command. This is called "batch SMTP" format, but it isn't really SMTP. The
    SMTP commands are just another way of passing envelope addresses in a
    non-interactive submission.

  * If the process runs Exim with the -bs option, the message is read
    interactively, using the SMTP protocol. A two-way pipe is normally used for
    passing data between the local process and the Exim process. This is "real"
    SMTP and is handled in the same way as SMTP over TCP/IP. For example, the
    ACLs for SMTP commands are used for this form of submission.

  * A local process may also make a TCP/IP call to the host's loopback address
    (127.0.0.1) or any other of its IP addresses. When receiving messages, Exim
    does not treat the loopback address specially. It treats all such
    connections in the same way as connections from other hosts.

In the three cases that do not involve TCP/IP, the sender address is
constructed from the login name of the user that called Exim and a default
qualification domain (which can be set by the qualify_domain configuration
option). For local or batch SMTP, a sender address that is passed using the
SMTP MAIL command is ignored. However, the system administrator may allow
certain users ("trusted users") to specify a different sender addresses
unconditionally, or all users to specify certain forms of different sender
address. The -f option or the SMTP MAIL command is used to specify these
different addresses. See section 5.2 for details of trusted users, and the 
untrusted_set_sender option for a way of allowing untrusted users to change
sender addresses.

Messages received by either of the non-interactive mechanisms are subject to
checking by the non-SMTP ACL if one is defined. Messages received using SMTP
(either over TCP/IP or interacting with a local process) can be checked by a
number of ACLs that operate at different times during the SMTP session. Either
individual recipients or the entire message can be rejected if local policy
requirements are not met. The local_scan() function (see chapter 46) is run for
all incoming messages.

Exim can be configured not to start a delivery process when a message is
received; this can be unconditional, or depend on the number of incoming SMTP
connections or the system load. In these situations, new messages wait on the
queue until a queue runner process picks them up. However, in standard
configurations under normal conditions, delivery is started as soon as a
message is received.


3.6 Handling an incoming message
--------------------------------

When Exim accepts a message, it writes two files in its spool directory. The
first contains the envelope information, the current status of the message, and
the header lines, and the second contains the body of the message. The names of
the two spool files consist of the message id, followed by "-H" for the file
containing the envelope and header, and "-D" for the data file.

By default, all these message files are held in a single directory called input
inside the general Exim spool directory. Some operating systems do not perform
very well if the number of files in a directory gets large; to improve
performance in such cases, the split_spool_directory option can be used. This
causes Exim to split up the input files into 62 sub-directories whose names are
single letters or digits. When this is done, the queue is processed one
sub-directory at a time instead of all at once, which can improve overall
performance even when there are not enough files in each directory to affect
file system performance.

The envelope information consists of the address of the message's sender and
the addresses of the recipients. This information is entirely separate from any
addresses contained in the header lines. The status of the message includes a
list of recipients who have already received the message. The format of the
first spool file is described in chapter 57.

Address rewriting that is specified in the rewrite section of the configuration
(see chapter 31) is done once and for all on incoming addresses, both in the
header lines and the envelope, at the time the message is accepted. If during
the course of delivery additional addresses are generated (for example, via
aliasing), these new addresses are rewritten as soon as they are generated. At
the time a message is actually delivered (transported) further rewriting can
take place; because this is a transport option, it can be different for
different forms of delivery. It is also possible to specify the addition or
removal of certain header lines at the time the message is delivered (see
chapters 15 and 24).


3.7 Life of a message
---------------------

A message remains in the spool directory until it is completely delivered to
its recipients or to an error address, or until it is deleted by an
administrator or by the user who originally created it. In cases when delivery
cannot proceed - for example when a message can neither be delivered to its
recipients nor returned to its sender, the message is marked "frozen" on the
spool, and no more deliveries are attempted.

An administrator can "thaw" such messages when the problem has been corrected,
and can also freeze individual messages by hand if necessary. In addition, an
administrator can force a delivery error, causing a bounce message to be sent.

There are options called ignore_bounce_errors_after and timeout_frozen_after,
which discard frozen messages after a certain time. The first applies only to
frozen bounces, the second to all frozen messages.

While Exim is working on a message, it writes information about each delivery
attempt to its main log file. This includes successful, unsuccessful, and
delayed deliveries for each recipient (see chapter 53). The log lines are also
written to a separate message log file for each message. These logs are solely
for the benefit of the administrator and are normally deleted along with the
spool files when processing of a message is complete. The use of individual
message logs can be disabled by setting no_message_logs; this might give an
improvement in performance on very busy systems.

All the information Exim itself needs to set up a delivery is kept in the first
spool file, along with the header lines. When a successful delivery occurs, the
address is immediately written at the end of a journal file, whose name is the
message id followed by "-J". At the end of a delivery run, if there are some
addresses left to be tried again later, the first spool file (the "-H" file) is
updated to indicate which these are, and the journal file is then deleted.
Updating the spool file is done by writing a new file and renaming it, to
minimize the possibility of data loss.

Should the system or Exim crash after a successful delivery but before the
spool file has been updated, the journal is left lying around. The next time
Exim attempts to deliver the message, it reads the journal file and updates the
spool file before proceeding. This minimizes the chances of double deliveries
caused by crashes.


3.8 Processing an address for delivery
--------------------------------------

The main delivery processing elements of Exim are called routers and transports
, and collectively these are known as drivers. Code for a number of them is
provided in the source distribution, and compile-time options specify which
ones are included in the binary. Runtime options specify which ones are
actually used for delivering messages.

Each driver that is specified in the runtime configuration is an instance of
that particular driver type. Multiple instances are allowed; for example, you
can set up several different smtp transports, each with different option values
that might specify different ports or different timeouts. Each instance has its
own identifying name. In what follows we will normally use the instance name
when discussing one particular instance (that is, one specific configuration of
the driver), and the generic driver name when discussing the driver's features
in general.

A router is a driver that operates on an address, either determining how its
delivery should happen, by assigning it to a specific transport, or converting
the address into one or more new addresses (for example, via an alias file). A
router may also explicitly choose to fail an address, causing it to be bounced.

A transport is a driver that transmits a copy of the message from Exim's spool
to some destination. There are two kinds of transport: for a local transport,
the destination is a file or a pipe on the local host, whereas for a remote
transport the destination is some other host. A message is passed to a specific
transport as a result of successful routing. If a message has several
recipients, it may be passed to a number of different transports.

An address is processed by passing it to each configured router instance in
turn, subject to certain preconditions, until a router accepts the address or
specifies that it should be bounced. We will describe this process in more
detail shortly. First, as a simple example, we consider how each recipient
address in a message is processed in a small configuration of three routers.

To make this a more concrete example, it is described in terms of some actual
routers, but remember, this is only an example. You can configure Exim's
routers in many different ways, and there may be any number of routers in a
configuration.

The first router that is specified in a configuration is often one that handles
addresses in domains that are not recognized specifically by the local host.
Typically these are addresses for arbitrary domains on the Internet. A
precondition is set up which looks for the special domains known to the host
(for example, its own domain name), and the router is run for addresses that do
not match. Typically, this is a router that looks up domains in the DNS in
order to find the hosts to which this address routes. If it succeeds, the
address is assigned to a suitable SMTP transport; if it does not succeed, the
router is configured to fail the address.

The second router is reached only when the domain is recognized as one that
"belongs" to the local host. This router does redirection - also known as
aliasing and forwarding. When it generates one or more new addresses from the
original, each of them is routed independently from the start. Otherwise, the
router may cause an address to fail, or it may simply decline to handle the
address, in which case the address is passed to the next router.

The final router in many configurations is one that checks to see if the
address belongs to a local mailbox. The precondition may involve a check to see
if the local part is the name of a login account, or it may look up the local
part in a file or a database. If its preconditions are not met, or if the
router declines, we have reached the end of the routers. When this happens, the
address is bounced.


3.9 Processing an address for verification
------------------------------------------

As well as being used to decide how to deliver to an address, Exim's routers
are also used for address verification. Verification can be requested as one of
the checks to be performed in an ACL for incoming messages, on both sender and
recipient addresses, and it can be tested using the -bv and -bvs command line
options.

When an address is being verified, the routers are run in "verify mode". This
does not affect the way the routers work, but it is a state that can be
detected. By this means, a router can be skipped or made to behave differently
when verifying. A common example is a configuration in which the first router
sends all messages to a message-scanning program unless they have been
previously scanned. Thus, the first router accepts all addresses without any
checking, making it useless for verifying. Normally, the no_verify option would
be set for such a router, causing it to be skipped in verify mode.


3.10 Running an individual router
---------------------------------

As explained in the example above, a number of preconditions are checked before
running a router. If any are not met, the router is skipped, and the address is
passed to the next router. When all the preconditions on a router are met, the
router is run. What happens next depends on the outcome, which is one of the
following:

  * accept: The router accepts the address, and either assigns it to a
    transport or generates one or more "child" addresses. Processing the
    original address ceases unless the unseen option is set on the router. This
    option can be used to set up multiple deliveries with different routing
    (for example, for keeping archive copies of messages). When unseen is set,
    the address is passed to the next router. Normally, however, an accept
    return marks the end of routing.

    Any child addresses generated by the router are processed independently,
    starting with the first router by default. It is possible to change this by
    setting the redirect_router option to specify which router to start at for
    child addresses. Unlike pass_router (see below) the router specified by 
    redirect_router may be anywhere in the router configuration.

  * pass: The router recognizes the address, but cannot handle it itself. It
    requests that the address be passed to another router. By default, the
    address is passed to the next router, but this can be changed by setting
    the pass_router option. However, (unlike redirect_router) the named router
    must be below the current router (to avoid loops).

  * decline: The router declines to accept the address because it does not
    recognize it at all. By default, the address is passed to the next router,
    but this can be prevented by setting the no_more option. When no_more is
    set, all the remaining routers are skipped. In effect, no_more converts 
    decline into fail.

  * fail: The router determines that the address should fail, and queues it for
    the generation of a bounce message. There is no further processing of the
    original address unless unseen is set on the router.

  * defer: The router cannot handle the address at the present time. (A
    database may be offline, or a DNS lookup may have timed out.) No further
    processing of the address happens in this delivery attempt. It is tried
    again next time the message is considered for delivery.

  * error: There is some error in the router (for example, a syntax error in
    its configuration). The action is as for defer.

If an address reaches the end of the routers without having been accepted by
any of them, it is bounced as unrouteable. The default error message in this
situation is "unrouteable address", but you can set your own message by making
use of the cannot_route_message option. This can be set for any router; the
value from the last router that "saw" the address is used.

Sometimes while routing you want to fail a delivery when some conditions are
met but others are not, instead of passing the address on for further routing.
You can do this by having a second router that explicitly fails the delivery
when the relevant conditions are met. The redirect router has a "fail" facility
for this purpose.


3.11 Duplicate addresses
------------------------

Once routing is complete, Exim scans the addresses that are assigned to local
and remote transports and discards any duplicates that it finds. During this
check, local parts are treated case-sensitively. This happens only when
actually delivering a message; when testing routers with -bt, all the routed
addresses are shown.


3.12 Router preconditions
-------------------------

The preconditions that are tested for each router are listed below, in the
order in which they are tested. The individual configuration options are
described in more detail in chapter 15.

 1. The local_part_prefix and local_part_suffix options can specify that the
    local parts handled by the router may or must have certain prefixes and/or
    suffixes. If a mandatory affix (prefix or suffix) is not present, the
    router is skipped. These conditions are tested first. When an affix is
    present, it is removed from the local part before further processing,
    including the evaluation of any other conditions.

 2. Routers can be designated for use only when not verifying an address, that
    is, only when routing it for delivery (or testing its delivery routing). If
    the verify option is set false, the router is skipped when Exim is
    verifying an address. Setting the verify option actually sets two options, 
    verify_sender and verify_recipient, which independently control the use of
    the router for sender and recipient verification. You can set these options
    directly if you want a router to be used for only one type of verification.
    Note that cutthrough delivery is classed as a recipient verification for
    this purpose.

 3. If the address_test option is set false, the router is skipped when Exim is
    run with the -bt option to test an address routing. This can be helpful
    when the first router sends all new messages to a scanner of some sort; it
    makes it possible to use -bt to test subsequent delivery routing without
    having to simulate the effect of the scanner.

 4. Routers can be designated for use only when verifying an address, as
    opposed to routing it for delivery. The verify_only option controls this.
    Again, cutthrough delivery counts as a verification.

 5. Individual routers can be explicitly skipped when running the routers to
    check an address given in the SMTP EXPN command (see the expn option).

 6. If the domains option is set, the domain of the address must be in the set
    of domains that it defines. A match verifies the variable $domain (which
    carries tainted data) and assigns an untainted value to the $domain_data
    variable. Such an untainted value is often needed in the transport. For
    specifics of the matching operation and the resulting untainted value,
    refer to section 10.3.

    When an untainted value is wanted, use this option rather than the generic 
    condition option.

 7. If the local_parts option is set, the local part of the address must be in
    the set of local parts that it defines. A match verifies the variable
    $local_part (which carries tainted data) and assigns an untainted value to
    the $local_part_data variable. Such an untainted value is often needed in
    the transport. For specifics of the matching operation and the resulting
    untainted value, refer to section 10.6.

    When an untainted value is wanted, use this option rather than the generic 
    condition option.

    If local_part_prefix or local_part_suffix is in use, the prefix or suffix
    is removed from the local part before this check. If you want to do
    precondition tests on local parts that include affixes, you can do so by
    using a condition option (see below) that uses the variables $local_part,
    $local_part_prefix, $local_part_prefix_v, $local_part_suffix and
    $local_part_suffix_v as necessary.

 8. If the check_local_user option is set, the local part must be the name of
    an account on the local host. If this check succeeds, the uid and gid of
    the local user are placed in $local_user_uid and $local_user_gid and the
    user's home directory is placed in $home; these values can be used in the
    remaining preconditions.

 9. If the router_home_directory option is set, it is expanded at this point,
    because it overrides the value of $home. If this expansion were left till
    later, the value of $home as set by check_local_user would be used in
    subsequent tests. Having two different values of $home in the same router
    could lead to confusion.

10. If the senders option is set, the envelope sender address must be in the
    set of addresses that it defines.

11. If the require_files option is set, the existence or non-existence of
    specified files is tested.

12. If the condition option is set, it is evaluated and tested. This option
    uses an expanded string to allow you to set up your own custom
    preconditions. Expanded strings are described in chapter 11.

    Note that while using this option for address matching technically works,
    it does not set any de-tainted values. Such values are often needed, either
    for router-specific options or for transport options. Using the domains and
    local_parts options is usually the most convenient way to obtain them.

Note that require_files comes near the end of the list, so you cannot use it to
check for the existence of a file in which to lookup up a domain, local part,
or sender. However, as these options are all expanded, you can use the exists
expansion condition to make such tests within each condition. The require_files
option is intended for checking files that the router may be going to use
internally, or which are needed by a specific transport (for example,
.procmailrc).


3.13 Delivery in detail
-----------------------

When a message is to be delivered, the sequence of events is as follows:

 1. If a system-wide filter file is specified, the message is passed to it. The
    filter may add recipients to the message, replace the recipients, discard
    the message, cause a new message to be generated, or cause the message
    delivery to fail. The format of the system filter file is the same as for
    Exim user filter files, described in the separate document entitled Exim's
    interfaces to mail filtering. (Note: Sieve cannot be used for system filter
    files.)

    Some additional features are available in system filters - see chapter 47
    for details. Note that a message is passed to the system filter only once
    per delivery attempt, however many recipients it has. However, if there are
    several delivery attempts because one or more addresses could not be
    immediately delivered, the system filter is run each time. The filter
    condition first_delivery can be used to detect the first run of the system
    filter.

 2. Each recipient address is offered to each configured router, in turn,
    subject to its preconditions, until one is able to handle it. If no router
    can handle the address, that is, if they all decline, the address is
    failed. Because routers can be targeted at particular domains, several
    locally handled domains can be processed entirely independently of each
    other.

 3. A router that accepts an address may assign it to a local or a remote
    transport. However, the transport is not run at this time. Instead, the
    address is placed on a list for the particular transport, which will be run
    later. Alternatively, the router may generate one or more new addresses
    (typically from alias, forward, or filter files). New addresses are fed
    back into this process from the top, but in order to avoid loops, a router
    ignores any address which has an identically-named ancestor that was
    processed by itself.

 4. When all the routing has been done, addresses that have been successfully
    handled are passed to their assigned transports. When local transports are
    doing real local deliveries, they handle only one address at a time, but if
    a local transport is being used as a pseudo-remote transport (for example,
    to collect batched SMTP messages for transmission by some other means)
    multiple addresses can be handled. Remote transports can always handle more
    than one address at a time, but can be configured not to do so, or to
    restrict multiple addresses to the same domain.

 5. Each local delivery to a file or a pipe runs in a separate process under a
    non-privileged uid, and these deliveries are run one at a time. Remote
    deliveries also run in separate processes, normally under a uid that is
    private to Exim ("the Exim user"), but in this case, several remote
    deliveries can be run in parallel. The maximum number of simultaneous
    remote deliveries for any one message is set by the remote_max_parallel
    option. The order in which deliveries are done is not defined, except that
    all local deliveries happen before any remote deliveries.

 6. When it encounters a local delivery during a queue run, Exim checks its
    retry database to see if there has been a previous temporary delivery
    failure for the address before running the local transport. If there was a
    previous failure, Exim does not attempt a new delivery until the retry time
    for the address is reached. However, this happens only for delivery
    attempts that are part of a queue run. Local deliveries are always
    attempted when delivery immediately follows message reception, even if
    retry times are set for them. This makes for better behaviour if one
    particular message is causing problems (for example, causing quota
    overflow, or provoking an error in a filter file).

 7. Remote transports do their own retry handling, since an address may be
    deliverable to one of a number of hosts, each of which may have a different
    retry time. If there have been previous temporary failures and no host has
    reached its retry time, no delivery is attempted, whether in a queue run or
    not. See chapter 32 for details of retry strategies.

 8. If there were any permanent errors, a bounce message is returned to an
    appropriate address (the sender in the common case), with details of the
    error for each failing address. Exim can be configured to send copies of
    bounce messages to other addresses.

 9. If one or more addresses suffered a temporary failure, the message is left
    on the queue, to be tried again later. Delivery of these addresses is said
    to be deferred.

10. When all the recipient addresses have either been delivered or bounced,
    handling of the message is complete. The spool files and message log are
    deleted, though the message log can optionally be preserved if required.


3.14 Retry mechanism
--------------------

Exim's mechanism for retrying messages that fail to get delivered at the first
attempt is the queue runner process. You must either run an Exim daemon that
uses the -q option with a time interval to start queue runners at regular
intervals or use some other means (such as cron) to start them. If you do not
arrange for queue runners to be run, messages that fail temporarily at the
first attempt will remain in your queue forever. A queue runner process works
its way through the queue, one message at a time, trying each delivery that has
passed its retry time. You can run several queue runners at once.

Exim uses a set of configured rules to determine when next to retry the failing
address (see chapter 32). These rules also specify when Exim should give up
trying to deliver to the address, at which point it generates a bounce message.
If no retry rules are set for a particular host, address, and error
combination, no retries are attempted, and temporary errors are treated as
permanent.

3.14.1 Temporary delivery failure

There are many reasons why a message may not be immediately deliverable to a
particular address. Failure to connect to a remote machine (because it, or the
connection to it, is down) is one of the most common. Temporary failures may be
detected during routing as well as during the transport stage of delivery.
Local deliveries may be delayed if NFS files are unavailable, or if a mailbox
is on a file system where the user is over quota. Exim can be configured to
impose its own quotas on local mailboxes; where system quotas are set they will
also apply.

If a host is unreachable for a period of time, a number of messages may be
waiting for it by the time it recovers, and sending them in a single SMTP
connection is clearly beneficial. Whenever a delivery to a remote host is
deferred, Exim makes a note in its hints database, and whenever a successful
SMTP delivery has happened, it looks to see if any other messages are waiting
for the same host. If any are found, they are sent over the same SMTP
connection, subject to a configuration limit as to the maximum number in any
one connection.

3.14.2 Permanent delivery failure

When a message cannot be delivered to some or all of its intended recipients, a
bounce message is generated. Temporary delivery failures turn into permanent
errors when their timeout expires. All the addresses that fail in a given
delivery attempt are listed in a single message. If the original message has
many recipients, it is possible for some addresses to fail in one delivery
attempt and others to fail subsequently, giving rise to more than one bounce
message. The wording of bounce messages can be customized by the administrator.
See chapter 50 for details.

Bounce messages contain an X-Failed-Recipients: header line that lists the
failed addresses, for the benefit of programs that try to analyse such messages
automatically.

A bounce message is normally sent to the sender of the original message, as
obtained from the message's envelope. For incoming SMTP messages, this is the
address given in the MAIL command. However, when an address is expanded via a
forward or alias file, an alternative address can be specified for delivery
failures of the generated addresses. For a mailing list expansion (see section
51.2) it is common to direct bounce messages to the manager of the list.

3.14.3 Failures to deliver bounce messages

If a bounce message (either locally generated or received from a remote host)
itself suffers a permanent delivery failure, the message is left in the queue,
but it is frozen, awaiting the attention of an administrator. There are options
that can be used to make Exim discard such failed messages, or to keep them for
only a short time (see timeout_frozen_after and ignore_bounce_errors_after).



===============================================================================
4. BUILDING AND INSTALLING EXIM


4.1 Unpacking
-------------

Exim is distributed as a gzipped or bzipped tar file which, when unpacked,
creates a directory with the name of the current release (for example,
exim-4.97) into which the following files are placed:

    ACKNOWLEDGMENTS contains some acknowledgments
    CHANGES         contains a reference to where changes are documented
    LICENCE         the GNU General Public Licence
    Makefile        top-level make file
    NOTICE          conditions for the use of Exim
    README          list of files, directories and simple build instructions

Other files whose names begin with README may also be present. The following
subdirectories are created:

    Local        an empty directory for local configuration files
    OS           OS-specific files
    doc          documentation files
    exim_monitor source files for the Exim monitor
    scripts      scripts used in the build process
    src          remaining source files
    util         independent utilities

The main utility programs are contained in the src directory and are built with
the Exim binary. The util directory contains a few optional scripts that may be
useful to some sites.


4.2 Multiple machine architectures and operating systems
--------------------------------------------------------

The building process for Exim is arranged to make it easy to build binaries for
a number of different architectures and operating systems from the same set of
source files. Compilation does not take place in the src directory. Instead, a 
build directory is created for each architecture and operating system. Symbolic
links to the sources are installed in this directory, which is where the actual
building takes place. In most cases, Exim can discover the machine architecture
and operating system for itself, but the defaults can be overridden if
necessary. A C99-capable compiler will be required for the build.


4.3 PCRE2 library
-----------------

Exim no longer has an embedded regular-expression library as the vast majority
of modern systems include PCRE2 as a system library, although you may need to
install the PCRE2 package or the PCRE2 development package for your operating
system. If your system has a normal PCRE2 installation the Exim build process
will need no further configuration. If the library or the headers are in an
unusual location you will need to either set the PCRE2_LIBS and INCLUDE
directives appropriately, or set PCRE2_CONFIG=yes to use the installed 
pcre-config command. If your operating system has no PCRE2 support then you
will need to obtain and build the current PCRE2 from https://github.com/
PhilipHazel/pcre2/releases. More information on PCRE2 is available at https://
www.pcre.org/.


4.4 DBM libraries
-----------------

Even if you do not use any DBM files in your configuration, Exim still needs a
DBM library in order to operate, because it uses indexed files for its hints
databases. Unfortunately, there are a number of DBM libraries in existence, and
different operating systems often have different ones installed.

If you are using Solaris, IRIX, one of the modern BSD systems, or a modern
Linux distribution, the DBM configuration should happen automatically, and you
may be able to ignore this section. Otherwise, you may have to learn more than
you would like about DBM libraries from what follows.

Licensed versions of Unix normally contain a library of DBM functions operating
via the ndbm interface, and this is what Exim expects by default. Free versions
of Unix seem to vary in what they contain as standard. In particular, some
early versions of Linux have no default DBM library, and different distributors
have chosen to bundle different libraries with their packaged versions.
However, the more recent releases seem to have standardized on the Berkeley DB
library.

Different DBM libraries have different conventions for naming the files they
use. When a program opens a file called dbmfile, there are several
possibilities:

 1. A traditional ndbm implementation, such as that supplied as part of
    Solaris, operates on two files called dbmfile.dir and dbmfile.pag.

 2. The GNU library, gdbm, operates on a single file. If used via its ndbm
    compatibility interface it makes two different hard links to it with names
    dbmfile.dir and dbmfile.pag, but if used via its native interface, the
    filename is used unmodified.

 3. The Berkeley DB package, if called via its ndbm compatibility interface,
    operates on a single file called dbmfile.db, but otherwise looks to the
    programmer exactly the same as the traditional ndbm implementation.

 4. If the Berkeley package is used in its native mode, it operates on a single
    file called dbmfile; the programmer's interface is somewhat different to
    the traditional ndbm interface.

 5. To complicate things further, there are several very different versions of
    the Berkeley DB package. Version 1.85 was stable for a very long time,
    releases 2.x and 3.x were current for a while, but the latest versions when
    Exim last revamped support were numbered 5.x. Maintenance of some of the
    earlier releases has ceased, and Exim no longer supports versions before 3.
    x. All versions of Berkeley DB could be obtained from http://
    www.sleepycat.com/, which is now a redirect to their new owner's page with
    far newer versions listed. It is probably wise to plan to move your storage
    configurations away from Berkeley DB format, as today there are smaller and
    simpler alternatives more suited to Exim's usage model.

 6. Yet another DBM library, called tdb, is available from https://
    sourceforge.net/projects/tdb/files/. It has its own interface, and also
    operates on a single file.

Exim and its utilities can be compiled to use any of these interfaces. In order
to use any version of the Berkeley DB package in native mode, you must set
USE_DB in an appropriate configuration file (typically Local/Makefile). For
example:

USE_DB=yes

Similarly, for gdbm you set USE_GDBM, and for tdb you set USE_TDB. An error is
diagnosed if you set more than one of these. You can set USE_NDBM if needed to
override an operating system default.

At the lowest level, the build-time configuration sets none of these options,
thereby assuming an interface of type (1). However, some operating system
configuration files (for example, those for the BSD operating systems and
Linux) assume type (4) by setting USE_DB as their default, and the
configuration files for Cygwin set USE_GDBM. Anything you set in Local/Makefile
, however, overrides these system defaults.

As well as setting USE_DB, USE_GDBM, or USE_TDB, it may also be necessary to
set DBMLIB, to cause inclusion of the appropriate library, as in one of these
lines:

DBMLIB = -ldb
DBMLIB = -ltdb
DBMLIB = -lgdbm -lgdbm_compat

The last of those was for a Linux having GDBM provide emulated NDBM facilities.
Settings like that will work if the DBM library is installed in the standard
place. Sometimes it is not, and the library's header file may also not be in
the default path. You may need to set INCLUDE to specify where the header file
is, and to specify the path to the library more fully in DBMLIB, as in this
example:

INCLUDE=-I/usr/local/include/db-4.1
DBMLIB=/usr/local/lib/db-4.1/libdb.a

There is further detailed discussion about the various DBM libraries in the
file doc/dbm.discuss.txt in the Exim distribution.


4.5 Pre-building configuration
------------------------------

Before building Exim, a local configuration file that specifies options
independent of any operating system has to be created with the name Local/
Makefile. A template for this file is supplied as the file src/EDITME, and it
contains full descriptions of all the option settings therein. These
descriptions are therefore not repeated here. If you are building Exim for the
first time, the simplest thing to do is to copy src/EDITME to Local/Makefile,
then read it and edit it appropriately.

There are three settings that you must supply, because Exim will not build
without them. They are the location of the runtime configuration file
(CONFIGURE_FILE), the directory in which Exim binaries will be installed
(BIN_DIRECTORY), and the identity of the Exim user (EXIM_USER and maybe
EXIM_GROUP as well). The value of CONFIGURE_FILE can in fact be a
colon-separated list of filenames; Exim uses the first of them that exists.

There are a few other parameters that can be specified either at build time or
at runtime, to enable the same binary to be used on a number of different
machines. However, if the locations of Exim's spool directory and log file
directory (if not within the spool directory) are fixed, it is recommended that
you specify them in Local/Makefile instead of at runtime, so that errors
detected early in Exim's execution (such as a malformed configuration file) can
be logged.

Exim's interfaces for calling virus and spam scanning software directly from
access control lists are not compiled by default. If you want to include these
facilities, you need to set

WITH_CONTENT_SCAN=yes

in your Local/Makefile. For details of the facilities themselves, see chapter
45.

If you are going to build the Exim monitor, a similar configuration process is
required. The file exim_monitor/EDITME must be edited appropriately for your
installation and saved under the name Local/eximon.conf. If you are happy with
the default settings described in exim_monitor/EDITME, Local/eximon.conf can be
empty, but it must exist.

This is all the configuration that is needed in straightforward cases for known
operating systems. However, the building process is set up so that it is easy
to override options that are set by default or by operating-system-specific
configuration files, for example, to change the C compiler, which defaults to 
gcc. See section 4.13 below for details of how to do this.


4.6 Support for iconv()
-----------------------

The contents of header lines in messages may be encoded according to the rules
described RFC 2047. This makes it possible to transmit characters that are not
in the ASCII character set, and to label them as being in a particular
character set. When Exim is inspecting header lines by means of the $h_
mechanism, it decodes them, and translates them into a specified character set
(default is set at build time). The translation is possible only if the
operating system supports the iconv() function.

However, some of the operating systems that supply iconv() do not support very
many conversions. The GNU libiconv library (available from https://www.gnu.org/
software/libiconv/) can be installed on such systems to remedy this deficiency,
as well as on systems that do not supply iconv() at all. After installing 
libiconv, you should add

HAVE_ICONV=yes

to your Local/Makefile and rebuild Exim.


4.7 Including TLS/SSL encryption support
----------------------------------------

Exim is usually built to support encrypted SMTP connections, using the STARTTLS
command as per RFC 2487. It can also support clients that expect to start a TLS
session immediately on connection to a non-standard port (see the 
tls_on_connect_ports runtime option and the -tls-on-connect command line
option).

If you want to build Exim with TLS support, you must first install either the
OpenSSL or GnuTLS library. There is no cryptographic code in Exim itself for
implementing SSL.

If you do not want TLS support you should set

DISABLE_TLS=yes

in Local/Makefile.

If OpenSSL is installed, you should set

USE_OPENSL=yes
TLS_LIBS=-lssl -lcrypto

in Local/Makefile. You may also need to specify the locations of the OpenSSL
library and include files. For example:

USE_OPENSSL=yes
TLS_LIBS=-L/usr/local/openssl/lib -lssl -lcrypto
TLS_INCLUDE=-I/usr/local/openssl/include/

If you have pkg-config available, then instead you can just use:

USE_OPENSSL=yes
USE_OPENSSL_PC=openssl

If GnuTLS is installed, you should set

USE_GNUTLS=yes
TLS_LIBS=-lgnutls -ltasn1 -lgcrypt

in Local/Makefile, and again you may need to specify the locations of the
library and include files. For example:

USE_GNUTLS=yes
TLS_LIBS=-L/usr/gnu/lib -lgnutls -ltasn1 -lgcrypt
TLS_INCLUDE=-I/usr/gnu/include

If you have pkg-config available, then instead you can just use:

USE_GNUTLS=yes
USE_GNUTLS_PC=gnutls

You do not need to set TLS_INCLUDE if the relevant directory is already
specified in INCLUDE. Details of how to configure Exim to make use of TLS are
given in chapter 43.


4.8 Use of tcpwrappers
----------------------

Exim can be linked with the tcpwrappers library in order to check incoming SMTP
calls using the tcpwrappers control files. This may be a convenient alternative
to Exim's own checking facilities for installations that are already making use
of tcpwrappers for other purposes. To do this, you should set USE_TCP_WRAPPERS
in Local/Makefile, arrange for the file tcpd.h to be available at compile time,
and also ensure that the library libwrap.a is available at link time, typically
by including -lwrap in EXTRALIBS_EXIM. For example, if tcpwrappers is installed
in /usr/local, you might have

USE_TCP_WRAPPERS=yes
CFLAGS=-O -I/usr/local/include
EXTRALIBS_EXIM=-L/usr/local/lib -lwrap

in Local/Makefile. The daemon name to use in the tcpwrappers control files is
"exim". For example, the line

exim : LOCAL  192.168.1.  .friendly.domain.example

in your /etc/hosts.allow file allows connections from the local host, from the
subnet 192.168.1.0/24, and from all hosts in friendly.domain.example. All other
connections are denied. The daemon name used by tcpwrappers can be changed at
build time by setting TCP_WRAPPERS_DAEMON_NAME in Local/Makefile, or by setting
tcp_wrappers_daemon_name in the configure file. Consult the tcpwrappers
documentation for further details.


4.9 Including support for IPv6
------------------------------

Exim contains code for use on systems that have IPv6 support. Setting
"HAVE_IPV6=YES" in Local/Makefile causes the IPv6 code to be included; it may
also be necessary to set IPV6_INCLUDE and IPV6_LIBS on systems where the IPv6
support is not fully integrated into the normal include and library files.

Two different types of DNS record for handling IPv6 addresses have been
defined. AAAA records (analogous to A records for IPv4) are in use, and are
currently seen as the mainstream. Another record type called A6 was proposed as
better than AAAA because it had more flexibility. However, it was felt to be
over-complex, and its status was reduced to "experimental". Exim used to have a
compile option for including A6 record support but this has now been withdrawn.


4.10 Dynamically loaded lookup module support
---------------------------------------------

On some platforms, Exim supports not compiling all lookup types directly into
the main binary, instead putting some into external modules which can be loaded
on demand. This permits packagers to build Exim with support for lookups with
extensive library dependencies without requiring all users to install all of
those dependencies. Most, but not all, lookup types can be built this way.

Set "LOOKUP_MODULE_DIR" to the directory into which the modules will be
installed; Exim will only load modules from that directory, as a security
measure. You will need to set "CFLAGS_DYNAMIC" if not already defined for your
OS; see OS/Makefile-Linux for an example. Some other requirements for adjusting
"EXTRALIBS" may also be necessary, see src/EDITME for details.

Then, for each module to be loaded dynamically, define the relevant "LOOKUP_"<
lookup_type> flags to have the value "2" instead of "yes". For example, this
will build in lsearch but load sqlite and mysql support on demand:

LOOKUP_LSEARCH=yes
LOOKUP_SQLITE=2
LOOKUP_MYSQL=2


4.11 The building process
-------------------------

Once Local/Makefile (and Local/eximon.conf, if required) have been created, run
make at the top level. It determines the architecture and operating system
types, and creates a build directory if one does not exist. For example, on a
Sun system running Solaris 8, the directory build-SunOS5-5.8-sparc is created. 
Symbolic links to relevant source files are installed in the build directory.

If this is the first time make has been run, it calls a script that builds a
make file inside the build directory, using the configuration files from the
Local directory. The new make file is then passed to another instance of make.
This does the real work, building a number of utility scripts, and then
compiling and linking the binaries for the Exim monitor (if configured), a
number of utility programs, and finally Exim itself. The command "make
makefile" can be used to force a rebuild of the make file in the build
directory, should this ever be necessary.

If you have problems building Exim, check for any comments there may be in the
README file concerning your operating system, and also take a look at the FAQ,
where some common problems are covered.


4.12 Output from "make"
-----------------------

The output produced by the make process for compile lines is often very
unreadable, because these lines can be very long. For this reason, the normal
output is suppressed by default, and instead output similar to that which
appears when compiling the 2.6 Linux kernel is generated: just a short line for
each module that is being compiled or linked. However, it is still possible to
get the full output, by calling make like this:

FULLECHO='' make -e

The value of FULLECHO defaults to "@", the flag character that suppresses
command reflection in make. When you ask for the full output, it is given in
addition to the short output.


4.13 Overriding build-time options for Exim
-------------------------------------------

The main make file that is created at the beginning of the building process
consists of the concatenation of a number of files which set configuration
values, followed by a fixed set of make instructions. If a value is set more
than once, the last setting overrides any previous ones. This provides a
convenient way of overriding defaults. The files that are concatenated are, in
order:

OS/Makefile-Default
OS/Makefile-<ostype>
Local/Makefile
Local/Makefile-<ostype>
Local/Makefile-<archtype>
Local/Makefile-<ostype>-<archtype>
OS/Makefile-Base

where <ostype> is the operating system type and <archtype> is the architecture
type. Local/Makefile is required to exist, and the building process fails if it
is absent. The other three Local files are optional, and are often not needed.

The values used for <ostype> and <archtype> are obtained from scripts called
scripts/os-type and scripts/arch-type respectively. If either of the
environment variables EXIM_OSTYPE or EXIM_ARCHTYPE is set, their values are
used, thereby providing a means of forcing particular settings. Otherwise, the
scripts try to get values from the uname command. If this fails, the shell
variables OSTYPE and ARCHTYPE are inspected. A number of ad hoc transformations
are then applied, to produce the standard names that Exim expects. You can run
these scripts directly from the shell in order to find out what values are
being used on your system.

OS/Makefile-Default contains comments about the variables that are set therein.
Some (but not all) are mentioned below. If there is something that needs
changing, review the contents of this file and the contents of the make file
for your operating system (OS/Makefile-<ostype>) to see what the default values
are.

If you need to change any of the values that are set in OS/Makefile-Default or
in OS/Makefile-<ostype>, or to add any new definitions, you do not need to
change the original files. Instead, you should make the changes by putting the
new values in an appropriate Local file. For example, when building Exim in
many releases of the Tru64-Unix (formerly Digital UNIX, formerly DEC-OSF1)
operating system, it is necessary to specify that the C compiler is called cc
rather than gcc. Also, the compiler must be called with the option -std1, to
make it recognize some of the features of Standard C that Exim uses. (Most
other compilers recognize Standard C by default.) To do this, you should create
a file called Local/Makefile-OSF1 containing the lines

CC=cc
CFLAGS=-std1

If you are compiling for just one operating system, it may be easier to put
these lines directly into Local/Makefile.

Keeping all your local configuration settings separate from the distributed
files makes it easy to transfer them to new versions of Exim simply by copying
the contents of the Local directory.

Exim contains support for doing LDAP, NIS, NIS+, and other kinds of file
lookup, but not all systems have these components installed, so the default is
not to include the relevant code in the binary. All the different kinds of file
and database lookup that Exim supports are implemented as separate code modules
which are included only if the relevant compile-time options are set. In the
case of LDAP, NIS, and NIS+, the settings for Local/Makefile are:

LOOKUP_LDAP=yes
LOOKUP_NIS=yes
LOOKUP_NISPLUS=yes

and similar settings apply to the other lookup types. They are all listed in
src/EDITME. In many cases the relevant include files and interface libraries
need to be installed before compiling Exim. However, there are some optional
lookup types (such as cdb) for which the code is entirely contained within
Exim, and no external include files or libraries are required. When a lookup
type is not included in the binary, attempts to configure Exim to use it cause
runtime configuration errors.

Many systems now use a tool called pkg-config to encapsulate information about
how to compile against a library; Exim has some initial support for being able
to use pkg-config for lookups and authenticators. For any given makefile
variable which starts "LOOKUP_" or "AUTH_", you can add a new variable with the
"_PC" suffix in the name and assign as the value the name of the package to be
queried. The results of querying via the pkg-config command will be added to
the appropriate Makefile variables with "+=" directives, so your version of 
make will need to support that syntax. For instance:

LOOKUP_SQLITE=yes
LOOKUP_SQLITE_PC=sqlite3
AUTH_GSASL=yes
AUTH_GSASL_PC=libgsasl
AUTH_HEIMDAL_GSSAPI=yes
AUTH_HEIMDAL_GSSAPI_PC=heimdal-gssapi

Exim can be linked with an embedded Perl interpreter, allowing Perl subroutines
to be called during string expansion. To enable this facility,

EXIM_PERL=perl.o

must be defined in Local/Makefile. Details of this facility are given in
chapter 12.

The location of the X11 libraries is something that varies a lot between
operating systems, and there may be different versions of X11 to cope with.
Exim itself makes no use of X11, but if you are compiling the Exim monitor, the
X11 libraries must be available. The following three variables are set in OS/
Makefile-Default:

X11=/usr/X11R6
XINCLUDE=-I$(X11)/include
XLFLAGS=-L$(X11)/lib

These are overridden in some of the operating-system configuration files. For
example, in OS/Makefile-SunOS5 there is

X11=/usr/openwin
XINCLUDE=-I$(X11)/include
XLFLAGS=-L$(X11)/lib -R$(X11)/lib

If you need to override the default setting for your operating system, place a
definition of all three of these variables into your Local/Makefile-<ostype>
file.

If you need to add any extra libraries to the link steps, these can be put in a
variable called EXTRALIBS, which appears in all the link commands, but by
default is not defined. In contrast, EXTRALIBS_EXIM is used only on the command
for linking the main Exim binary, and not for any associated utilities.

There is also DBMLIB, which appears in the link commands for binaries that use
DBM functions (see also section 4.4). Finally, there is EXTRALIBS_EXIMON, which
appears only in the link step for the Exim monitor binary, and which can be
used, for example, to include additional X11 libraries.

The make file copes with rebuilding Exim correctly if any of the configuration
files are edited. However, if an optional configuration file is deleted, it is
necessary to touch the associated non-optional file (that is, Local/Makefile or
Local/eximon.conf) before rebuilding.


4.14 OS-specific header files
-----------------------------

The OS directory contains a number of files with names of the form os.h-
<ostype>. These are system-specific C header files that should not normally
need to be changed. There is a list of macro settings that are recognized in
the file OS/os.configuring, which should be consulted if you are porting Exim
to a new operating system.


4.15 Overriding build-time options for the monitor
--------------------------------------------------

A similar process is used for overriding things when building the Exim monitor,
where the files that are involved are

OS/eximon.conf-Default
OS/eximon.conf-<ostype>
Local/eximon.conf
Local/eximon.conf-<ostype>
Local/eximon.conf-<archtype>
Local/eximon.conf-<ostype>-<archtype>

As with Exim itself, the final three files need not exist, and in this case the
OS/eximon.conf-<ostype> file is also optional. The default values in OS/
eximon.conf-Default can be overridden dynamically by setting environment
variables of the same name, preceded by EXIMON_. For example, setting
EXIMON_LOG_DEPTH in the environment overrides the value of LOG_DEPTH at
runtime.


4.16 Installing Exim binaries and scripts
-----------------------------------------

The command "make install" runs the exim_install script with no arguments. The
script copies binaries and utility scripts into the directory whose name is
specified by the BIN_DIRECTORY setting in Local/Makefile. The install script
copies files only if they are newer than the files they are going to replace.
The Exim binary is required to be owned by root and have the setuid bit set,
for normal configurations. Therefore, you must run "make install" as root so
that it can set up the Exim binary in this way. However, in some special
situations (for example, if a host is doing no local deliveries) it may be
possible to run Exim without making the binary setuid root (see chapter 56 for
details).

Exim's runtime configuration file is named by the CONFIGURE_FILE setting in
Local/Makefile. If this names a single file, and the file does not exist, the
default configuration file src/configure.default is copied there by the
installation script. If a runtime configuration file already exists, it is left
alone. If CONFIGURE_FILE is a colon-separated list, naming several alternative
files, no default is installed.

One change is made to the default configuration file when it is installed: the
default configuration contains a router that references a system aliases file.
The path to this file is set to the value specified by SYSTEM_ALIASES_FILE in
Local/Makefile (/etc/aliases by default). If the system aliases file does not
exist, the installation script creates it, and outputs a comment to the user.

The created file contains no aliases, but it does contain comments about the
aliases a site should normally have. Mail aliases have traditionally been kept
in /etc/aliases. However, some operating systems are now using /etc/mail/
aliases. You should check if yours is one of these, and change Exim's
configuration if necessary.

The default configuration uses the local host's name as the only local domain,
and is set up to do local deliveries into the shared directory /var/mail,
running as the local user. System aliases and .forward files in users' home
directories are supported, but no NIS or NIS+ support is configured. Domains
other than the name of the local host are routed using the DNS, with delivery
over SMTP.

It is possible to install Exim for special purposes (such as building a binary
distribution) in a private part of the file system. You can do this by a
command such as

make DESTDIR=/some/directory/ install

This has the effect of pre-pending the specified directory to all the file
paths, except the name of the system aliases file that appears in the default
configuration. (If a default alias file is created, its name is modified.) For
backwards compatibility, ROOT is used if DESTDIR is not set, but this usage is
deprecated.

Running make install does not copy the Exim 4 conversion script convert4r4. You
will probably run this only once if you are upgrading from Exim 3. None of the
documentation files in the doc directory are copied, except for the info files
when you have set INFO_DIRECTORY, as described in section 4.17 below.

For the utility programs, old versions are renamed by adding the suffix .O to
their names. The Exim binary itself, however, is handled differently. It is
installed under a name that includes the version number and the compile number,
for example, exim-4.97-1. The script then arranges for a symbolic link called
exim to point to the binary. If you are updating a previous version of Exim,
the script takes care to ensure that the name exim is never absent from the
directory (as seen by other processes).

If you want to see what the make install will do before running it for real,
you can pass the -n option to the installation script by this command:

make INSTALL_ARG=-n install

The contents of the variable INSTALL_ARG are passed to the installation script.
You do not need to be root to run this test. Alternatively, you can run the
installation script directly, but this must be from within the build directory.
For example, from the top-level Exim directory you could use this command:

(cd build-SunOS5-5.5.1-sparc; ../scripts/exim_install -n)

There are two other options that can be supplied to the installation script.

  * -no_chown bypasses the call to change the owner of the installed binary to
    root, and the call to make it a setuid binary.

  * -no_symlink bypasses the setting up of the symbolic link exim to the
    installed binary.

INSTALL_ARG can be used to pass these options to the script. For example:

make INSTALL_ARG=-no_symlink install

The installation script can also be given arguments specifying which files are
to be copied. For example, to install just the Exim binary, and nothing else,
without creating the symbolic link, you could use:

make INSTALL_ARG='-no_symlink exim' install


4.17 Installing info documentation
----------------------------------

Not all systems use the GNU info system for documentation, and for this reason,
the Texinfo source of Exim's documentation is not included in the main
distribution. Instead it is available separately from the FTP site (see section
1.5).

If you have defined INFO_DIRECTORY in Local/Makefile and the Texinfo source of
the documentation is found in the source tree, running "make install"
automatically builds the info files and installs them.


4.18 Setting up the spool directory
-----------------------------------

When it starts up, Exim tries to create its spool directory if it does not
exist. The Exim uid and gid are used for the owner and group of the spool
directory. Sub-directories are automatically created in the spool directory as
necessary.


4.19 Testing
------------

Having installed Exim, you can check that the runtime configuration file is
syntactically valid by running the following command, which assumes that the
Exim binary directory is within your PATH environment variable:

exim -bV

If there are any errors in the configuration file, Exim outputs error messages.
Otherwise it outputs the version number and build date, the DBM library that is
being used, and information about which drivers and other optional code modules
are included in the binary. Some simple routing tests can be done by using the
address testing option. For example,

exim -bt <local username>

should verify that it recognizes a local mailbox, and

exim -bt <remote address>

a remote one. Then try getting it to deliver mail, both locally and remotely.
This can be done by passing messages directly to Exim, without going through a
user agent. For example:

exim -v postmaster@your.domain.example
From: user@your.domain.example
To: postmaster@your.domain.example
Subject: Testing Exim

This is a test message.
^D

The -v option causes Exim to output some verification of what it is doing. In
this case you should see copies of three log lines, one for the message's
arrival, one for its delivery, and one containing "Completed".

If you encounter problems, look at Exim's log files (mainlog and paniclog) to
see if there is any relevant information there. Another source of information
is running Exim with debugging turned on, by specifying the -d option. If a
message is stuck on Exim's spool, you can force a delivery with debugging
turned on by a command of the form

exim -d -M <exim-message-id>

You must be root or an "admin user" in order to do this. The -d option produces
rather a lot of output, but you can cut this down to specific areas. For
example, if you use -d-all+route only the debugging information relevant to
routing is included. (See the -d option in chapter 5 for more details.)

One specific problem that has shown up on some sites is the inability to do
local deliveries into a shared mailbox directory, because it does not have the
"sticky bit" set on it. By default, Exim tries to create a lock file before
writing to a mailbox file, and if it cannot create the lock file, the delivery
is deferred. You can get round this either by setting the "sticky bit" on the
directory, or by setting a specific group for local deliveries and allowing
that group to create files in the directory (see the comments above the 
local_delivery transport in the default configuration file). Another approach
is to configure Exim not to use lock files, but just to rely on fcntl() locking
instead. However, you should do this only if all user agents also use fcntl()
locking. For further discussion of locking issues, see chapter 26.

One thing that cannot be tested on a system that is already running an MTA is
the receipt of incoming SMTP mail on the standard SMTP port. However, the -oX
option can be used to run an Exim daemon that listens on some other port, or 
inetd can be used to do this. The -bh option and the exim_checkaccess utility
can be used to check out policy controls on incoming SMTP mail.

Testing a new version on a system that is already running Exim can most easily
be done by building a binary with a different CONFIGURE_FILE setting. From
within the runtime configuration, all other file and directory names that Exim
uses can be altered, in order to keep it entirely clear of the production
version.


4.20 Replacing another MTA with Exim
------------------------------------

Building and installing Exim for the first time does not of itself put it in
general use. The name by which the system's MTA is called by mail user agents
is either /usr/sbin/sendmail, or /usr/lib/sendmail (depending on the operating
system), and it is necessary to make this name point to the exim binary in
order to get the user agents to pass messages to Exim. This is normally done by
renaming any existing file and making /usr/sbin/sendmail or /usr/lib/sendmail a
symbolic link to the exim binary. It is a good idea to remove any setuid
privilege and executable status from the old MTA. It is then necessary to stop
and restart the mailer daemon, if one is running.

Some operating systems have introduced alternative ways of switching MTAs. For
example, if you are running FreeBSD, you need to edit the file /etc/mail/
mailer.conf instead of setting up a symbolic link as just described. A typical
example of the contents of this file for running Exim is as follows:

sendmail            /usr/exim/bin/exim
send-mail           /usr/exim/bin/exim
mailq               /usr/exim/bin/exim -bp
newaliases          /usr/bin/true

Once you have set up the symbolic link, or edited /etc/mail/mailer.conf, your
Exim installation is "live". Check it by sending a message from your favourite
user agent.

You should consider what to tell your users about the change of MTA. Exim may
have different capabilities to what was previously running, and there are
various operational differences such as the text of messages produced by
command line options and in bounce messages. If you allow your users to make
use of Exim's filtering capabilities, you should make the document entitled 
Exim's interface to mail filtering available to them.


4.21 Running the daemon
-----------------------

The most common command line for launching the Exim daemon looks like

exim -bd -q5m

This starts a daemon which

  * listens for incoming smtp connections, launching handler processes for each
    new one

  * starts a queue-runner process every five minutes, to inspect queued
    messages and run delivery attempts on any that have arrived at their retry
    time

Should a queue run take longer than the time between queue-runner starts, they
will run in parallel. Numbers of jobs of the various types are subject to
policy controls defined in the configuration.


4.22 Upgrading Exim
-------------------

If you are already running Exim on your host, building and installing a new
version automatically makes it available to MUAs, or any other programs that
call the MTA directly. However, if you are running an Exim daemon, you do need 
to send it a HUP signal, to make it re-execute itself, and thereby pick up the
new binary. You do not need to stop processing mail in order to install a new
version of Exim. The install script does not modify an existing runtime
configuration file.


4.23 Stopping the Exim daemon on Solaris
----------------------------------------

The standard command for stopping the mailer daemon on Solaris is

/etc/init.d/sendmail stop

If /usr/lib/sendmail has been turned into a symbolic link, this script fails to
stop Exim because it uses the command ps -e and greps the output for the text
"sendmail"; this is not present because the actual program name (that is,
"exim") is given by the ps command with these options. A solution is to replace
the line that finds the process id with something like

pid=`cat /var/spool/exim/exim-daemon.pid`

to obtain the daemon's pid directly from the file that Exim saves it in.

Note, however, that stopping the daemon does not "stop Exim". Messages can
still be received from local processes, and if automatic delivery is configured
(the normal case), deliveries will still occur.



===============================================================================
5. THE EXIM COMMAND LINE

Exim's command line takes the standard Unix form of a sequence of options, each
starting with a hyphen character, followed by a number of arguments. The
options are compatible with the main options of Sendmail, and there are also
some additional options, some of which are compatible with Smail 3. Certain
combinations of options do not make sense, and provoke an error if used. The
form of the arguments depends on which options are set.


5.1 Setting options by program name
-----------------------------------

If Exim is called under the name mailq, it behaves as if the option -bp were
present before any other options. The -bp option requests a listing of the
contents of the mail queue on the standard output. This feature is for
compatibility with some systems that contain a command of that name in one of
the standard libraries, symbolically linked to /usr/sbin/sendmail or /usr/lib/
sendmail.

If Exim is called under the name rsmtp it behaves as if the option -bS were
present before any other options, for compatibility with Smail. The -bS option
is used for reading in a number of messages in batched SMTP format.

If Exim is called under the name rmail it behaves as if the -i and -oee options
were present before any other options, for compatibility with Smail. The name 
rmail is used as an interface by some UUCP systems.

If Exim is called under the name runq it behaves as if the option -q were
present before any other options, for compatibility with Smail. The -q option
causes a single queue runner process to be started.

If Exim is called under the name newaliases it behaves as if the option -bi
were present before any other options, for compatibility with Sendmail. This
option is used for rebuilding Sendmail's alias file. Exim does not have the
concept of a single alias file, but can be configured to run a given command if
called with the -bi option.


5.2 Trusted and admin users
---------------------------

Some Exim options are available only to trusted users and others are available
only to admin users. In the description below, the phrases "Exim user" and
"Exim group" mean the user and group defined by EXIM_USER and EXIM_GROUP in
Local/Makefile or set by the exim_user and exim_group options. These do not
necessarily have to use the name "exim".

  * The trusted users are root, the Exim user, any user listed in the 
    trusted_users configuration option, and any user whose current group or any
    supplementary group is one of those listed in the trusted_groups
    configuration option. Note that the Exim group is not automatically
    trusted.

    Trusted users are always permitted to use the -f option or a leading
    "From " line to specify the envelope sender of a message that is passed to
    Exim through the local interface (see the -bm and -f options below). See
    the untrusted_set_sender option for a way of permitting non-trusted users
    to set envelope senders.

    For a trusted user, there is never any check on the contents of the From:
    header line, and a Sender: line is never added. Furthermore, any existing 
    Sender: line in incoming local (non-TCP/IP) messages is not removed.

    Trusted users may also specify a host name, host address, interface
    address, protocol name, ident value, and authentication data when
    submitting a message locally. Thus, they are able to insert messages into
    Exim's queue locally that have the characteristics of messages received
    from a remote host. Untrusted users may in some circumstances use -f, but
    can never set the other values that are available to trusted users.

  * The admin users are root, the Exim user, and any user that is a member of
    the Exim group or of any group listed in the admin_groups configuration
    option. The current group does not have to be one of these groups.

    Admin users are permitted to list the queue, and to carry out certain
    operations on messages, for example, to force delivery failures. It is also
    necessary to be an admin user in order to see the full information provided
    by the Exim monitor, and full debugging output.

    By default, the use of the -M, -q, -R, and -S options to cause Exim to
    attempt delivery of messages on its queue is restricted to admin users.
    However, this restriction can be relaxed by setting the prod_requires_admin
    option false (that is, specifying no_prod_requires_admin).

    Similarly, the use of the -bp option to list all the messages in the queue
    is restricted to admin users unless queue_list_requires_admin is set false.

Warning: If you configure your system so that admin users are able to edit
Exim's configuration file, you are giving those users an easy way of getting
root. There is further discussion of this issue at the start of chapter 6.


5.3 Command line options
------------------------

Exim's command line options are described in alphabetical order below. If none
of the options that specifies a specific action (such as starting the daemon or
a queue runner, or testing an address, or receiving a message in a specific
format, or listing the queue) are present, and there is at least one argument
on the command line, -bm (accept a local message on the standard input, with
the arguments specifying the recipients) is assumed. Otherwise, Exim outputs a
brief message about itself and exits.

-- --

    This is a pseudo-option whose only purpose is to terminate the options and
    therefore to cause subsequent command line items to be treated as arguments
    rather than options, even if they begin with hyphens.

--help

    This option causes Exim to output a few sentences stating what it is. The
    same output is generated if the Exim binary is called with no options and
    no arguments.

--version

    This option is an alias for -bV and causes version information to be
    displayed.

-Ac, -Am

    These options are used by Sendmail for selecting configuration files and
    are ignored by Exim.

-B <type>

    This is a Sendmail option for selecting 7 or 8 bit processing. Exim is
    8-bit clean; it ignores this option.

-bd

    This option runs Exim as a daemon, awaiting incoming SMTP connections.
    Usually the -bd option is combined with the -q<time> option, to specify
    that the daemon should also initiate periodic queue runs.

    The -bd option can be used only by an admin user. If either of the -d
    (debugging) or -v (verifying) options are set, the daemon does not
    disconnect from the controlling terminal. When running this way, it can be
    stopped by pressing ctrl-C.

    By default, Exim listens for incoming connections to the standard SMTP port
    on all the host's running interfaces. However, it is possible to listen on
    other ports, on multiple ports, and only on specific interfaces. Chapter 13
    contains a description of the options that control this.

    When a listening daemon is started without the use of -oX (that is, without
    overriding the normal configuration), it writes its process id to a file
    called exim-daemon.pid in Exim's spool directory. This location can be
    overridden by setting PID_FILE_PATH in Local/Makefile. The file is written
    while Exim is still running as root.

    When -oX is used on the command line to start a listening daemon, the
    process id is not written to the normal pid file path. However, -oP can be
    used to specify a path on the command line if a pid file is required.

    The SIGHUP signal can be used to cause the daemon to re-execute itself.
    This should be done whenever Exim's configuration file, or any file that is
    incorporated into it by means of the .include facility, is changed, and
    also whenever a new version of Exim is installed. It is not necessary to do
    this when other files that are referenced from the configuration (for
    example, alias files) are changed, because these are reread each time they
    are used.

    Either a SIGTERM or a SIGINT signal should be used to cause the daemon to
    cleanly shut down. Subprocesses handling recceiving or delivering messages,
    or for scanning the queue, will not be affected by the termination of the
    daemon process.

-bdf

    This option has the same effect as -bd except that it never disconnects
    from the controlling terminal, even when no debugging is specified.

-be

    Run Exim in expansion testing mode. Exim discards its root privilege, to
    prevent ordinary users from using this mode to read otherwise inaccessible
    files. If no arguments are given, Exim runs interactively, prompting for
    lines of data. Otherwise, it processes each argument in turn.

    If Exim was built with USE_READLINE=yes in Local/Makefile, it tries to load
    the libreadline library dynamically whenever the -be option is used without
    command line arguments. If successful, it uses the readline() function,
    which provides extensive line-editing facilities, for reading the test
    data. A line history is supported.

    Long expansion expressions can be split over several lines by using
    backslash continuations. As in Exim's runtime configuration, white space at
    the start of continuation lines is ignored. Each argument or data line is
    passed through the string expansion mechanism, and the result is output.
    Variable values from the configuration file (for example, $qualify_domain)
    are available, but no message-specific values (such as $message_exim_id)
    are set, because no message is being processed (but see -bem and -Mset).

    Note: If you use this mechanism to test lookups, and you change the data
    files or databases you are using, you must exit and restart Exim before
    trying the same lookup again. Otherwise, because each Exim process caches
    the results of lookups, you will just get the same result as before.

    Macro processing is done on lines before string-expansion: new macros can
    be defined and macros will be expanded. Because macros in the config file
    are often used for secrets, those are only available to admin users.

    The word "set" at the start of a line, followed by a single space, is
    recognised specially as defining a value for a variable. The syntax is
    otherwise the same as the ACL modifier "set =".

-bem <filename>

    This option operates like -be except that it must be followed by the name
    of a file. For example:

    exim -bem /tmp/testmessage

    The file is read as a message (as if receiving a locally-submitted non-SMTP
    message) before any of the test expansions are done. Thus, message-specific
    variables such as $message_size and $header_from: are available. However,
    no Received: header is added to the message. If the -t option is set,
    recipients are read from the headers in the normal way, and are shown in
    the $recipients variable. Note that recipients cannot be given on the
    command line, because further arguments are taken as strings to expand
    (just like -be).

-bF <filename>

    This option is the same as -bf except that it assumes that the filter being
    tested is a system filter. The additional commands that are available only
    in system filters are recognized.

-bf <filename>

    This option runs Exim in user filter testing mode; the file is the filter
    file to be tested, and a test message must be supplied on the standard
    input. If there are no message-dependent tests in the filter, an empty file
    can be supplied.

    If you want to test a system filter file, use -bF instead of -bf. You can
    use both -bF and -bf on the same command, in order to test a system filter
    and a user filter in the same run. For example:

    exim -bF /system/filter -bf /user/filter </test/message

    This is helpful when the system filter adds header lines or sets filter
    variables that are used by the user filter.

    If the test filter file does not begin with one of the special lines

    # Exim filter
    # Sieve filter

    it is taken to be a normal .forward file, and is tested for validity under
    that interpretation. See sections 22.4 to 22.6 for a description of the
    possible contents of non-filter redirection lists.

    The result of an Exim command that uses -bf, provided no errors are
    detected, is a list of the actions that Exim would try to take if presented
    with the message for real. More details of filter testing are given in the
    separate document entitled Exim's interfaces to mail filtering.

    When testing a filter file, the envelope sender can be set by the -f
    option, or by a "From " line at the start of the test message. Various
    parameters that would normally be taken from the envelope recipient address
    of the message can be set by means of additional command line options (see
    the next four options).

-bfd <domain>

    This sets the domain of the recipient address when a filter file is being
    tested by means of the -bf option. The default is the value of
    $qualify_domain.

-bfl <local part>

    This sets the local part of the recipient address when a filter file is
    being tested by means of the -bf option. The default is the username of the
    process that calls Exim. A local part should be specified with any prefix
    or suffix stripped, because that is how it appears to the filter when a
    message is actually being delivered.

-bfp <prefix>

    This sets the prefix of the local part of the recipient address when a
    filter file is being tested by means of the -bf option. The default is an
    empty prefix.

-bfs <suffix>

    This sets the suffix of the local part of the recipient address when a
    filter file is being tested by means of the -bf option. The default is an
    empty suffix.

-bh <IP address>

    This option runs a fake SMTP session as if from the given IP address, using
    the standard input and output. The IP address may include a port number at
    the end, after a full stop. For example:

    exim -bh 10.9.8.7.1234
    exim -bh fe80::a00:20ff:fe86:a061.5678

    When an IPv6 address is given, it is converted into canonical form. In the
    case of the second example above, the value of $sender_host_address after
    conversion to the canonical form is
    "fe80:0000:0000:0a00:20ff:fe86:a061.5678".

    Comments as to what is going on are written to the standard error file.
    These include lines beginning with "LOG" for anything that would have been
    logged. This facility is provided for testing configuration options for
    incoming messages, to make sure they implement the required policy. For
    example, you can test your relay controls using -bh.

    Warning 1: You can test features of the configuration that rely on ident
    (RFC 1413) information by using the -oMt option. However, Exim cannot
    actually perform an ident callout when testing using -bh because there is
    no incoming SMTP connection.

    Warning 2: Address verification callouts (see section 44.22) are also
    skipped when testing using -bh. If you want these callouts to occur, use 
    -bhc instead.

    Messages supplied during the testing session are discarded, and nothing is
    written to any of the real log files. There may be pauses when DNS (and
    other) lookups are taking place, and of course these may time out. The -oMi
    option can be used to specify a specific IP interface and port if this is
    important, and -oMaa and -oMai can be used to set parameters as if the SMTP
    session were authenticated.

    The exim_checkaccess utility is a "packaged" version of -bh whose output
    just states whether a given recipient address from a given host is
    acceptable or not. See section 54.8.

    Features such as authentication and encryption, where the client input is
    not plain text, cannot easily be tested with -bh. Instead, you should use a
    specialized SMTP test program such as swaks.

-bhc <IP address>

    This option operates in the same way as -bh, except that address
    verification callouts are performed if required. This includes consulting
    and updating the callout cache database.

-bi

    Sendmail interprets the -bi option as a request to rebuild its alias file.
    Exim does not have the concept of a single alias file, and so it cannot
    mimic this behaviour. However, calls to /usr/lib/sendmail with the -bi
    option tend to appear in various scripts such as NIS make files, so the
    option must be recognized.

    If -bi is encountered, the command specified by the bi_command
    configuration option is run, under the uid and gid of the caller of Exim.
    If the -oA option is used, its value is passed to the command as an
    argument. The command set by bi_command may not contain arguments. The
    command can use the exim_dbmbuild utility, or some other means, to rebuild
    alias files if this is required. If the bi_command option is not set,
    calling Exim with -bi is a no-op.

-bI:help

    We shall provide various options starting "-bI:" for querying Exim for
    information. The output of many of these will be intended for machine
    consumption. This one is not. The -bI:help option asks Exim for a synopsis
    of supported options beginning "-bI:". Use of any of these options shall
    cause Exim to exit after producing the requested output.

-bI:dscp

    This option causes Exim to emit an alphabetically sorted list of all
    recognised DSCP names.

-bI:sieve

    This option causes Exim to emit an alphabetically sorted list of all
    supported Sieve protocol extensions on stdout, one per line. This is
    anticipated to be useful for ManageSieve (RFC 5804) implementations, in
    providing that protocol's "SIEVE" capability response line. As the precise
    list may depend upon compile-time build options, which this option will
    adapt to, this is the only way to guarantee a correct response.

-bm

    This option runs an Exim receiving process that accepts an incoming,
    locally-generated message on the standard input. The recipients are given
    as the command arguments (except when -t is also present - see below). Each
    argument can be a comma-separated list of RFC 2822 addresses. This is the
    default option for selecting the overall action of an Exim call; it is
    assumed if no other conflicting option is present.

    If any addresses in the message are unqualified (have no domain), they are
    qualified by the values of the qualify_domain or qualify_recipient options,
    as appropriate. The -bnq option (see below) provides a way of suppressing
    this for special cases.

    Policy checks on the contents of local messages can be enforced by means of
    the non-SMTP ACL. See chapter 44 for details.

    The return code is zero if the message is successfully accepted. Otherwise,
    the action is controlled by the -oex option setting - see below.

    The format of the message must be as defined in RFC 2822, except that, for
    compatibility with Sendmail and Smail, a line in one of the forms

    From sender Fri Jan  5 12:55 GMT 1997
    From sender Fri, 5 Jan 97 12:55:01

    (with the weekday optional, and possibly with additional text after the
    date) is permitted to appear at the start of the message. There appears to
    be no authoritative specification of the format of this line. Exim
    recognizes it by matching against the regular expression defined by the 
    uucp_from_pattern option, which can be changed if necessary.

    The specified sender is treated as if it were given as the argument to the 
    -f option, but if a -f option is also present, its argument is used in
    preference to the address taken from the message. The caller of Exim must
    be a trusted user for the sender of a message to be set in this way.

-bmalware <filename>

    This debugging option causes Exim to scan the given file or directory
    (depending on the used scanner interface), using the malware scanning
    framework. The option of av_scanner influences this option, so if 
    av_scanner's value is dependent upon an expansion then the expansion should
    have defaults which apply to this invocation. ACLs are not invoked, so if 
    av_scanner references an ACL variable then that variable will never be
    populated and -bmalware will fail.

    Exim will have changed working directory before resolving the filename, so
    using fully qualified pathnames is advisable. Exim will be running as the
    Exim user when it tries to open the file, rather than as the invoking user.
    This option requires admin privileges.

    The -bmalware option will not be extended to be more generally useful,
    there are better tools for file-scanning. This option exists to help
    administrators verify their Exim and AV scanner configuration.

-bnq

    By default, Exim automatically qualifies unqualified addresses (those
    without domains) that appear in messages that are submitted locally (that
    is, not over TCP/IP). This qualification applies both to addresses in
    envelopes, and addresses in header lines. Sender addresses are qualified
    using qualify_domain, and recipient addresses using qualify_recipient
    (which defaults to the value of qualify_domain).

    Sometimes, qualification is not wanted. For example, if -bS (batch SMTP) is
    being used to re-submit messages that originally came from remote hosts
    after content scanning, you probably do not want to qualify unqualified
    addresses in header lines. (Such lines will be present only if you have not
    enabled a header syntax check in the appropriate ACL.)

    The -bnq option suppresses all qualification of unqualified addresses in
    messages that originate on the local host. When this is used, unqualified
    addresses in the envelope provoke errors (causing message rejection) and
    unqualified addresses in header lines are left alone.

-bP

    If this option is given with no arguments, it causes the values of all
    Exim's main configuration options to be written to the standard output. The
    values of one or more specific options can be requested by giving their
    names as arguments, for example:

    exim -bP qualify_domain hold_domains

    However, any option setting that is preceded by the word "hide" in the
    configuration file is not shown in full, except to an admin user. For other
    users, the output is as in this example:

    mysql_servers = <value not displayable>

    If config is given as an argument, the config is output, as it was parsed,
    any include file resolved, any comment removed.

    If config_file is given as an argument, the name of the runtime
    configuration file is output. (configure_file works too, for backward
    compatibility.) If a list of configuration files was supplied, the value
    that is output here is the name of the file that was actually used.

    If the -n flag is given, then for most modes of -bP operation the name will
    not be output.

    If log_file_path or pid_file_path are given, the names of the directories
    where log files and daemon pid files are written are output, respectively.
    If these values are unset, log files are written in a sub-directory of the
    spool directory called log, and the pid file is written directly into the
    spool directory.

    If -bP is followed by a name preceded by "+", for example,

    exim -bP +local_domains

    it searches for a matching named list of any type (domain, host, address,
    or local part) and outputs what it finds.

    If one of the words router, transport, or authenticator is given, followed
    by the name of an appropriate driver instance, the option settings for that
    driver are output. For example:

    exim -bP transport local_delivery

    The generic driver options are output first, followed by the driver's
    private options. A list of the names of drivers of a particular type can be
    obtained by using one of the words router_list, transport_list, or 
    authenticator_list, and a complete list of all drivers with their option
    settings can be obtained by using routers, transports, or authenticators.

    If environment is given as an argument, the set of environment variables is
    output, line by line. Using the -n flag suppresses the value of the
    variables.

    If invoked by an admin user, then macro, macro_list and macros are
    available, similarly to the drivers. Because macros are sometimes used for
    storing passwords, this option is restricted. The output format is one item
    per line. For the "-bP macro <name>" form, if no such macro is found the
    exit status will be nonzero.

-bp

    This option requests a listing of the contents of the mail queue on the
    standard output. If the -bp option is followed by a list of message ids,
    just those messages are listed. By default, this option can be used only by
    an admin user. However, the queue_list_requires_admin option can be set
    false to allow any user to see the queue.

    Each message in the queue is displayed as in the following example:

    25m  2.9K 0t5C6f-0000c8-00 <alice@wonderland.fict.example>
              red.king@looking-glass.fict.example
              <other addresses>

    The first line contains the length of time the message has been in the
    queue (in this case 25 minutes), the size of the message (2.9K), the unique
    local identifier for the message, and the message sender, as contained in
    the envelope. For bounce messages, the sender address is empty, and appears
    as "<>". If the message was submitted locally by an untrusted user who
    overrode the default sender address, the user's login name is shown in
    parentheses before the sender address.

    If the message is frozen (attempts to deliver it are suspended) then the
    text "*** frozen ***" is displayed at the end of this line.

    The recipients of the message (taken from the envelope, not the headers)
    are displayed on subsequent lines. Those addresses to which the message has
    already been delivered are marked with the letter D. If an original address
    gets expanded into several addresses via an alias or forward file, the
    original is displayed with a D only when deliveries for all of its child
    addresses are complete.

-bpa

    This option operates like -bp, but in addition it shows delivered addresses
    that were generated from the original top level address(es) in each message
    by alias or forwarding operations. These addresses are flagged with "+D"
    instead of just "D".

-bpc

    This option counts the number of messages in the queue, and writes the
    total to the standard output. It is restricted to admin users, unless 
    queue_list_requires_admin is set false.

-bpi

    This option operates like -bp, but only outputs message ids (one per line).

-bpr

    This option operates like -bp, but the output is not sorted into
    chronological order of message arrival. This can speed it up when there are
    lots of messages in the queue, and is particularly useful if the output is
    going to be post-processed in a way that doesn't need the sorting.

-bpra

    This option is a combination of -bpr and -bpa.

-bpri

    This option is a combination of -bpr and -bpi.

-bpru

    This option is a combination of -bpr and -bpu.

-bpu

    This option operates like -bp but shows only undelivered top-level
    addresses for each message displayed. Addresses generated by aliasing or
    forwarding are not shown, unless the message was deferred after processing
    by a router with the one_time option set.

-brt

    This option is for testing retry rules, and it must be followed by up to
    three arguments. It causes Exim to look for a retry rule that matches the
    values and to write it to the standard output. For example:

    exim -brt bach.comp.mus.example
    Retry rule: *.comp.mus.example  F,2h,15m; F,4d,30m;

    See chapter 32 for a description of Exim's retry rules. The first argument,
    which is required, can be a complete address in the form local_part@domain,
    or it can be just a domain name. If the second argument contains a dot, it
    is interpreted as an optional second domain name; if no retry rule is found
    for the first argument, the second is tried. This ties in with Exim's
    behaviour when looking for retry rules for remote hosts - if no rule is
    found that matches the host, one that matches the mail domain is sought.
    Finally, an argument that is the name of a specific delivery error, as used
    in setting up retry rules, can be given. For example:

    exim -brt haydn.comp.mus.example quota_3d
    Retry rule: *@haydn.comp.mus.example quota_3d  F,1h,15m

-brw

    This option is for testing address rewriting rules, and it must be followed
    by a single argument, consisting of either a local part without a domain,
    or a complete address with a fully qualified domain. Exim outputs how this
    address would be rewritten for each possible place it might appear. See
    chapter 31 for further details.

-bS

    This option is used for batched SMTP input, which is an alternative
    interface for non-interactive local message submission. A number of
    messages can be submitted in a single run. However, despite its name, this
    is not really SMTP input. Exim reads each message's envelope from SMTP
    commands on the standard input, but generates no responses. If the caller
    is trusted, or untrusted_set_sender is set, the senders in the SMTP MAIL
    commands are believed; otherwise the sender is always the caller of Exim.

    The message itself is read from the standard input, in SMTP format (leading
    dots doubled), terminated by a line containing just a single dot. An error
    is provoked if the terminating dot is missing. A further message may then
    follow.

    As for other local message submissions, the contents of incoming batch SMTP
    messages can be checked using the non-SMTP ACL (see chapter 44).
    Unqualified addresses are automatically qualified using qualify_domain and 
    qualify_recipient, as appropriate, unless the -bnq option is used.

    Some other SMTP commands are recognized in the input. HELO and EHLO act as
    RSET; VRFY, EXPN, ETRN, and HELP act as NOOP; QUIT quits, ignoring the rest
    of the standard input.

    If any error is encountered, reports are written to the standard output and
    error streams, and Exim gives up immediately. The return code is 0 if no
    error was detected; it is 1 if one or more messages were accepted before
    the error was detected; otherwise it is 2.

    More details of input using batched SMTP are given in section 49.5.

-bs

    This option causes Exim to accept one or more messages by reading SMTP
    commands on the standard input, and producing SMTP replies on the standard
    output. SMTP policy controls, as defined in ACLs (see chapter 44) are
    applied. Some user agents use this interface as a way of passing
    locally-generated messages to the MTA.

    In this usage, if the caller of Exim is trusted, or untrusted_set_sender is
    set, the senders of messages are taken from the SMTP MAIL commands.
    Otherwise the content of these commands is ignored and the sender is set up
    as the calling user. Unqualified addresses are automatically qualified
    using qualify_domain and qualify_recipient, as appropriate, unless the -bnq
    option is used.

    The -bs option is also used to run Exim from inetd, as an alternative to
    using a listening daemon. Exim can distinguish the two cases by checking
    whether the standard input is a TCP/IP socket. When Exim is called from 
    inetd, the source of the mail is assumed to be remote, and the comments
    above concerning senders and qualification do not apply. In this situation,
    Exim behaves in exactly the same way as it does when receiving a message
    via the listening daemon.

-bt

    This option runs Exim in address testing mode, in which each argument is
    taken as a recipient address to be tested for deliverability. The results
    are written to the standard output. If a test fails, and the caller is not
    an admin user, no details of the failure are output, because these might
    contain sensitive information such as usernames and passwords for database
    lookups.

    If no arguments are given, Exim runs in an interactive manner, prompting
    with a right angle bracket for addresses to be tested.

    Unlike the -be test option, you cannot arrange for Exim to use the readline
    () function, because it is running as root and there are security issues.

    Each address is handled as if it were the recipient address of a message
    (compare the -bv option). It is passed to the routers and the result is
    written to the standard output. However, any router that has 
    no_address_test set is bypassed. This can make -bt easier to use for
    genuine routing tests if your first router passes everything to a scanner
    program.

    The return code is 2 if any address failed outright; it is 1 if no address
    failed outright but at least one could not be resolved for some reason.
    Return code 0 is given only when all addresses succeed.

    Note: When actually delivering a message, Exim removes duplicate recipient
    addresses after routing is complete, so that only one delivery takes place.
    This does not happen when testing with -bt; the full results of routing are
    always shown.

    Warning: -bt can only do relatively simple testing. If any of the routers
    in the configuration makes any tests on the sender address of a message, 
    you can use the -f option to set an appropriate sender when running -bt
    tests. Without it, the sender is assumed to be the calling user at the
    default qualifying domain. However, if you have set up (for example)
    routers whose behaviour depends on the contents of an incoming message, you
    cannot test those conditions using -bt. The -N option provides a possible
    way of doing such tests.

-bV

    This option causes Exim to write the current version number, compilation
    number, and compilation date of the exim binary to the standard output. It
    also lists the DBM library that is being used, the optional modules (such
    as specific lookup types), the drivers that are included in the binary, and
    the name of the runtime configuration file that is in use.

    As part of its operation, -bV causes Exim to read and syntax check its
    configuration file. However, this is a static check only. It cannot check
    values that are to be expanded. For example, although a misspelt ACL verb
    is detected, an error in the verb's arguments is not. You cannot rely on 
    -bV alone to discover (for example) all the typos in the configuration;
    some realistic testing is needed. The -bh and -N options provide more
    dynamic testing facilities.

-bv

    This option runs Exim in address verification mode, in which each argument
    is taken as a recipient address to be verified by the routers. (This does
    not involve any verification callouts). During normal operation,
    verification happens mostly as a consequence processing a verify condition
    in an ACL (see chapter 44). If you want to test an entire ACL, possibly
    including callouts, see the -bh and -bhc options.

    If verification fails, and the caller is not an admin user, no details of
    the failure are output, because these might contain sensitive information
    such as usernames and passwords for database lookups.

    If no arguments are given, Exim runs in an interactive manner, prompting
    with a right angle bracket for addresses to be verified.

    Unlike the -be test option, you cannot arrange for Exim to use the readline
    () function, because it is running as exim and there are security issues.

    Verification differs from address testing (the -bt option) in that routers
    that have no_verify set are skipped, and if the address is accepted by a
    router that has fail_verify set, verification fails. The address is
    verified as a recipient if -bv is used; to test verification for a sender
    address, -bvs should be used.

    If the -v option is not set, the output consists of a single line for each
    address, stating whether it was verified or not, and giving a reason in the
    latter case. Without -v, generating more than one address by redirection
    causes verification to end successfully, without considering the generated
    addresses. However, if just one address is generated, processing continues,
    and the generated address must verify successfully for the overall
    verification to succeed.

    When -v is set, more details are given of how the address has been handled,
    and in the case of address redirection, all the generated addresses are
    also considered. Verification may succeed for some and fail for others.

    The return code is 2 if any address failed outright; it is 1 if no address
    failed outright but at least one could not be resolved for some reason.
    Return code 0 is given only when all addresses succeed.

    If any of the routers in the configuration makes any tests on the sender
    address of a message, you should use the -f option to set an appropriate
    sender when running -bv tests. Without it, the sender is assumed to be the
    calling user at the default qualifying domain.

-bvs

    This option acts like -bv, but verifies the address as a sender rather than
    a recipient address. This affects any rewriting and qualification that
    might happen.

-bw

    This option runs Exim as a daemon, awaiting incoming SMTP connections,
    similarly to the -bd option. All port specifications on the command-line
    and in the configuration file are ignored. Queue-running may not be
    specified.

    In this mode, Exim expects to be passed a socket as fd 0 (stdin) which is
    listening for connections. This permits the system to start up and have
    inetd (or equivalent) listen on the SMTP ports, starting an Exim daemon for
    each port only when the first connection is received.

    If the option is given as -bw<time> then the time is a timeout, after which
    the daemon will exit, which should cause inetd to listen once more.

-C <filelist>

    This option causes Exim to find the runtime configuration file from the
    given list instead of from the list specified by the CONFIGURE_FILE
    compile-time setting. Usually, the list will consist of just a single
    filename, but it can be a colon-separated list of names. In this case, the
    first file that exists is used. Failure to open an existing file stops Exim
    from proceeding any further along the list, and an error is generated.

    When this option is used by a caller other than root, and the list is
    different from the compiled-in list, Exim gives up its root privilege
    immediately, and runs with the real and effective uid and gid set to those
    of the caller. However, if a TRUSTED_CONFIG_LIST file is defined in Local/
    Makefile, that file contains a list of full pathnames, one per line, for
    configuration files which are trusted. Root privilege is retained for any
    configuration file so listed, as long as the caller is the Exim user (or
    the user specified in the CONFIGURE_OWNER option, if any), and as long as
    the configuration file is not writeable by inappropriate users or groups.

    Leaving TRUSTED_CONFIG_LIST unset precludes the possibility of testing a
    configuration using -C right through message reception and delivery, even
    if the caller is root. The reception works, but by that time, Exim is
    running as the Exim user, so when it re-executes to regain privilege for
    the delivery, the use of -C causes privilege to be lost. However, root can
    test reception and delivery using two separate commands (one to put a
    message in the queue, using -odq, and another to do the delivery, using -M
    ).

    If ALT_CONFIG_PREFIX is defined in Local/Makefile, it specifies a prefix
    string with which any file named in a -C command line option must start. In
    addition, the filename must not contain the sequence "/../". However, if
    the value of the -C option is identical to the value of CONFIGURE_FILE in
    Local/Makefile, Exim ignores -C and proceeds as usual. There is no default
    setting for ALT_CONFIG_PREFIX; when it is unset, any filename can be used
    with -C.

    ALT_CONFIG_PREFIX can be used to confine alternative configuration files to
    a directory to which only root has access. This prevents someone who has
    broken into the Exim account from running a privileged Exim with an
    arbitrary configuration file.

    The -C facility is useful for ensuring that configuration files are
    syntactically correct, but cannot be used for test deliveries, unless the
    caller is privileged, or unless it is an exotic configuration that does not
    require privilege. No check is made on the owner or group of the files
    specified by this option.

-D<macro>=<value>

    This option can be used to override macro definitions in the configuration
    file (see section 6.4). However, like -C, if it is used by an unprivileged
    caller, it causes Exim to give up its root privilege. If DISABLE_D_OPTION
    is defined in Local/Makefile, the use of -D is completely disabled, and its
    use causes an immediate error exit.

    If WHITELIST_D_MACROS is defined in Local/Makefile then it should be a
    colon-separated list of macros which are considered safe and, if -D only
    supplies macros from this list, and the values are acceptable, then Exim
    will not give up root privilege if the caller is root, the Exim run-time
    user, or the CONFIGURE_OWNER, if set. This is a transition mechanism and is
    expected to be removed in the future. Acceptable values for the macros
    satisfy the regexp: "^[A-Za-z0-9_/.-]*$"

    The entire option (including equals sign if present) must all be within one
    command line item. -D can be used to set the value of a macro to the empty
    string, in which case the equals sign is optional. These two commands are
    synonymous:

    exim -DABC  ...
    exim -DABC= ...

    To include spaces in a macro definition item, quotes must be used. If you
    use quotes, spaces are permitted around the macro name and the equals sign.
    For example:

    exim '-D ABC = something' ...

    -D may be repeated up to 10 times on a command line. Only macro names up to
    22 letters long can be set.

-d<debug options>

    This option causes debugging information to be written to the standard
    error stream. It is restricted to admin users because debugging output may
    show database queries that contain password information. Also, the details
    of users' filter files should be protected. If a non-admin user uses -d,
    Exim writes an error message to the standard error stream and exits with a
    non-zero return code.

    When -d is used, -v is assumed. If -d is given on its own, a lot of
    standard debugging data is output. This can be reduced, or increased to
    include some more rarely needed information, by directly following -d with
    a string made up of names preceded by plus or minus characters. These add
    or remove sets of debugging data, respectively. For example, -d+filter adds
    filter debugging, whereas -d-all+filter selects only filter debugging. Note
    that no spaces are allowed in the debug setting. The available debugging
    categories are:

        acl          ACL interpretation
        auth         authenticators
        deliver      general delivery logic
        dns          DNS lookups (see also resolver)
        dnsbl        DNS black list (aka RBL) code
        exec         arguments for execv() calls
        expand       detailed debugging for string expansions
        filter       filter handling
        hints_lookup hints data lookups
        host_lookup  all types of name-to-IP address handling
        ident        ident lookup
        interface    lists of local interfaces
        lists        matching things in lists
        load         system load checks
        local_scan   can be used by local_scan() (see chapter 46)
        lookup       general lookup code and all lookups
        memory       memory handling
        noutf8       modifier: avoid UTF-8 line-drawing
        pid          modifier: add pid to debug output lines
        process_info setting info for the process log
        queue_run    queue runs
        receive      general message reception logic
        resolver     turn on the DNS resolver's debugging output
        retry        retry handling
        rewrite      address rewriting"
        route        address routing
        timestamp    modifier: add timestamp to debug output lines
        tls          TLS logic
        transport    transports
        uid          changes of uid/gid and looking up uid/gid
        verify       address verification logic
        all          almost all of the above (see below), and also -v

    The "all" option excludes "memory" when used as "+all", but includes it for
    "-all". The reason for this is that "+all" is something that people tend to
    use when generating debug output for Exim maintainers. If "+memory" is
    included, an awful lot of output that is very rarely of interest is
    generated, so it now has to be explicitly requested. However, "-all" does
    turn everything off.

    The "resolver" option produces output only if the DNS resolver was compiled
    with DEBUG enabled. This is not the case in some operating systems. Also,
    unfortunately, debugging output from the DNS resolver is written to stdout
    rather than stderr.

    The default (-d with no argument) omits "expand", "filter", "interface",
    "load", "memory", "pid", "resolver", and "timestamp". However, the "pid"
    selector is forced when debugging is turned on for a daemon, which then
    passes it on to any re-executed Exims. Exim also automatically adds the pid
    to debug lines when several remote deliveries are run in parallel.

    The "timestamp" selector causes the current time to be inserted at the
    start of all debug output lines. This can be useful when trying to track
    down delays in processing.

    The "noutf8" selector disables the use of UTF-8 line-drawing characters to
    group related information. When disabled. ascii-art is used instead. Using
    the "+all" option does not set this modifier,

    If the debug_print option is set in any driver, it produces output whenever
    any debugging is selected, or if -v is used.

-dd<debug options>

    This option behaves exactly like -d except when used on a command that
    starts a daemon process. In that case, debugging is turned off for the
    subprocesses that the daemon creates. Thus, it is useful for monitoring the
    behaviour of the daemon without creating as much output as full debugging
    does.

-dropcr

    This is an obsolete option that is now a no-op. It used to affect the way
    Exim handled CR and LF characters in incoming messages. What happens now is
    described in section 48.2.

-E

    This option specifies that an incoming message is a locally-generated
    delivery failure report. It is used internally by Exim when handling
    delivery failures and is not intended for external use. Its only effect is
    to stop Exim generating certain messages to the postmaster, as otherwise
    message cascades could occur in some situations. As part of the same
    option, a message id may follow the characters -E. If it does, the log
    entry for the receipt of the new message contains the id, following "R=",
    as a cross-reference.

-ex

    There are a number of Sendmail options starting with -oe which seem to be
    called by various programs without the leading o in the option. For
    example, the vacation program uses -eq. Exim treats all options of the form
    -ex as synonymous with the corresponding -oex options.

-F <string>

    This option sets the sender's full name for use when a locally-generated
    message is being accepted. In the absence of this option, the user's gecos
    entry from the password data is used. As users are generally permitted to
    alter their gecos entries, no security considerations are involved. White
    space between -F and the <string> is optional.

-f <address>

    This option sets the address of the envelope sender of a locally-generated
    message (also known as the return path). The option can normally be used
    only by a trusted user, but untrusted_set_sender can be set to allow
    untrusted users to use it.

    Processes running as root or the Exim user are always trusted. Other
    trusted users are defined by the trusted_users or trusted_groups options.
    In the absence of -f, or if the caller is not trusted, the sender of a
    local message is set to the caller's login name at the default qualify
    domain.

    There is one exception to the restriction on the use of -f: an empty sender
    can be specified by any user, trusted or not, to create a message that can
    never provoke a bounce. An empty sender can be specified either as an empty
    string, or as a pair of angle brackets with nothing between them, as in
    these examples of shell commands:

    exim -f '<>' user@domain
    exim -f "" user@domain

    In addition, the use of -f is not restricted when testing a filter file
    with -bf or when testing or verifying addresses using the -bt or -bv
    options.

    Allowing untrusted users to change the sender address does not of itself
    make it possible to send anonymous mail. Exim still checks that the From:
    header refers to the local user, and if it does not, it adds a Sender:
    header, though this can be overridden by setting no_local_from_check.

    White space between -f and the <address> is optional (that is, they can be
    given as two arguments or one combined argument). The sender of a
    locally-generated message can also be set (when permitted) by an initial
    "From " line in the message - see the description of -bm above - but if -f
    is also present, it overrides "From ".

-G

    This option is equivalent to an ACL applying:

    control = suppress_local_fixups

    for every message received. Note that Sendmail will complain about such bad
    formatting, where Exim silently just does not fix it up. This may change in
    future.

    As this affects audit information, the caller must be a trusted user to use
    this option.

-h <number>

    This option is accepted for compatibility with Sendmail, but has no effect.
    (In Sendmail it overrides the "hop count" obtained by counting Received:
    headers.)

-i

    This option, which has the same effect as -oi, specifies that a dot on a
    line by itself should not terminate an incoming, non-SMTP message. Solaris
    2.4 (SunOS 5.4) Sendmail has a similar -i processing option https://
    docs.oracle.com/cd/E19457-01/801-6680-1M/801-6680-1M.pdf, p. 1M-529), and
    therefore a -oi command line option, which both are used by its mailx
    command.

-L <tag>

    This option is equivalent to setting syslog_processname in the config file
    and setting log_file_path to "syslog". Its use is restricted to
    administrators. The configuration file has to be read and parsed, to
    determine access rights, before this is set and takes effect, so early
    configuration file errors will not honour this flag.

    The tag should not be longer than 32 characters.

-M <message id> <message id> ...

    This option requests Exim to run a delivery attempt on each message in
    turn. If any of the messages are frozen, they are automatically thawed
    before the delivery attempt. The settings of queue_domains, 
    queue_smtp_domains, and hold_domains are ignored.

    Retry hints for any of the addresses are overridden - Exim tries to deliver
    even if the normal retry time has not yet been reached. This option
    requires the caller to be an admin user. However, there is an option called
    prod_requires_admin which can be set false to relax this restriction (and
    also the same requirement for the -q, -R, and -S options).

    The deliveries happen synchronously, that is, the original Exim process
    does not terminate until all the delivery attempts have finished. No output
    is produced unless there is a serious error. If you want to see what is
    happening, use the -v option as well, or inspect Exim's main log.

-Mar <message id> <address> <address> ...

    This option requests Exim to add the addresses to the list of recipients of
    the message ("ar" for "add recipients"). The first argument must be a
    message id, and the remaining ones must be email addresses. However, if the
    message is active (in the middle of a delivery attempt), it is not altered.
    This option can be used only by an admin user.

-MC <transport> <hostname> <host IP> <sequence number> <message id>

    This option is not intended for use by external callers. It is used
    internally by Exim to invoke another instance of itself to deliver a
    waiting message using an existing SMTP connection, which is passed as the
    standard input. Details are given in chapter 49. This must be the final
    option, and the caller must be root or the Exim user in order to use it.

-MCA

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option. It signifies that
    the connection to the remote host has been authenticated.

-MCD

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option. It signifies that
    the remote host supports the ESMTP DSN extension.

-MCd

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -d option to pass on an
    information string on the purpose of the process.

-MCG <queue name>

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option. It signifies that an
    alternate queue is used, named by the following argument.

-MCK

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option. It signifies that a
    remote host supports the ESMTP CHUNKING extension.

-MCL

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option. It signifies that
    the server to which Exim is connected advertised limits on numbers of
    mails, recipients or recipient domains. The limits are given by the
    following three arguments.

-MCP

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option. It signifies that
    the server to which Exim is connected supports pipelining.

-MCp

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option. It signifies that
    the connection t a remote server is via a SOCKS proxy, using addresses and
    ports given by the following four arguments.

-MCQ <process id> <pipe fd>

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option when the original
    delivery was started by a queue runner. It passes on the process id of the
    queue runner, together with the file descriptor number of an open pipe.
    Closure of the pipe signals the final completion of the sequence of
    processes that are passing messages through the same SMTP connection.

-MCq <recipient address> <size>

    This option is not intended for use by external callers. It is used
    internally by Exim to implement quota checking for local users.

-MCS

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option, and passes on the
    fact that the ESMTP SIZE option should be used on messages delivered down
    the existing connection.

-MCT

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option, and passes on the
    fact that the host to which Exim is connected supports TLS encryption.

-MCr <SNI>, -MCs <SNI>

    These options are not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MCt option, and passes on the
    fact that a TLS Server Name Indication was sent as part of the channel
    establishment. The argument gives the SNI string. The "r" variant indicates
    a DANE-verified connection.

-MCt <IP address> <port> <cipher>

    This option is not intended for use by external callers. It is used
    internally by Exim in conjunction with the -MC option, and passes on the
    fact that the connection is being proxied by a parent process for handling
    TLS encryption. The arguments give the local address and port being
    proxied, and the TLS cipher.

-Mc <message id> <message id> ...

    This option requests Exim to run a delivery attempt on each message, in
    turn, but unlike the -M option, it does check for retry hints, and respects
    any that are found. This option is not very useful to external callers. It
    is provided mainly for internal use by Exim when it needs to re-invoke
    itself in order to regain root privilege for a delivery (see chapter 56).
    However, -Mc can be useful when testing, in order to run a delivery that
    respects retry times and other options such as hold_domains that are
    overridden when -M is used. Such a delivery does not count as a queue run.
    If you want to run a specific delivery as if in a queue run, you should use
    -q with a message id argument. A distinction between queue run deliveries
    and other deliveries is made in one or two places.

-Mes <message id> <address>

    This option requests Exim to change the sender address in the message to
    the given address, which must be a fully qualified address or "<>" ("es"
    for "edit sender"). There must be exactly two arguments. The first argument
    must be a message id, and the second one an email address. However, if the
    message is active (in the middle of a delivery attempt), its status is not
    altered. This option can be used only by an admin user.

-Mf <message id> <message id> ...

    This option requests Exim to mark each listed message as "frozen". This
    prevents any delivery attempts taking place until the message is "thawed",
    either manually or as a result of the auto_thaw configuration option.
    However, if any of the messages are active (in the middle of a delivery
    attempt), their status is not altered. This option can be used only by an
    admin user.

-Mg <message id> <message id> ...

    This option requests Exim to give up trying to deliver the listed messages,
    including any that are frozen. However, if any of the messages are active,
    their status is not altered. For non-bounce messages, a delivery error
    message is sent to the sender. Bounce messages are just discarded. This
    option can be used only by an admin user.

-MG <queue name> <message id> <message id> ...

    This option requests that each listed message be moved from its current
    queue to the given named queue. The destination queue name argument is
    required, but can be an empty string to define the default queue. If the
    messages are not currently located in the default queue, a -qG<name> option
    will be required to define the source queue.

-Mmad <message id> <message id> ...

    This option requests Exim to mark all the recipient addresses in the
    messages as already delivered ("mad" for "mark all delivered"). However, if
    any message is active (in the middle of a delivery attempt), its status is
    not altered. This option can be used only by an admin user.

-Mmd <message id> <address> <address> ...

    This option requests Exim to mark the given addresses as already delivered
    ("md" for "mark delivered"). The first argument must be a message id, and
    the remaining ones must be email addresses. These are matched to recipient
    addresses in the message in a case-sensitive manner. If the message is
    active (in the middle of a delivery attempt), its status is not altered.
    This option can be used only by an admin user.

-Mrm <message id> <message id> ...

    This option requests Exim to remove the given messages from the queue. No
    bounce messages are sent; each message is simply forgotten. However, if any
    of the messages are active, their status is not altered. This option can be
    used only by an admin user or by the user who originally caused the message
    to be placed in the queue.

-Mset <message id>

    This option is useful only in conjunction with -be (that is, when testing
    string expansions). Exim loads the given message from its spool before
    doing the test expansions, thus setting message-specific variables such as
    $message_size and the header variables. The $recipients variable is made
    available. This feature is provided to make it easier to test expansions
    that make use of these variables. However, this option can be used only by
    an admin user. See also -bem.

-Mt <message id> <message id> ...

    This option requests Exim to "thaw" any of the listed messages that are
    "frozen", so that delivery attempts can resume. However, if any of the
    messages are active, their status is not altered. This option can be used
    only by an admin user.

-Mvb <message id>

    This option causes the contents of the message body (-D) spool file to be
    written to the standard output. This option can be used only by an admin
    user.

-Mvc <message id>

    This option causes a copy of the complete message (header lines plus body)
    to be written to the standard output in RFC 2822 format. This option can be
    used only by an admin user.

-Mvh <message id>

    This option causes the contents of the message headers (-H) spool file to
    be written to the standard output. This option can be used only by an admin
    user.

-Mvl <message id>

    This option causes the contents of the message log spool file to be written
    to the standard output. This option can be used only by an admin user.

-m

    This is a synonym for -om that is accepted by Sendmail (https://
    docs.oracle.com/cd/E19457-01/801-6680-1M/801-6680-1M.pdf p. 1M-258), so
    Exim treats it that way too.

-N

    This is a debugging option that inhibits delivery of a message at the
    transport level. It implies -v. Exim goes through many of the motions of
    delivery - it just doesn't actually transport the message, but instead
    behaves as if it had successfully done so. However, it does not make any
    updates to the retry database, and the log entries for deliveries are
    flagged with "*>" rather than "=>".

    Because -N discards any message to which it applies, only root or the Exim
    user are allowed to use it with -bd, -q, -R or -M. In other words, an
    ordinary user can use it only when supplying an incoming message to which
    it will apply. Although transportation never fails when -N is set, an
    address may be deferred because of a configuration problem on a transport,
    or a routing problem. Once -N has been used for a delivery attempt, it
    sticks to the message, and applies to any subsequent delivery attempts that
    may happen for that message.

-n

    This option is interpreted by Sendmail to mean "no aliasing". For normal
    modes of operation, it is ignored by Exim. When combined with -bP it makes
    the output more terse (suppresses option names, environment values and
    config pretty printing).

-O <data>

    This option is interpreted by Sendmail to mean "set option". It is ignored
    by Exim.

-oA <file name>

    This option is used by Sendmail in conjunction with -bi to specify an
    alternative alias filename. Exim handles -bi differently; see the
    description above.

-oB <n>

    This is a debugging option which limits the maximum number of messages that
    can be delivered down one SMTP connection, overriding the value set in any 
    smtp transport. If <n> is omitted, the limit is set to 1.

-odb

    This option applies to all modes in which Exim accepts incoming messages,
    including the listening daemon. It requests "background" delivery of such
    messages, which means that the accepting process automatically starts a
    delivery process for each message received, but does not wait for the
    delivery processes to finish.

    When all the messages have been received, the reception process exits,
    leaving the delivery processes to finish in their own time. The standard
    output and error streams are closed at the start of each delivery process.
    This is the default action if none of the -od options are present.

    If one of the queueing options in the configuration file (queue_only or 
    queue_only_file, for example) is in effect, -odb overrides it if 
    queue_only_override is set true, which is the default setting. If 
    queue_only_override is set false, -odb has no effect.

-odf

    This option requests "foreground" (synchronous) delivery when Exim has
    accepted a locally-generated message. (For the daemon it is exactly the
    same as -odb.) A delivery process is automatically started to deliver the
    message, and Exim waits for it to complete before proceeding.

    The original Exim reception process does not finish until the delivery
    process for the final message has ended. The standard error stream is left
    open during deliveries.

    However, like -odb, this option has no effect if queue_only_override is
    false and one of the queueing options in the configuration file is in
    effect.

    If there is a temporary delivery error during foreground delivery, the
    message is left in the queue for later delivery, and the original reception
    process exits. See chapter 52 for a way of setting up a restricted
    configuration that never queues messages.

-odi

    This option is synonymous with -odf. It is provided for compatibility with
    Sendmail.

-odq

    This option applies to all modes in which Exim accepts incoming messages,
    including the listening daemon. It specifies that the accepting process
    should not automatically start a delivery process for each message
    received. Messages are placed in the queue, and remain there until a
    subsequent queue runner process encounters them. There are several
    configuration options (such as queue_only) that can be used to queue
    incoming messages under certain conditions. This option overrides all of
    them and also -odqs. It always forces queueing.

-odqs

    This option is a hybrid between -odb/-odi and -odq. However, like -odb and 
    -odi, this option has no effect if queue_only_override is false and one of
    the queueing options in the configuration file is in effect.

    When -odqs does operate, a delivery process is started for each incoming
    message, in the background by default, but in the foreground if -odi is
    also present. The recipient addresses are routed, and local deliveries are
    done in the normal way. However, if any SMTP deliveries are required, they
    are not done at this time, so the message remains in the queue until a
    subsequent queue runner process encounters it. Because routing was done,
    Exim knows which messages are waiting for which hosts, and so a number of
    messages for the same host can be sent in a single SMTP connection. The 
    queue_smtp_domains configuration option has the same effect for specific
    domains. See also the -qq option.

-oee

    If an error is detected while a non-SMTP message is being received (for
    example, a malformed address), the error is reported to the sender in a
    mail message.

    Provided this error message is successfully sent, the Exim receiving
    process exits with a return code of zero. If not, the return code is 2 if
    the problem is that the original message has no recipients, or 1 for any
    other error. This is the default -oex option if Exim is called as rmail.

-oem

    This is the same as -oee, except that Exim always exits with a non-zero
    return code, whether or not the error message was successfully sent. This
    is the default -oex option, unless Exim is called as rmail.

-oep

    If an error is detected while a non-SMTP message is being received, the
    error is reported by writing a message to the standard error file (stderr).
    The return code is 1 for all errors.

-oeq

    This option is supported for compatibility with Sendmail, but has the same
    effect as -oep.

-oew

    This option is supported for compatibility with Sendmail, but has the same
    effect as -oem.

-oi

    This option, which has the same effect as -i, specifies that a dot on a
    line by itself should not terminate an incoming, non-SMTP message.
    Otherwise, a single dot does terminate, though Exim does no special
    processing for other lines that start with a dot. This option is set by
    default if Exim is called as rmail. See also -ti.

-oitrue

    This option is treated as synonymous with -oi.

-oMa <host address>

    A number of options starting with -oM can be used to set values associated
    with remote hosts on locally-submitted messages (that is, messages not
    received over TCP/IP). These options can be used by any caller in
    conjunction with the -bh, -be, -bf, -bF, -bt, or -bv testing options. In
    other circumstances, they are ignored unless the caller is trusted.

    The -oMa option sets the sender host address. This may include a port
    number at the end, after a full stop (period). For example:

    exim -bs -oMa 10.9.8.7.1234

    An alternative syntax is to enclose the IP address in square brackets,
    followed by a colon and the port number:

    exim -bs -oMa [10.9.8.7]:1234

    The IP address is placed in the $sender_host_address variable, and the
    port, if present, in $sender_host_port. If both -oMa and -bh are present on
    the command line, the sender host IP address is taken from whichever one is
    last.

-oMaa <name>

    See -oMa above for general remarks about the -oM options. The -oMaa option
    sets the value of $sender_host_authenticated (the authenticator name). See
    chapter 33 for a discussion of SMTP authentication. This option can be used
    with -bh and -bs to set up an authenticated SMTP session without actually
    using the SMTP AUTH command.

-oMai <string>

    See -oMa above for general remarks about the -oM options. The -oMai option
    sets the value of $authenticated_id (the id that was authenticated). This
    overrides the default value (the caller's login id, except with -bh, where
    there is no default) for messages from local sources. See chapter 33 for a
    discussion of authenticated ids.

-oMas <address>

    See -oMa above for general remarks about the -oM options. The -oMas option
    sets the authenticated sender value in $authenticated_sender. It overrides
    the sender address that is created from the caller's login id for messages
    from local sources, except when -bh is used, when there is no default. For
    both -bh and -bs, an authenticated sender that is specified on a MAIL
    command overrides this value. See chapter 33 for a discussion of
    authenticated senders.

-oMi <interface address>

    See -oMa above for general remarks about the -oM options. The -oMi option
    sets the IP interface address value. A port number may be included, using
    the same syntax as for -oMa. The interface address is placed in
    $received_ip_address and the port number, if present, in $received_port.

-oMm <message reference>

    See -oMa above for general remarks about the -oM options. The -oMm option
    sets the message reference, e.g. message-id, and is logged during delivery.
    This is useful when some kind of audit trail is required to tie messages
    together. The format of the message reference is checked and will abort if
    the format is invalid. The option will only be accepted if exim is running
    in trusted mode, not as any regular user.

    The best example of a message reference is when Exim sends a bounce
    message. The message reference is the message-id of the original message
    for which Exim is sending the bounce.

-oMr <protocol name>

    See -oMa above for general remarks about the -oM options. The -oMr option
    sets the received protocol value that is stored in $received_protocol.
    However, it does not apply (and is ignored) when -bh or -bs is used. For 
    -bh, the protocol is forced to one of the standard SMTP protocol names (see
    the description of $received_protocol in section 11.9). For -bs, the
    protocol is always "local-" followed by one of those same names. For -bS
    (batched SMTP) however, the protocol can be set by -oMr. Repeated use of
    this option is not supported.

-oMs <host name>

    See -oMa above for general remarks about the -oM options. The -oMs option
    sets the sender host name in $sender_host_name. When this option is
    present, Exim does not attempt to look up a host name from an IP address;
    it uses the name it is given.

-oMt <ident string>

    See -oMa above for general remarks about the -oM options. The -oMt option
    sets the sender ident value in $sender_ident. The default setting for local
    callers is the login id of the calling process, except when -bh is used,
    when there is no default.

-om

    In Sendmail, this option means "me too", indicating that the sender of a
    message should receive a copy of the message if the sender appears in an
    alias expansion. Exim always does this, so the option does nothing.

-oo

    This option is ignored. In Sendmail it specifies "old style headers",
    whatever that means.

-oP <path>

    This option is useful only in conjunction with -bd or -q with a time value.
    The option specifies the file to which the process id of the daemon is
    written. When -oX is used with -bd, or when -q with a time is used without 
    -bd, this is the only way of causing Exim to write a pid file, because in
    those cases, the normal pid file is not used.

-oPX

    This option is not intended for general use. The daemon uses it when
    terminating due to a SIGTEM, possibly in combination with -oP <path>. It
    causes the pid file to be removed.

-or <time>

    This option sets a timeout value for incoming non-SMTP messages. If it is
    not set, Exim will wait forever for the standard input. The value can also
    be set by the receive_timeout option. The format used for specifying times
    is described in section 6.16.

-os <time>

    This option sets a timeout value for incoming SMTP messages. The timeout
    applies to each SMTP command and block of data. The value can also be set
    by the smtp_receive_timeout option; it defaults to 5 minutes. The format
    used for specifying times is described in section 6.16.

-ov

    This option has exactly the same effect as -v.

-oX <number or string>

    This option is relevant only when the -bd (start listening daemon) option
    is also given. It controls which ports and interfaces the daemon uses.
    Details of the syntax, and how it interacts with configuration file
    options, are given in chapter 13. When -oX is used to start a daemon, no
    pid file is written unless -oP is also present to specify a pid filename.

-oY

    This option controls the creation of an inter-process communications
    endpoint by the Exim daemon. It is only relevant when the -bd (start
    listening daemon) option is also given. Normally the daemon creates this
    socket, unless a -oX and no -oP option is also present.

    If this option is given then the socket will not be created. This is
    required if the system is running multiple daemons, in which case it should
    be used on all. The features supported by the socket will not be available
    in such cases.

    The socket is currently used for

      o fast ramp-up of queue runner processes

      o caching compiled regexes

      o obtaining a current queue size

-pd

    This option applies when an embedded Perl interpreter is linked with Exim
    (see chapter 12). It overrides the setting of the perl_at_start option,
    forcing the starting of the interpreter to be delayed until it is needed.

-ps

    This option applies when an embedded Perl interpreter is linked with Exim
    (see chapter 12). It overrides the setting of the perl_at_start option,
    forcing the starting of the interpreter to occur as soon as Exim is
    started.

-p<rval>:<sval>

    For compatibility with Sendmail, this option is equivalent to

    -oMr <rval> -oMs <sval>

    It sets the incoming protocol and host name (for trusted callers). The host
    name and its colon can be omitted when only the protocol is to be set. Note
    the Exim already has two private options, -pd and -ps, that refer to
    embedded Perl. It is therefore impossible to set a protocol value of "d" or
    "s" using this option (but that does not seem a real limitation). Repeated
    use of this option is not supported.

-q

    This option is normally restricted to admin users. However, there is a
    configuration option called prod_requires_admin which can be set false to
    relax this restriction (and also the same requirement for the -M, -R, and 
    -S options).

    If other commandline options do not specify an action, the -q option starts
    one queue runner process. This scans the queue of waiting messages, and
    runs a delivery process for each one in turn. It waits for each delivery
    process to finish before starting the next one. A delivery process may not
    actually do any deliveries if the retry times for the addresses have not
    been reached. Use -qf (see below) if you want to override this.

    If the delivery process spawns other processes to deliver other messages
    down passed SMTP connections, the queue runner waits for these to finish
    before proceeding.

    When all the queued messages have been considered, the original queue
    runner process terminates. In other words, a single pass is made over the
    waiting mail, one message at a time. Use -q with a time (see below) if you
    want this to be repeated periodically.

    Exim processes the waiting messages in an unpredictable order. It isn't
    very random, but it is likely to be different each time, which is all that
    matters. If one particular message screws up a remote MTA, other messages
    to the same MTA have a chance of getting through if they get tried first.

    It is possible to cause the messages to be processed in lexical message id
    order, which is essentially the order in which they arrived, by setting the
    queue_run_in_order option, but this is not recommended for normal use.

-q<qflags>

    The -q option may be followed by one or more flag letters that change its
    behaviour. They are all optional, but if more than one is present, they
    must appear in the correct order. Each flag is described in a separate item
    below.

-qq...

    An option starting with -qq requests a two-stage queue run. In the first
    stage, the queue is scanned as if the queue_smtp_domains option matched
    every domain. Addresses are routed, local deliveries happen, but no remote
    transports are run.

    Performance will be best if the queue_run_in_order option is false. If that
    is so and the queue_fast_ramp option is true and a daemon-notifier socket
    is available then in the first phase of the run, once a threshold number of
    messages are routed for a given host, a delivery process is forked in
    parallel with the rest of the scan.

    The hints database that remembers which messages are waiting for specific
    hosts is updated, as if delivery to those hosts had been deferred.

    After the first queue scan complete, a second, normal queue scan is done,
    with routing and delivery taking place as normal. Messages that are routed
    to the same host should mostly be delivered down a single SMTP connection
    because of the hints that were set up during the first queue scan.

    Two-phase queue runs should be used on systems which, even intermittently,
    have a large queue (such as mailing-list operators). They may also be
    useful for hosts that are connected to the Internet intermittently.

-q[q]i...

    If the i flag is present, the queue runner runs delivery processes only for
    those messages that haven't previously been tried. (i stands for "initial
    delivery".) This can be helpful if you are putting messages in the queue
    using -odq and want a queue runner just to process the new messages.

-q[q][i]f...

    If one f flag is present, a delivery attempt is forced for each non-frozen
    message, whereas without f only those non-frozen addresses that have passed
    their retry times are tried.

-q[q][i]ff...

    If ff is present, a delivery attempt is forced for every message, whether
    frozen or not.

-q[q][i][f[f]]l

    The l (the letter "ell") flag specifies that only local deliveries are to
    be done. If a message requires any remote deliveries, it remains in the
    queue for later delivery.

-q[q][i][f[f]][l][G<name>[/<time>]]]

    If the G flag and a name is present, the queue runner operates on the queue
    with the given name rather than the default queue. The name should not
    contain a / character. For a periodic queue run (see below) append to the
    name a slash and a time value.

    If other commandline options specify an action, a -qG<name> option will
    specify a queue to operate on. For example:

    exim -bp -qGquarantine
    mailq -qGquarantine
    exim -qGoffpeak -Rf @special.domain.example

-q<qflags> <start id> <end id>

    When scanning the queue, Exim can be made to skip over messages whose ids
    are lexically less than a given value by following the -q option with a
    starting message id. For example:

    exim -q 0t5C6f-0000c8-00

    Messages that arrived earlier than "0t5C6f-0000c8-00" are not inspected. If
    a second message id is given, messages whose ids are lexically greater than
    it are also skipped. If the same id is given twice, for example,

    exim -q 0t5C6f-0000c8-00 0t5C6f-0000c8-00

    just one delivery process is started, for that message. This differs from 
    -M in that retry data is respected, and it also differs from -Mc in that it
    counts as a delivery from a queue run. Note that the selection mechanism
    does not affect the order in which the messages are scanned. There are also
    other ways of selecting specific sets of messages for delivery in a queue
    run - see -R and -S.

-q<qflags><time>

    When a time value is present, the -q option causes Exim to run as a daemon,
    starting a queue runner process at intervals specified by the given time
    value (whose format is described in section 6.16). This form of the -q
    option is commonly combined with the -bd option, in which case a single
    daemon process handles both functions. A common way of starting up a
    combined daemon at system boot time is to use a command such as

    /usr/exim/bin/exim -bd -q30m

    Such a daemon listens for incoming SMTP calls, and also starts a queue
    runner process every 30 minutes.

    It is possible to set up runners for multiple named queues within one
    daemon, For example:

    exim -qGhipri/2m -q10m -qqGmailinglist/1h

    When a daemon is started by -q with a time value, but without -bd, no pid
    file is written unless one is explicitly requested by the -oP option.

-qR<rsflags> <string>

    This option is synonymous with -R. It is provided for Sendmail
    compatibility.

-qS<rsflags> <string>

    This option is synonymous with -S.

-R<rsflags> <string>

    The <rsflags> may be empty, in which case the white space before the string
    is optional, unless the string is f, ff, r, rf, or rff, which are the
    possible values for <rsflags>. White space is required if <rsflags> is not
    empty.

    This option is similar to -q with no time value, that is, it causes Exim to
    perform a single queue run, except that, when scanning the messages on the
    queue, Exim processes only those that have at least one undelivered
    recipient address containing the given string, which is checked in a
    case-independent way. If the <rsflags> start with r, <string> is
    interpreted as a regular expression; otherwise it is a literal string.

    If you want to do periodic queue runs for messages with specific
    recipients, you can combine -R with -q and a time value. For example:

    exim -q25m -R @special.domain.example

    This example does a queue run for messages with recipients in the given
    domain every 25 minutes. Any additional flags that are specified with -q
    are applied to each queue run.

    Once a message is selected for delivery by this mechanism, all its
    addresses are processed. For the first selected message, Exim overrides any
    retry information and forces a delivery attempt for each undelivered
    address. This means that if delivery of any address in the first message is
    successful, any existing retry information is deleted, and so delivery
    attempts for that address in subsequently selected messages (which are
    processed without forcing) will run. However, if delivery of any address
    does not succeed, the retry information is updated, and in subsequently
    selected messages, the failing address will be skipped.

    If the <rsflags> contain f or ff, the delivery forcing applies to all
    selected messages, not just the first; frozen messages are included when ff
    is present.

    The -R option makes it straightforward to initiate delivery of all messages
    to a given domain after a host has been down for some time. When the SMTP
    command ETRN is accepted by its ACL (see chapter 44), its default effect is
    to run Exim with the -R option, but it can be configured to run an
    arbitrary command instead.

-r

    This is a documented (for Sendmail) obsolete alternative name for -f.

-S<rsflags> <string>

    This option acts like -R except that it checks the string against each
    message's sender instead of against the recipients. If -R is also set, both
    conditions must be met for a message to be selected. If either of the
    options has f or ff in its flags, the associated action is taken.

-Tqt <times>

    This is an option that is exclusively for use by the Exim testing suite. It
    is not recognized when Exim is run normally. It allows for the setting up
    of explicit "queue times" so that various warning/retry features can be
    tested.

-t

    When Exim is receiving a locally-generated, non-SMTP message on its
    standard input, the -t option causes the recipients of the message to be
    obtained from the To:, Cc:, and Bcc: header lines in the message instead of
    from the command arguments. The addresses are extracted before any
    rewriting takes place and the Bcc: header line, if present, is then
    removed.

    If the command has any arguments, they specify addresses to which the
    message is not to be delivered. That is, the argument addresses are removed
    from the recipients list obtained from the headers. This is compatible with
    Smail 3 and in accordance with the documented behaviour of several versions
    of Sendmail, as described in man pages on a number of operating systems
    (e.g. Solaris 8, IRIX 6.5, HP-UX 11). However, some versions of Sendmail 
    add argument addresses to those obtained from the headers, and the O'Reilly
    Sendmail book documents it that way. Exim can be made to add argument
    addresses instead of subtracting them by setting the option 
    extract_addresses_remove_arguments false.

    If there are any Resent- header lines in the message, Exim extracts
    recipients from all Resent-To:, Resent-Cc:, and Resent-Bcc: header lines
    instead of from To:, Cc:, and Bcc:. This is for compatibility with Sendmail
    and other MTAs. (Prior to release 4.20, Exim gave an error if -t was used
    in conjunction with Resent- header lines.)

    RFC 2822 talks about different sets of Resent- header lines (for when a
    message is resent several times). The RFC also specifies that they should
    be added at the front of the message, and separated by Received: lines. It
    is not at all clear how -t should operate in the present of multiple sets,
    nor indeed exactly what constitutes a "set". In practice, it seems that
    MUAs do not follow the RFC. The Resent- lines are often added at the end of
    the header, and if a message is resent more than once, it is common for the
    original set of Resent- headers to be renamed as X-Resent- when a new set
    is added. This removes any possible ambiguity.

-ti

    This option is exactly equivalent to -t -i. It is provided for
    compatibility with Sendmail.

-tls-on-connect

    This option is available when Exim is compiled with TLS support. It forces
    all incoming SMTP connections to behave as if the incoming port is listed
    in the tls_on_connect_ports option. See section 13.4 and chapter 43 for
    further details.

-U

    Sendmail uses this option for "initial message submission", and its
    documentation states that in future releases, it may complain about
    syntactically invalid messages rather than fixing them when this flag is
    not set. Exim ignores this option.

-v

    This option causes Exim to write information to the standard error stream,
    describing what it is doing. In particular, it shows the log lines for
    receiving and delivering a message, and if an SMTP connection is made, the
    SMTP dialogue is shown. Some of the log lines shown may not actually be
    written to the log if the setting of log_selector discards them. Any
    relevant selectors are shown with each log line. If none are shown, the
    logging is unconditional.

-x

    AIX uses -x for a private purpose ("mail from a local mail program has
    National Language Support extended characters in the body of the mail
    item"). It sets -x when calling the MTA from its mail command. Exim ignores
    this option.

-X <logfile>

    This option is interpreted by Sendmail to cause debug information to be
    sent to the named file. It is ignored by Exim.

-z <log-line>

    This option writes its argument to Exim's logfile. Use is restricted to
    administrators; the intent is for operational notes. Quotes should be used
    to maintain a multi-word item as a single argument, under most shells.



===============================================================================
6. THE EXIM RUNTIME CONFIGURATION FILE

Exim uses a single runtime configuration file that is read whenever an Exim
binary is executed. Note that in normal operation, this happens frequently,
because Exim is designed to operate in a distributed manner, without central
control.

If a syntax error is detected while reading the configuration file, Exim writes
a message on the standard error, and exits with a non-zero return code. The
message is also written to the panic log. Note: Only simple syntax errors can
be detected at this time. The values of any expanded options are not checked
until the expansion happens, even when the expansion does not actually alter
the string.

The name of the configuration file is compiled into the binary for security
reasons, and is specified by the CONFIGURE_FILE compilation option. In most
configurations, this specifies a single file. However, it is permitted to give
a colon-separated list of filenames, in which case Exim uses the first existing
file in the list.

The runtime configuration file must be owned by root or by the user that is
specified at compile time by the CONFIGURE_OWNER option (if set). The
configuration file must not be world-writeable, or group-writeable unless its
group is the root group or the one specified at compile time by the
CONFIGURE_GROUP option.

Warning: In a conventional configuration, where the Exim binary is setuid to
root, anybody who is able to edit the runtime configuration file has an easy
way to run commands as root. If you specify a user or group in the
CONFIGURE_OWNER or CONFIGURE_GROUP options, then that user and/or any users who
are members of that group will trivially be able to obtain root privileges.

Up to Exim version 4.72, the runtime configuration file was also permitted to
be writeable by the Exim user and/or group. That has been changed in Exim 4.73
since it offered a simple privilege escalation for any attacker who managed to
compromise the Exim user account.

A default configuration file, which will work correctly in simple situations,
is provided in the file src/configure.default. If CONFIGURE_FILE defines just
one filename, the installation process copies the default configuration to a
new file of that name if it did not previously exist. If CONFIGURE_FILE is a
list, no default is automatically installed. Chapter 7 is a "walk-through"
discussion of the default configuration.


6.1 Using a different configuration file
----------------------------------------

A one-off alternate configuration can be specified by the -C command line
option, which may specify a single file or a list of files. However, when -C is
used, Exim gives up its root privilege, unless called by root (or unless the
argument for -C is identical to the built-in value from CONFIGURE_FILE), or is
listed in the TRUSTED_CONFIG_LIST file and the caller is the Exim user or the
user specified in the CONFIGURE_OWNER setting. -C is useful mainly for checking
the syntax of configuration files before installing them. No owner or group
checks are done on a configuration file specified by -C, if root privilege has
been dropped.

Even the Exim user is not trusted to specify an arbitrary configuration file
with the -C option to be used with root privileges, unless that file is listed
in the TRUSTED_CONFIG_LIST file. This locks out the possibility of testing a
configuration using -C right through message reception and delivery, even if
the caller is root. The reception works, but by that time, Exim is running as
the Exim user, so when it re-execs to regain privilege for the delivery, the
use of -C causes privilege to be lost. However, root can test reception and
delivery using two separate commands (one to put a message in the queue, using 
-odq, and another to do the delivery, using -M).

If ALT_CONFIG_PREFIX is defined in Local/Makefile, it specifies a prefix string
with which any file named in a -C command line option must start. In addition,
the filename must not contain the sequence "/../". There is no default setting
for ALT_CONFIG_PREFIX; when it is unset, any filename can be used with -C.

One-off changes to a configuration can be specified by the -D command line
option, which defines and overrides values for macros used inside the
configuration file. However, like -C, the use of this option by a
non-privileged user causes Exim to discard its root privilege. If
DISABLE_D_OPTION is defined in Local/Makefile, the use of -D is completely
disabled, and its use causes an immediate error exit.

The WHITELIST_D_MACROS option in Local/Makefile permits the binary builder to
declare certain macro names trusted, such that root privilege will not
necessarily be discarded. WHITELIST_D_MACROS defines a colon-separated list of
macros which are considered safe and, if -D only supplies macros from this
list, and the values are acceptable, then Exim will not give up root privilege
if the caller is root, the Exim run-time user, or the CONFIGURE_OWNER, if set.
This is a transition mechanism and is expected to be removed in the future.
Acceptable values for the macros satisfy the regexp: "^[A-Za-z0-9_/.-]*$"

Some sites may wish to use the same Exim binary on different machines that
share a file system, but to use different configuration files on each machine.
If CONFIGURE_FILE_USE_NODE is defined in Local/Makefile, Exim first looks for a
file whose name is the configuration filename followed by a dot and the
machine's node name, as obtained from the uname() function. If this file does
not exist, the standard name is tried. This processing occurs for each filename
in the list given by CONFIGURE_FILE or -C.

In some esoteric situations different versions of Exim may be run under
different effective uids and the CONFIGURE_FILE_USE_EUID is defined to help
with this. See the comments in src/EDITME for details.


6.2 Configuration file format
-----------------------------

Exim's configuration file is divided into a number of different parts. General
option settings must always appear at the start of the file. The other parts
are all optional, and may appear in any order. Each part other than the first
is introduced by the word "begin" followed by at least one literal space, and
the name of the part. The optional parts are:

  * ACL: Access control lists for controlling incoming SMTP mail (see chapter
    44).

  * authenticators: Configuration settings for the authenticator drivers. These
    are concerned with the SMTP AUTH command (see chapter 33).

  * routers: Configuration settings for the router drivers. Routers process
    addresses and determine how the message is to be delivered (see chapters 15
    -22).

  * transports: Configuration settings for the transport drivers. Transports
    define mechanisms for copying messages to destinations (see chapters 24-30
    ).

  * retry: Retry rules, for use when a message cannot be delivered immediately.
    If there is no retry section, or if it is empty (that is, no retry rules
    are defined), Exim will not retry deliveries. In this situation, temporary
    errors are treated the same as permanent errors. Retry rules are discussed
    in chapter 32.

  * rewrite: Global address rewriting rules, for use when a message arrives and
    when new addresses are generated during delivery. Rewriting is discussed in
    chapter 31.

  * local_scan: Private options for the local_scan() function. If you want to
    use this feature, you must set

    LOCAL_SCAN_HAS_OPTIONS=yes

    in Local/Makefile before building Exim. Details of the local_scan()
    facility are given in chapter 46.

Leading and trailing white space in configuration lines is always ignored.

Blank lines in the file, and lines starting with a # character (ignoring
leading white space) are treated as comments and are ignored. Note: A #
character other than at the beginning of a line is not treated specially, and
does not introduce a comment.

Any non-comment line can be continued by ending it with a backslash. Note that
the general rule for white space means that trailing white space after the
backslash and leading white space at the start of continuation lines is
ignored. Comment lines beginning with # (but not empty lines) may appear in the
middle of a sequence of continuation lines.

A convenient way to create a configuration file is to start from the default,
which is supplied in src/configure.default, and add, delete, or change settings
as required.

The ACLs, retry rules, and rewriting rules have their own syntax which is
described in chapters 44, 32, and 31, respectively. The other parts of the
configuration file have some syntactic items in common, and these are described
below, from section 6.11 onwards. Before that, the inclusion, macro, and
conditional facilities are described.


6.3 File inclusions in the configuration file
---------------------------------------------

You can include other files inside Exim's runtime configuration file by using
this syntax:

.include <filename>
.include_if_exists <filename>

on a line by itself. Double quotes round the filename are optional. If you use
the first form, a configuration error occurs if the file does not exist; the
second form does nothing for non-existent files. The first form allows a
relative name. It is resolved relative to the directory of the including file.
For the second form an absolute filename is required.

Includes may be nested to any depth, but remember that Exim reads its
configuration file often, so it is a good idea to keep them to a minimum. If
you change the contents of an included file, you must HUP the daemon, because
an included file is read only when the configuration itself is read.

The processing of inclusions happens early, at a physical line level, so, like
comment lines, an inclusion can be used in the middle of an option setting, for
example:

hosts_lookup = a.b.c \
               .include /some/file

Include processing happens after macro processing (see below). Its effect is to
process the lines of the included file as if they occurred inline where the
inclusion appears.


6.4 Macros in the configuration file
------------------------------------

If a line in the main part of the configuration (that is, before the first
"begin" line) begins with an upper case letter, it is taken as a macro
definition, and must be of the form

<name> = <rest of line>

The name must consist of letters, digits, and underscores, and need not all be
in upper case, though that is recommended. The rest of the line, including any
continuations, is the replacement text, and has leading and trailing white
space removed. Quotes are not removed. The replacement text can never end with
a backslash character, but this doesn't seem to be a serious limitation.

Macros may also be defined between router, transport, authenticator, or ACL
definitions. They may not, however, be defined within an individual driver or
ACL, or in the local_scan, retry, or rewrite sections of the configuration.


6.5 Macro substitution
----------------------

Once a macro is defined, all subsequent lines in the file (and any included
files) are scanned for the macro name; if there are several macros, the line is
scanned for each, in turn, in the order in which the macros are defined. The
replacement text is not re-scanned for the current macro, though it is scanned
for subsequently defined macros. For this reason, a macro name may not contain
the name of a previously defined macro as a substring. You could, for example,
define

ABCD_XYZ = <something>
ABCD = <something else>

but putting the definitions in the opposite order would provoke a configuration
error. Macro expansion is applied to individual physical lines from the file,
before checking for line continuation or file inclusion (see above). If a line
consists solely of a macro name, and the expansion of the macro is empty, the
line is ignored. A macro at the start of a line may turn the line into a
comment line or a ".include" line.


6.6 Redefining macros
---------------------

Once defined, the value of a macro can be redefined later in the configuration
(or in an included file). Redefinition is specified by using == instead of =.
For example:

MAC =  initial value
...
MAC == updated value

Redefinition does not alter the order in which the macros are applied to the
subsequent lines of the configuration file. It is still the same order in which
the macros were originally defined. All that changes is the macro's value.
Redefinition makes it possible to accumulate values. For example:

MAC =  initial value
...
MAC == MAC and something added

This can be helpful in situations where the configuration file is built from a
number of other files.


6.7 Overriding macro values
---------------------------

The values set for macros in the configuration file can be overridden by the -D
command line option, but Exim gives up its root privilege when -D is used,
unless called by root or the Exim user. A definition on the command line using
the -D option causes all definitions and redefinitions within the file to be
ignored.


6.8 Example of macro usage
--------------------------

As an example of macro usage, consider a configuration where aliases are looked
up in a MySQL database. It helps to keep the file less cluttered if long
strings such as SQL statements are defined separately as macros, for example:

ALIAS_QUERY = select mailbox from user where \
              login='${quote_mysql:$local_part}';

This can then be used in a redirect router setting like this:

data = ${lookup mysql{ALIAS_QUERY}}

In earlier versions of Exim macros were sometimes used for domain, host, or
address lists. In Exim 4 these are handled better by named lists - see section
10.2.4.


6.9 Builtin macros
------------------

Exim defines some macros depending on facilities available, which may differ
due to build-time definitions and from one release to another. All of these
macros start with an underscore. They can be used to conditionally include
parts of a configuration (see below).

The following classes of macros are defined:

 _HAVE_*                      build-time defines
 _DRIVER_ROUTER_*             router drivers
 _DRIVER_TRANSPORT_*          transport drivers
 _DRIVER_AUTHENTICATOR_*      authenticator drivers
 _EXP_COND_*                  expansion conditions
 _EXP_ITEM_*                  expansion items
 _EXP_OP_*                    expansion operators
 _EXP_VAR_*                   expansion variables
 _LOG_*                       log_selector values
 _OPT_MAIN_*                  main config options
 _OPT_ROUTERS_*               generic router options
 _OPT_TRANSPORTS_*            generic transport options
 _OPT_AUTHENTICATORS_*        generic authenticator options
 _OPT_ROUTER_*_*              private router options
 _OPT_TRANSPORT_*_*           private transport options
 _OPT_AUTHENTICATOR_*_*       private authenticator options

Use an "exim -bP macros" command to get the list of macros.


6.10 Conditional skips in the configuration file
------------------------------------------------

You can use the directives ".ifdef", ".ifndef", ".elifdef", ".elifndef",
".else", and ".endif" to dynamically include or exclude portions of the
configuration file. The processing happens whenever the file is read (that is,
when an Exim binary starts to run).

The implementation is very simple. Instances of the first four directives must
be followed by text that includes the names of one or macros. The condition
that is tested is whether or not any macro substitution has taken place in the
line. Thus:

.ifdef AAA
message_size_limit = 50M
.else
message_size_limit = 100M
.endif

sets a message size limit of 50M if the macro "AAA" is defined (or "A" or
"AA"), and 100M otherwise. If there is more than one macro named on the line,
the condition is true if any of them are defined. That is, it is an "or"
condition. To obtain an "and" condition, you need to use nested ".ifdef"s.

Although you can use a macro expansion to generate one of these directives, it
is not very useful, because the condition "there was a macro substitution in
this line" will always be true.

Text following ".else" and ".endif" is ignored, and can be used as comment to
clarify complicated nestings.


6.11 Common option syntax
-------------------------

For the main set of options, driver options, and local_scan() options, each
setting is on a line by itself, and starts with a name consisting of lower-case
letters and underscores. Many options require a data value, and in these cases
the name must be followed by an equals sign (with optional white space) and
then the value. For example:

qualify_domain = mydomain.example.com

Some option settings may contain sensitive data, for example, passwords for
accessing databases. To stop non-admin users from using the -bP command line
option to read these values, you can precede the option settings with the word
"hide". For example:

hide mysql_servers = localhost/users/admin/secret-password

For non-admin users, such options are displayed like this:

mysql_servers = <value not displayable>

If "hide" is used on a driver option, it hides the value of that option on all
instances of the same driver.

The following sections describe the syntax used for the different data types
that are found in option settings.


6.12 Boolean options
--------------------

Options whose type is given as boolean are on/off switches. There are two
different ways of specifying such options: with and without a data value. If
the option name is specified on its own without data, the switch is turned on;
if it is preceded by "no_" or "not_" the switch is turned off. However, boolean
options may be followed by an equals sign and one of the words "true", "false",
"yes", or "no", as an alternative syntax. For example, the following two
settings have exactly the same effect:

queue_only
queue_only = true

The following two lines also have the same (opposite) effect:

no_queue_only
queue_only = false

You can use whichever syntax you prefer.


6.13 Integer values
-------------------

If an option's type is given as "integer", the value can be given in decimal,
hexadecimal, or octal. If it starts with a digit greater than zero, a decimal
number is assumed. Otherwise, it is treated as an octal number unless it starts
with the characters "0x", in which case the remainder is interpreted as a
hexadecimal number.

If an integer value is followed by the letter K, it is multiplied by 1024; if
it is followed by the letter M, it is multiplied by 1024x1024; if by the letter
G, 1024x1024x1024. When the values of integer option settings are output,
values which are an exact multiple of 1024 or 1024x1024 are sometimes, but not
always, printed using the letters K and M. The printing style is independent of
the actual input format that was used.


6.14 Octal integer values
-------------------------

If an option's type is given as "octal integer", its value is always
interpreted as an octal number, whether or not it starts with the digit zero.
Such options are always output in octal.


6.15 Fixed point numbers
------------------------

If an option's type is given as "fixed-point", its value must be a decimal
integer, optionally followed by a decimal point and up to three further digits.


6.16 Time intervals
-------------------

A time interval is specified as a sequence of numbers, each followed by one of
the following letters, with no intervening white space:

    s seconds
    m minutes
    h hours
    d days
    w weeks

For example, "3h50m" specifies 3 hours and 50 minutes. The values of time
intervals are output in the same format. Exim does not restrict the values; it
is perfectly acceptable, for example, to specify "90m" instead of "1h30m".


6.17 String values
------------------

If an option's type is specified as "string", the value can be specified with
or without double-quotes. If it does not start with a double-quote, the value
consists of the remainder of the line plus any continuation lines, starting at
the first character after any leading white space, with trailing white space
removed, and with no interpretation of the characters in the string. Because
Exim removes comment lines (those beginning with #) at an early stage, they can
appear in the middle of a multi-line string. The following two settings are
therefore equivalent:

trusted_users = uucp:mail
trusted_users = uucp:\
                # This comment line is ignored
                mail

If a string does start with a double-quote, it must end with a closing
double-quote, and any backslash characters other than those used for line
continuation are interpreted as escape characters, as follows:

    "\\"              single backslash
    "\n"              newline
    "\r"              carriage return
    "\t"              tab
    "\"<octal digits> up to 3 octal digits specify one character
    "\x"<hex digits>  up to 2 hexadecimal digits specify one character

If a backslash is followed by some other character, including a double-quote
character, that character replaces the pair.

Quoting is necessary only if you want to make use of the backslash escapes to
insert special characters, or if you need to specify a value with leading or
trailing spaces. These cases are rare, so quoting is almost never needed in
current versions of Exim. In versions of Exim before 3.14, quoting was required
in order to continue lines, so you may come across older configuration files
and examples that apparently quote unnecessarily.


6.18 Expanded strings
---------------------

Some strings in the configuration file are subjected to string expansion, by
which means various parts of the string may be changed according to the
circumstances (see chapter 11). The input syntax for such strings is as just
described; in particular, the handling of backslashes in quoted strings is done
as part of the input process, before expansion takes place. However, backslash
is also an escape character for the expander, so any backslashes that are
required for that reason must be doubled if they are within a quoted
configuration string.


6.19 User and group names
-------------------------

User and group names are specified as strings, using the syntax described
above, but the strings are interpreted specially. A user or group name must
either consist entirely of digits, or be a name that can be looked up using the
getpwnam() or getgrnam() function, as appropriate.


6.20 List construction
----------------------

The data for some configuration options is a list of items, with colon as the
default separator. Many of these options are shown with type "string list" in
the descriptions later in this document. Others are listed as "domain list",
"host list", "address list", or "local part list". Syntactically, they are all
the same; however, those other than "string list" are subject to particular
kinds of interpretation, as described in chapter 10.

In all these cases, the entire list is treated as a single string as far as the
input syntax is concerned. The trusted_users setting in section 6.17 above is
an example. If a colon is actually needed in an item in a list, it must be
entered as two colons. Leading and trailing white space on each item in a list
is ignored. This makes it possible to include items that start with a colon,
and in particular, certain forms of IPv6 address. For example, the list

local_interfaces = 127.0.0.1 : ::::1

contains two IP addresses, the IPv4 address 127.0.0.1 and the IPv6 address ::1.

Note: Although leading and trailing white space is ignored in individual list
items, it is not ignored when parsing the list. The spaces around the first
colon in the example above are necessary. If they were not there, the list
would be interpreted as the two items 127.0.0.1:: and 1.


6.21 Changing list separators
-----------------------------

Doubling colons in IPv6 addresses is an unwelcome chore, so a mechanism was
introduced to allow the separator character to be changed. If a list begins
with a left angle bracket, followed by any punctuation character, that
character is used instead of colon as the list separator. For example, the list
above can be rewritten to use a semicolon separator like this:

local_interfaces = <; 127.0.0.1 ; ::1

This facility applies to all lists, with the exception of the list in 
log_file_path. It is recommended that the use of non-colon separators be
confined to circumstances where they really are needed.

It is also possible to use newline and other control characters (those with
code values less than 32, plus DEL) as separators in lists. Such separators
must be provided literally at the time the list is processed. For options that
are string-expanded, you can write the separator using a normal escape
sequence. This will be processed by the expander before the string is
interpreted as a list. For example, if a newline-separated list of domains is
generated by a lookup, you can process it directly by a line such as this:

domains = <\n ${lookup mysql{.....}}

This avoids having to change the list separator in such data. You are unlikely
to want to use a control character as a separator in an option that is not
expanded, because the value is literal text. However, it can be done by giving
the value in quotes. For example:

local_interfaces = "<\n 127.0.0.1 \n ::1"

Unlike printing character separators, which can be included in list items by
doubling, it is not possible to include a control character as data when it is
set as the separator. Two such characters in succession are interpreted as
enclosing an empty list item.


6.22 Empty items in lists
-------------------------

An empty item at the end of a list is always ignored. In other words, trailing
separator characters are ignored. Thus, the list in

senders = user@domain :

contains only a single item. If you want to include an empty string as one item
in a list, it must not be the last item. For example, this list contains three
items, the second of which is empty:

senders = user1@domain : : user2@domain

Note: There must be white space between the two colons, as otherwise they are
interpreted as representing a single colon data character (and the list would
then contain just one item). If you want to specify a list that contains just
one, empty item, you can do it as in this example:

senders = :

In this case, the first item is empty, and the second is discarded because it
is at the end of the list.


6.23 Format of driver configurations
------------------------------------

There are separate parts in the configuration for defining routers, transports,
and authenticators. In each part, you are defining a number of driver
instances, each with its own set of options. Each driver instance is defined by
a sequence of lines like this:

<instance name>:
  <option>
  ...
  <option>

In the following example, the instance name is localuser, and it is followed by
three options settings:

localuser:
  driver = accept
  check_local_user
  transport = local_delivery

For each driver instance, you specify which Exim code module it uses - by the
setting of the driver option - and (optionally) some configuration settings.
For example, in the case of transports, if you want a transport to deliver with
SMTP you would use the smtp driver; if you want to deliver to a local file you
would use the appendfile driver. Each of the drivers is described in detail in
its own separate chapter later in this manual.

You can have several routers, transports, or authenticators that are based on
the same underlying driver (each must have a different instance name).

The order in which routers are defined is important, because addresses are
passed to individual routers one by one, in order. The order in which
transports are defined does not matter at all. The order in which
authenticators are defined is used only when Exim, as a client, is searching
them to find one that matches an authentication mechanism offered by the
server.

Within a driver instance definition, there are two kinds of option: generic and
private. The generic options are those that apply to all drivers of the same
type (that is, all routers, all transports or all authenticators). The driver
option is a generic option that must appear in every definition. The private
options are special for each driver, and none need appear, because they all
have default values.

The options may appear in any order, except that the driver option must precede
any private options, since these depend on the particular driver. For this
reason, it is recommended that driver always be the first option.

Driver instance names, which are used for reference in log entries and
elsewhere, can be any sequence of letters, digits, and underscores (starting
with a letter) and must be unique among drivers of the same type. A router and
a transport (for example) can each have the same name, but no two router
instances can have the same name. The name of a driver instance should not be
confused with the name of the underlying driver module. For example, the
configuration lines:

remote_smtp:
  driver = smtp

create an instance of the smtp transport driver whose name is remote_smtp. The
same driver code can be used more than once, with different instance names and
different option settings each time. A second instance of the smtp transport,
with different options, might be defined thus:

special_smtp:
  driver = smtp
  port = 1234
  command_timeout = 10s

The names remote_smtp and special_smtp would be used to reference these
transport instances from routers, and these names would appear in log lines.

Comment lines may be present in the middle of driver specifications. The full
list of option settings for any particular driver instance, including all the
defaulted values, can be extracted by making use of the -bP command line
option.



===============================================================================
7. THE DEFAULT CONFIGURATION FILE

The default configuration file supplied with Exim as src/configure.default is
sufficient for a host with simple mail requirements. As an introduction to the
way Exim is configured, this chapter "walks through" the default configuration,
giving brief explanations of the settings. Detailed descriptions of the options
are given in subsequent chapters. The default configuration file itself
contains extensive comments about ways you might want to modify the initial
settings. However, note that there are many options that are not mentioned at
all in the default configuration.


7.1 Macros
----------

All macros should be defined before any options.

One macro is specified, but commented out, in the default configuration:

# ROUTER_SMARTHOST=MAIL.HOSTNAME.FOR.CENTRAL.SERVER.EXAMPLE

If all off-site mail is expected to be delivered to a "smarthost", then set the
hostname here and uncomment the macro. This will affect which router is used
later on. If this is left commented out, then Exim will perform direct-to-MX
deliveries using a dnslookup router.

In addition to macros defined here, Exim includes a number of built-in macros
to enable configuration to be guarded by a binary built with support for a
given feature. See section 6.9 for more details.


7.2 Main configuration settings
-------------------------------

The main (global) configuration option settings section must always come first
in the file, after the macros. The first thing you'll see in the file, after
some initial comments, is the line

# primary_hostname =

This is a commented-out setting of the primary_hostname option. Exim needs to
know the official, fully qualified name of your host, and this is where you can
specify it. However, in most cases you do not need to set this option. When it
is unset, Exim uses the uname() system function to obtain the host name.

The first three non-comment configuration lines are as follows:

domainlist local_domains    = @
domainlist relay_to_domains =
hostlist   relay_from_hosts = 127.0.0.1

These are not, in fact, option settings. They are definitions of two named
domain lists and one named host list. Exim allows you to give names to lists of
domains, hosts, and email addresses, in order to make it easier to manage the
configuration file (see section 10.2.4).

The first line defines a domain list called local_domains; this is used later
in the configuration to identify domains that are to be delivered on the local
host.

There is just one item in this list, the string "@". This is a special form of
entry which means "the name of the local host". Thus, if the local host is
called a.host.example, mail to any.user@a.host.example is expected to be
delivered locally. Because the local host's name is referenced indirectly, the
same configuration file can be used on different hosts.

The second line defines a domain list called relay_to_domains, but the list
itself is empty. Later in the configuration we will come to the part that
controls mail relaying through the local host; it allows relaying to any
domains in this list. By default, therefore, no relaying on the basis of a mail
domain is permitted.

The third line defines a host list called relay_from_hosts. This list is used
later in the configuration to permit relaying from any host or IP address that
matches the list. The default contains just the IP address of the IPv4 loopback
interface, which means that processes on the local host are able to submit mail
for relaying by sending it over TCP/IP to that interface. No other hosts are
permitted to submit messages for relaying.

Just to be sure there's no misunderstanding: at this point in the configuration
we aren't actually setting up any controls. We are just defining some domains
and hosts that will be used in the controls that are specified later.

The next two configuration lines are genuine option settings:

acl_smtp_rcpt = acl_check_rcpt
acl_smtp_data = acl_check_data

These options specify Access Control Lists (ACLs) that are to be used during an
incoming SMTP session for every recipient of a message (every RCPT command),
and after the contents of the message have been received, respectively. The
names of the lists are acl_check_rcpt and acl_check_data, and we will come to
their definitions below, in the ACL section of the configuration. The RCPT ACL
controls which recipients are accepted for an incoming message - if a
configuration does not provide an ACL to check recipients, no SMTP mail can be
accepted. The DATA ACL allows the contents of a message to be checked.

Two commented-out option settings are next:

# av_scanner = clamd:/tmp/clamd
# spamd_address = 127.0.0.1 783

These are example settings that can be used when Exim is compiled with the
content-scanning extension. The first specifies the interface to the virus
scanner, and the second specifies the interface to SpamAssassin. Further
details are given in chapter 45.

Three more commented-out option settings follow:

# tls_advertise_hosts = *
# tls_certificate = /etc/ssl/exim.crt
# tls_privatekey = /etc/ssl/exim.pem

These are example settings that can be used when Exim is compiled with support
for TLS (aka SSL) as described in section 4.7. The first one specifies the list
of clients that are allowed to use TLS when connecting to this server; in this
case, the wildcard means all clients. The other options specify where Exim
should find its TLS certificate and private key, which together prove the
server's identity to any clients that connect. More details are given in
chapter 43.

Another two commented-out option settings follow:

# daemon_smtp_ports = 25 : 465 : 587
# tls_on_connect_ports = 465

These options provide better support for roaming users who wish to use this
server for message submission. They are not much use unless you have turned on
TLS (as described in the previous paragraph) and authentication (about which
more in section 7.8). Mail submission from mail clients (MUAs) should be
separate from inbound mail to your domain (MX delivery) for various good
reasons (eg, ability to impose much saner TLS protocol and ciphersuite
requirements without unintended consequences). RFC 6409 (previously 4409)
specifies use of port 587 for SMTP Submission, which uses STARTTLS, so this is
the "submission" port. RFC 8314 specifies use of port 465 as the "submissions"
protocol, which should be used in preference to 587. You should also consider
deploying SRV records to help clients find these ports. Older names for
"submissions" are "smtps" and "ssmtp".

Two more commented-out options settings follow:

# qualify_domain =
# qualify_recipient =

The first of these specifies a domain that Exim uses when it constructs a
complete email address from a local login name. This is often needed when Exim
receives a message from a local process. If you do not set qualify_domain, the
value of primary_hostname is used. If you set both of these options, you can
have different qualification domains for sender and recipient addresses. If you
set only the first one, its value is used in both cases.

The following line must be uncommented if you want Exim to recognize addresses
of the form user@[10.11.12.13] that is, with a "domain literal" (an IP address
within square brackets) instead of a named domain.

# allow_domain_literals

The RFCs still require this form, but many people think that in the modern
Internet it makes little sense to permit mail to be sent to specific hosts by
quoting their IP addresses. This ancient format has been used by people who try
to abuse hosts by using them for unwanted relaying. However, some people
believe there are circumstances (for example, messages addressed to postmaster)
where domain literals are still useful.

The next configuration line is a kind of trigger guard:

never_users = root

It specifies that no delivery must ever be run as the root user. The normal
convention is to set up root as an alias for the system administrator. This
setting is a guard against slips in the configuration. The list of users
specified by never_users is not, however, the complete list; the build-time
configuration in Local/Makefile has an option called FIXED_NEVER_USERS
specifying a list that cannot be overridden. The contents of never_users are
added to this list. By default FIXED_NEVER_USERS also specifies root.

When a remote host connects to Exim in order to send mail, the only information
Exim has about the host's identity is its IP address. The next configuration
line,

host_lookup = *

specifies that Exim should do a reverse DNS lookup on all incoming connections,
in order to get a host name. This improves the quality of the logging
information, but if you feel it is too expensive, you can remove it entirely,
or restrict the lookup to hosts on "nearby" networks. Note that it is not
always possible to find a host name from an IP address, because not all DNS
reverse zones are maintained, and sometimes DNS servers are unreachable.

The next two lines are concerned with ident callbacks, as defined by RFC 1413
(hence their names):

rfc1413_hosts = *
rfc1413_query_timeout = 0s

These settings cause Exim to avoid ident callbacks for all incoming SMTP calls.
Few hosts offer RFC1413 service these days; calls have to be terminated by a
timeout and this needlessly delays the startup of an incoming SMTP connection.
If you have hosts for which you trust RFC1413 and need this information, you
can change this.

This line enables an efficiency SMTP option. It is negotiated by clients and
not expected to cause problems but can be disabled if needed.

prdr_enable = true

When Exim receives messages over SMTP connections, it expects all addresses to
be fully qualified with a domain, as required by the SMTP definition. However,
if you are running a server to which simple clients submit messages, you may
find that they send unqualified addresses. The two commented-out options:

# sender_unqualified_hosts =
# recipient_unqualified_hosts =

show how you can specify hosts that are permitted to send unqualified sender
and recipient addresses, respectively.

The log_selector option is used to increase the detail of logging over the
default:

log_selector = +smtp_protocol_error +smtp_syntax_error \
               +tls_certificate_verified

The percent_hack_domains option is also commented out:

# percent_hack_domains =

It provides a list of domains for which the "percent hack" is to operate. This
is an almost obsolete form of explicit email routing. If you do not know
anything about it, you can safely ignore this topic.

The next two settings in the main part of the default configuration are
concerned with messages that have been "frozen" on Exim's queue. When a message
is frozen, Exim no longer continues to try to deliver it. Freezing occurs when
a bounce message encounters a permanent failure because the sender address of
the original message that caused the bounce is invalid, so the bounce cannot be
delivered. This is probably the most common case, but there are also other
conditions that cause freezing, and frozen messages are not always bounce
messages.

ignore_bounce_errors_after = 2d
timeout_frozen_after = 7d

The first of these options specifies that failing bounce messages are to be
discarded after 2 days in the queue. The second specifies that any frozen
message (whether a bounce message or not) is to be timed out (and discarded)
after a week. In this configuration, the first setting ensures that no failing
bounce message ever lasts a week.

Exim queues it's messages in a spool directory. If you expect to have large
queues, you may consider using this option. It splits the spool directory into
subdirectories to avoid file system degradation from many files in a single
directory, resulting in better performance. Manual manipulation of queued
messages becomes more complex (though fortunately not often needed).

# split_spool_directory = true

In an ideal world everybody follows the standards. For non-ASCII messages RFC
2047 is a standard, allowing a maximum line length of 76 characters. Exim
adheres that standard and won't process messages which violate this standard.
(Even ${rfc2047:...} expansions will fail.) In particular, the Exim maintainers
have had multiple reports of problems from Russian administrators of issues
until they disable this check, because of some popular, yet buggy, mail
composition software.

# check_rfc2047_length = false

If you need to be strictly RFC compliant you may wish to disable the 8BITMIME
advertisement. Use this, if you exchange mails with systems that are not 8-bit
clean.

# accept_8bitmime = false

Libraries you use may depend on specific environment settings. This imposes a
security risk (e.g. PATH). There are two lists: keep_environment for the
variables to import as they are, and add_environment for variables we want to
set to a fixed value. Note that TZ is handled separately, by the timezone
runtime option and by the TIMEZONE_DEFAULT buildtime option.

# keep_environment = ^LDAP
# add_environment = PATH=/usr/bin::/bin


7.3 ACL configuration
---------------------

In the default configuration, the ACL section follows the main configuration.
It starts with the line

begin acl

and it contains the definitions of two ACLs, called acl_check_rcpt and 
acl_check_data, that were referenced in the settings of acl_smtp_rcpt and 
acl_smtp_data above.

The first ACL is used for every RCPT command in an incoming SMTP message. Each
RCPT command specifies one of the message's recipients. The ACL statements are
considered in order, until the recipient address is either accepted or
rejected. The RCPT command is then accepted or rejected, according to the
result of the ACL processing.

acl_check_rcpt:

This line, consisting of a name terminated by a colon, marks the start of the
ACL, and names it.

accept  hosts = :

This ACL statement accepts the recipient if the sending host matches the list.
But what does that strange list mean? It doesn't actually contain any host
names or IP addresses. The presence of the colon puts an empty item in the
list; Exim matches this only if the incoming message did not come from a remote
host, because in that case, the remote hostname is empty. The colon is
important. Without it, the list itself is empty, and can never match anything.

What this statement is doing is to accept unconditionally all recipients in
messages that are submitted by SMTP from local processes using the standard
input and output (that is, not using TCP/IP). A number of MUAs operate in this
manner.

deny    domains       = +local_domains
        local_parts   = ^[.] : ^.*[@%!/|]
        message       = Restricted characters in address

deny    domains       = !+local_domains
        local_parts   = ^[./|] : ^.*[@%!] : ^.*/\\.\\./
        message       = Restricted characters in address

These statements are concerned with local parts that contain any of the
characters "@", "%", "!", "/", "|", or dots in unusual places. Although these
characters are entirely legal in local parts (in the case of "@" and leading
dots, only if correctly quoted), they do not commonly occur in Internet mail
addresses.

The first three have in the past been associated with explicitly routed
addresses (percent is still sometimes used - see the percent_hack_domains
option). Addresses containing these characters are regularly tried by spammers
in an attempt to bypass relaying restrictions, and also by open relay testing
programs. Unless you really need them it is safest to reject these characters
at this early stage. This configuration is heavy-handed in rejecting these
characters for all messages it accepts from remote hosts. This is a deliberate
policy of being as safe as possible.

The first rule above is stricter, and is applied to messages that are addressed
to one of the local domains handled by this host. This is implemented by the
first condition, which restricts it to domains that are listed in the 
local_domains domain list. The "+" character is used to indicate a reference to
a named list. In this configuration, there is just one domain in local_domains,
but in general there may be many.

The second condition on the first statement uses two regular expressions to
block local parts that begin with a dot or contain "@", "%", "!", "/", or "|".
If you have local accounts that include these characters, you will have to
modify this rule.

Empty components (two dots in a row) are not valid in RFC 2822, but Exim allows
them because they have been encountered in practice. (Consider the common
convention of local parts constructed as "
first-initial.second-initial.family-name" when applied to someone like the
author of Exim, who has no second initial.) However, a local part starting with
a dot or containing "/../" can cause trouble if it is used as part of a
filename (for example, for a mailing list). This is also true for local parts
that contain slashes. A pipe symbol can also be troublesome if the local part
is incorporated unthinkingly into a shell command line.

The second rule above applies to all other domains, and is less strict. This
allows your own users to send outgoing messages to sites that use slashes and
vertical bars in their local parts. It blocks local parts that begin with a
dot, slash, or vertical bar, but allows these characters within the local part.
However, the sequence "/../" is barred. The use of "@", "%", and "!" is
blocked, as before. The motivation here is to prevent your users (or your
users' viruses) from mounting certain kinds of attack on remote sites.

accept  local_parts   = postmaster
        domains       = +local_domains

This statement, which has two conditions, accepts an incoming address if the
local part is postmaster and the domain is one of those listed in the 
local_domains domain list. The "+" character is used to indicate a reference to
a named list. In this configuration, there is just one domain in local_domains,
but in general there may be many.

The presence of this statement means that mail to postmaster is never blocked
by any of the subsequent tests. This can be helpful while sorting out problems
in cases where the subsequent tests are incorrectly denying access.

require verify        = sender

This statement requires the sender address to be verified before any subsequent
ACL statement can be used. If verification fails, the incoming recipient
address is refused. Verification consists of trying to route the address, to
see if a bounce message could be delivered to it. In the case of remote
addresses, basic verification checks only the domain, but callouts can be used
for more verification if required. Section 44.21 discusses the details of
address verification.

accept  hosts         = +relay_from_hosts
        control       = submission

This statement accepts the address if the message is coming from one of the
hosts that are defined as being allowed to relay through this host. Recipient
verification is omitted here, because in many cases the clients are dumb MUAs
that do not cope well with SMTP error responses. For the same reason, the
second line specifies "submission mode" for messages that are accepted. This is
described in detail in section 48.1; it causes Exim to fix messages that are
deficient in some way, for example, because they lack a Date: header line. If
you are actually relaying out from MTAs, you should probably add recipient
verification here, and disable submission mode.

accept  authenticated = *
        control       = submission

This statement accepts the address if the client host has authenticated itself.
Submission mode is again specified, on the grounds that such messages are most
likely to come from MUAs. The default configuration does not define any
authenticators, though it does include some nearly complete commented-out
examples described in 7.8. This means that no client can in fact authenticate
until you complete the authenticator definitions.

require message = relay not permitted
        domains = +local_domains : +relay_to_domains

This statement rejects the address if its domain is neither a local domain nor
one of the domains for which this host is a relay.

require verify = recipient

This statement requires the recipient address to be verified; if verification
fails, the address is rejected.

# deny    dnslists    = black.list.example
#         message     = rejected because $sender_host_address \
#                       is in a black list at $dnslist_domain\n\
#                       $dnslist_text
#
# warn    dnslists    = black.list.example
#         add_header  = X-Warning: $sender_host_address is in \
#                       a black list at $dnslist_domain
#         log_message = found in $dnslist_domain

These commented-out lines are examples of how you could configure Exim to check
sending hosts against a DNS black list. The first statement rejects messages
from blacklisted hosts, whereas the second just inserts a warning header line.

# require verify = csa

This commented-out line is an example of how you could turn on client SMTP
authorization (CSA) checking. Such checks do DNS lookups for special SRV
records.

accept

The final statement in the first ACL unconditionally accepts any recipient
address that has successfully passed all the previous tests.

acl_check_data:

This line marks the start of the second ACL, and names it. Most of the contents
of this ACL are commented out:

# deny    malware   = *
#         message   = This message contains a virus \
#                     ($malware_name).

These lines are examples of how to arrange for messages to be scanned for
viruses when Exim has been compiled with the content-scanning extension, and a
suitable virus scanner is installed. If the message is found to contain a
virus, it is rejected with the given custom error message.

# warn    spam      = nobody
#         message   = X-Spam_score: $spam_score\n\
#                     X-Spam_score_int: $spam_score_int\n\
#                     X-Spam_bar: $spam_bar\n\
#                     X-Spam_report: $spam_report

These lines are an example of how to arrange for messages to be scanned by
SpamAssassin when Exim has been compiled with the content-scanning extension,
and SpamAssassin has been installed. The SpamAssassin check is run with
"nobody" as its user parameter, and the results are added to the message as a
series of extra header line. In this case, the message is not rejected,
whatever the spam score.

accept

This final line in the DATA ACL accepts the message unconditionally.


7.4 Router configuration
------------------------

The router configuration comes next in the default configuration, introduced by
the line

begin routers

Routers are the modules in Exim that make decisions about where to send
messages. An address is passed to each router, in turn, until it is either
accepted, or failed. This means that the order in which you define the routers
matters. Each router is fully described in its own chapter later in this
manual. Here we give only brief overviews.

# domain_literal:
#   driver = ipliteral
#   domains = !+local_domains
#   transport = remote_smtp

This router is commented out because the majority of sites do not want to
support domain literal addresses (those of the form user@[10.9.8.7]). If you
uncomment this router, you also need to uncomment the setting of 
allow_domain_literals in the main part of the configuration.

Which router is used next depends upon whether or not the ROUTER_SMARTHOST
macro has been defined, per

.ifdef ROUTER_SMARTHOST
smarthost:
#...
.else
dnslookup:
#...
.endif

If ROUTER_SMARTHOST has been defined, either at the top of the file or on the
command-line, then we route all non-local mail to that smarthost; otherwise,
we'll perform DNS lookups for direct-to-MX lookup. Any mail which is to a local
domain will skip these routers because of the domains option.

smarthost:
  driver = manualroute
  domains = ! +local_domains
  transport = smarthost_smtp
  route_data = ROUTER_SMARTHOST
  ignore_target_hosts = <; 0.0.0.0 ; 127.0.0.0/8 ; ::1
  no_more

This router only handles mail which is not to any local domains; this is
specified by the line

domains = ! +local_domains

The domains option lists the domains to which this router applies, but the
exclamation mark is a negation sign, so the router is used only for domains
that are not in the domain list called local_domains (which was defined at the
start of the configuration). The plus sign before local_domains indicates that
it is referring to a named list. Addresses in other domains are passed on to
the following routers.

The name of the router driver is manualroute because we are manually specifying
how mail should be routed onwards, instead of using DNS MX. While the name of
this router instance is arbitrary, the driver option must be one of the driver
modules that is in the Exim binary.

With no pre-conditions other than domains, all mail for non-local domains will
be handled by this router, and the no_more setting will ensure that no other
routers will be used for messages matching the pre-conditions. See 3.12 for
more on how the pre-conditions apply. For messages which are handled by this
router, we provide a hostname to deliver to in route_data and the macro
supplies the value; the address is then queued for the smarthost_smtp
transport.

dnslookup:
  driver = dnslookup
  domains = ! +local_domains
  transport = remote_smtp
  ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
  no_more

The domains option behaves as per smarthost, above.

The name of the router driver is dnslookup, and is specified by the driver
option. Do not be confused by the fact that the name of this router instance is
the same as the name of the driver. The instance name is arbitrary, but the
name set in the driver option must be one of the driver modules that is in the
Exim binary.

The dnslookup router routes addresses by looking up their domains in the DNS in
order to obtain a list of hosts to which the address is routed. If the router
succeeds, the address is queued for the remote_smtp transport, as specified by
the transport option. If the router does not find the domain in the DNS, no
further routers are tried because of the no_more setting, so the address fails
and is bounced.

The ignore_target_hosts option specifies a list of IP addresses that are to be
entirely ignored. This option is present because a number of cases have been
encountered where MX records in the DNS point to host names whose IP addresses
are 0.0.0.0 or are in the 127 subnet (typically 127.0.0.1). Completely ignoring
these IP addresses causes Exim to fail to route the email address, so it
bounces. Otherwise, Exim would log a routing problem, and continue to try to
deliver the message periodically until the address timed out.

system_aliases:
  driver = redirect
  allow_fail
  allow_defer
  data = ${lookup{$local_part}lsearch{/etc/aliases}}
# user = exim
  file_transport = address_file
  pipe_transport = address_pipe

Control reaches this and subsequent routers only for addresses in the local
domains. This router checks to see whether the local part is defined as an
alias in the /etc/aliases file, and if so, redirects it according to the data
that it looks up from that file. If no data is found for the local part, the
value of the data option is empty, causing the address to be passed to the next
router.

/etc/aliases is a conventional name for the system aliases file that is often
used. That is why it is referenced by from the default configuration file.
However, you can change this by setting SYSTEM_ALIASES_FILE in Local/Makefile
before building Exim.

userforward:
  driver = redirect
  check_local_user
# local_part_suffix = +* : -*
# local_part_suffix_optional
  file = $home/.forward
# allow_filter
  no_verify
  no_expn
  check_ancestor
  file_transport = address_file
  pipe_transport = address_pipe
  reply_transport = address_reply

This is the most complicated router in the default configuration. It is another
redirection router, but this time it is looking for forwarding data set up by
individual users. The check_local_user setting specifies a check that the local
part of the address is the login name of a local user. If it is not, the router
is skipped. The two commented options that follow check_local_user, namely:

# local_part_suffix = +* : -*
# local_part_suffix_optional

show how you can specify the recognition of local part suffixes. If the first
is uncommented, a suffix beginning with either a plus or a minus sign, followed
by any sequence of characters, is removed from the local part and placed in the
variable $local_part_suffix. The second suffix option specifies that the
presence of a suffix in the local part is optional. When a suffix is present,
the check for a local login uses the local part with the suffix removed.

When a local user account is found, the file called .forward in the user's home
directory is consulted. If it does not exist, or is empty, the router declines.
Otherwise, the contents of .forward are interpreted as redirection data (see
chapter 22 for more details).

Traditional .forward files contain just a list of addresses, pipes, or files.
Exim supports this by default. However, if allow_filter is set (it is commented
out by default), the contents of the file are interpreted as a set of Exim or
Sieve filtering instructions, provided the file begins with "#Exim filter" or "
#Sieve filter", respectively. User filtering is discussed in the separate
document entitled Exim's interfaces to mail filtering.

The no_verify and no_expn options mean that this router is skipped when
verifying addresses, or when running as a consequence of an SMTP EXPN command.
There are two reasons for doing this:

 1. Whether or not a local user has a .forward file is not really relevant when
    checking an address for validity; it makes sense not to waste resources
    doing unnecessary work.

 2. More importantly, when Exim is verifying addresses or handling an EXPN
    command during an SMTP session, it is running as the Exim user, not as
    root. The group is the Exim group, and no additional groups are set up. It
    may therefore not be possible for Exim to read users' .forward files at
    this time.

The setting of check_ancestor prevents the router from generating a new address
that is the same as any previous address that was redirected. (This works round
a problem concerning a bad interaction between aliasing and forwarding - see
section 22.5).

The final three option settings specify the transports that are to be used when
forwarding generates a direct delivery to a file, or to a pipe, or sets up an
auto-reply, respectively. For example, if a .forward file contains

a.nother@elsewhere.example, /home/spqr/archive

the delivery to /home/spqr/archive is done by running the address_file
transport.

localuser:
  driver = accept
  check_local_user
# local_part_suffix = +* : -*
# local_part_suffix_optional
  transport = local_delivery

The final router sets up delivery into local mailboxes, provided that the local
part is the name of a local login, by accepting the address and assigning it to
the local_delivery transport. Otherwise, we have reached the end of the
routers, so the address is bounced. The commented suffix settings fulfil the
same purpose as they do for the userforward router.


7.5 Transport configuration
---------------------------

Transports define mechanisms for actually delivering messages. They operate
only when referenced from routers, so the order in which they are defined does
not matter. The transports section of the configuration starts with

begin transports

Two remote transports and four local transports are defined.

remote_smtp:
  driver = smtp
  message_size_limit = ${if > {$max_received_linelength}{998} {1}{0}}
.ifdef _HAVE_PRDR
  hosts_try_prdr = *
.endif

This transport is used for delivering messages over SMTP connections. The list
of remote hosts comes from the router. The message_size_limit usage is a hack
to avoid sending on messages with over-long lines.

The hosts_try_prdr option enables an efficiency SMTP option. It is negotiated
between client and server and not expected to cause problems but can be
disabled if needed. The built-in macro _HAVE_PRDR guards the use of the 
hosts_try_prdr configuration option.

The other remote transport is used when delivering to a specific smarthost with
whom there must be some kind of existing relationship, instead of the usual
federated system.

smarthost_smtp:
  driver = smtp
  message_size_limit = ${if > {$max_received_linelength}{998} {1}{0}}
  multi_domain
  #
.ifdef _HAVE_TLS
  # Comment out any of these which you have to, then file a Support
  # request with your smarthost provider to get things fixed:
  hosts_require_tls = *
  tls_verify_hosts = *
  # As long as tls_verify_hosts is enabled, this this will have no effect,
  # but if you have to comment it out then this will at least log whether
  # you succeed or not:
  tls_try_verify_hosts = *
  #
  # The SNI name should match the name which we'll expect to verify;
  # many mail systems don't use SNI and this doesn't matter, but if it does,
  # we need to send a name which the remote site will recognize.
  # This _should_ be the name which the smarthost operators specified as
  # the hostname for sending your mail to.
  tls_sni = ROUTER_SMARTHOST
  #
.ifdef _HAVE_OPENSSL
  tls_require_ciphers = HIGH:!aNULL:@STRENGTH
.endif
.ifdef _HAVE_GNUTLS
  tls_require_ciphers = SECURE192:-VERS-SSL3.0:-VERS-TLS1.0:-VERS-TLS1.1
.endif
.endif
.ifdef _HAVE_PRDR
  hosts_try_prdr = *
.endif

After the same message_size_limit hack, we then specify that this Transport can
handle messages to multiple domains in one run. The assumption here is that
you're routing all non-local mail to the same place and that place is happy to
take all messages from you as quickly as possible. All other options depend
upon built-in macros; if Exim was built without TLS support then no other
options are defined. If TLS is available, then we configure "stronger than
default" TLS ciphersuites and versions using the tls_require_ciphers option,
where the value to be used depends upon the library providing TLS. Beyond that,
the options adopt the stance that you should have TLS support available from
your smarthost on today's Internet, so we turn on requiring TLS for the mail to
be delivered, and requiring that the certificate be valid, and match the
expected hostname. The tls_sni option can be used by service providers to
select an appropriate certificate to present to you and here we re-use the
ROUTER_SMARTHOST macro, because that is unaffected by CNAMEs present in DNS.
You want to specify the hostname which you'll expect to validate for, and that
should not be subject to insecure tampering via DNS results.

For the hosts_try_prdr option see the previous transport.

All other options are defaulted.

local_delivery:
  driver = appendfile
  file = /var/mail/$local_part_data
  delivery_date_add
  envelope_to_add
  return_path_add
# group = mail
# mode = 0660

This appendfile transport is used for local delivery to user mailboxes in
traditional BSD mailbox format.

We prefer to avoid using $local_part directly to define the mailbox filename,
as it is provided by a potential bad actor. Instead we use $local_part_data,
the result of looking up $local_part in the user database (done by using 
check_local_user in the the router).

By default appendfile runs under the uid and gid of the local user, which
requires the sticky bit to be set on the /var/mail directory. Some systems use
the alternative approach of running mail deliveries under a particular group
instead of using the sticky bit. The commented options show how this can be
done.

Exim adds three headers to the message as it delivers it: Delivery-date:, 
Envelope-to: and Return-path:. This action is requested by the three
similarly-named options above.

address_pipe:
  driver = pipe
  return_output

This transport is used for handling deliveries to pipes that are generated by
redirection (aliasing or users' .forward files). The return_output option
specifies that any output on stdout or stderr generated by the pipe is to be
returned to the sender.

address_file:
  driver = appendfile
  delivery_date_add
  envelope_to_add
  return_path_add

This transport is used for handling deliveries to files that are generated by
redirection. The name of the file is not specified in this instance of 
appendfile, because it comes from the redirect router.

address_reply:
  driver = autoreply

This transport is used for handling automatic replies generated by users'
filter files.


7.6 Default retry rule
----------------------

The retry section of the configuration file contains rules which affect the way
Exim retries deliveries that cannot be completed at the first attempt. It is
introduced by the line

begin retry

In the default configuration, there is just one rule, which applies to all
errors:

*   *   F,2h,15m; G,16h,1h,1.5; F,4d,6h

This causes any temporarily failing address to be retried every 15 minutes for
2 hours, then at intervals starting at one hour and increasing by a factor of
1.5 until 16 hours have passed, then every 6 hours up to 4 days. If an address
is not delivered after 4 days of temporary failure, it is bounced. The time is
measured from first failure, not from the time the message was received.

If the retry section is removed from the configuration, or is empty (that is,
if no retry rules are defined), Exim will not retry deliveries. This turns
temporary errors into permanent errors.


7.7 Rewriting configuration
---------------------------

The rewriting section of the configuration, introduced by

begin rewrite

contains rules for rewriting addresses in messages as they arrive. There are no
rewriting rules in the default configuration file.


7.8 Authenticators configuration
--------------------------------

The authenticators section of the configuration, introduced by

begin authenticators

defines mechanisms for the use of the SMTP AUTH command. The default
configuration file contains two commented-out example authenticators which
support plaintext username/password authentication using the standard PLAIN
mechanism and the traditional but non-standard LOGIN mechanism, with Exim
acting as the server. PLAIN and LOGIN are enough to support most MUA software.

The example PLAIN authenticator looks like this:

#PLAIN:
#  driver                  = plaintext
#  server_set_id           = $auth2
#  server_prompts          = :
#  server_condition        = Authentication is not yet configured
#  server_advertise_condition = ${if def:tls_in_cipher }

And the example LOGIN authenticator looks like this:

#LOGIN:
#  driver                  = plaintext
#  server_set_id           = $auth1
#  server_prompts          = <| Username: | Password:
#  server_condition        = Authentication is not yet configured
#  server_advertise_condition = ${if def:tls_in_cipher }

The server_set_id option makes Exim remember the authenticated username in
$authenticated_id, which can be used later in ACLs or routers. The 
server_prompts option configures the plaintext authenticator so that it
implements the details of the specific authentication mechanism, i.e. PLAIN or
LOGIN. The server_advertise_condition setting controls when Exim offers
authentication to clients; in the examples, this is only when TLS or SSL has
been started, so to enable the authenticators you also need to add support for
TLS as described in section 7.2.

The server_condition setting defines how to verify that the username and
password are correct. In the examples it just produces an error message. To
make the authenticators work, you can use a string expansion expression like
one of the examples in chapter 34.

Beware that the sequence of the parameters to PLAIN and LOGIN differ; the
usercode and password are in different positions. Chapter 34 covers both.



===============================================================================
8. REGULAR EXPRESSIONS

Exim supports the use of regular expressions in many of its options. It uses
the PCRE2 regular expression library; this provides regular expression matching
that is compatible with Perl 5. The syntax and semantics of regular expressions
is discussed in online Perl manpages, in many Perl reference books, and also in
Jeffrey Friedl's Mastering Regular Expressions, which is published by O'Reilly
(see http://www.oreilly.com/catalog/regex2/).

The documentation for the syntax and semantics of the regular expressions that
are supported by PCRE2 is included in the PCRE2 distribution, and no further
description is included here. The PCRE2 functions are called from Exim using
the default option settings (that is, with no PCRE2 options set), except that
the PCRE2_CASELESS option is set when the matching is required to be
case-insensitive.

In most cases, when a regular expression is required in an Exim configuration,
it has to start with a circumflex, in order to distinguish it from plain text
or an "ends with" wildcard. In this example of a configuration setting, the
second item in the colon-separated list is a regular expression.

domains = a.b.c : ^\\d{3} : *.y.z : ...

The doubling of the backslash is required because of string expansion that
precedes interpretation - see section 11.1 for more discussion of this issue,
and a way of avoiding the need for doubling backslashes. The regular expression
that is eventually used in this example contains just one backslash. The
circumflex is included in the regular expression, and has the normal effect of
"anchoring" it to the start of the string that is being matched.

There are, however, two cases where a circumflex is not required for the
recognition of a regular expression: these are the match condition in a string
expansion, and the matches condition in an Exim filter file. In these cases,
the relevant string is always treated as a regular expression; if it does not
start with a circumflex, the expression is not anchored, and can match anywhere
in the subject string.

In all cases, if you want a regular expression to match at the end of a string,
you must code the $ metacharacter to indicate this. For example:

domains = ^\\d{3}\\.example

matches the domain 123.example, but it also matches 123.example.com. You need
to use:

domains = ^\\d{3}\\.example\$

if you want example to be the top-level domain. The backslash before the $ is
needed because string expansion also interprets dollar characters.



===============================================================================
9. FILE AND DATABASE LOOKUPS

Exim can be configured to look up data in files or databases as it processes
messages. Two different kinds of syntax are used:

 1. A string that is to be expanded may contain explicit lookup requests. These
    cause parts of the string to be replaced by data that is obtained from the
    lookup. Lookups of this type are conditional expansion items. Different
    results can be defined for the cases of lookup success and failure. See
    chapter 11, where string expansions are described in detail. The key for
    the lookup is specified as part of the string to be expanded.

 2. Lists of domains, hosts, and email addresses can contain lookup requests as
    a way of avoiding excessively long linear lists. In this case, the data
    that is returned by the lookup is often (but not always) discarded; whether
    the lookup succeeds or fails is what really counts. These kinds of list are
    described in chapter 10. Depending on the lookup type (see below) the key
    for the lookup may need to be specified as above or may be implicit, given
    by the context in which the list is being checked.

String expansions, lists, and lookups interact with each other in such a way
that there is no order in which to describe any one of them that does not
involve references to the others. Each of these three chapters makes more sense
if you have read the other two first. If you are reading this for the first
time, be aware that some of it will make a lot more sense after you have read
chapters 10 and 11.


9.1 Examples of different lookup syntax
---------------------------------------

It is easy to confuse the two different kinds of lookup, especially as the
lists that may contain the second kind are always expanded before being
processed as lists. Therefore, they may also contain lookups of the first kind.
Be careful to distinguish between the following two examples:

domains = ${lookup{$sender_host_address}lsearch{/some/file}}
domains = lsearch;/some/file

  * The first uses a string expansion, the result of which must be a domain
    list. The key for an expansion-style lookup must be given explicitly. No
    strings have been specified for a successful or a failing lookup; the
    defaults in this case are the looked-up data and an empty string,
    respectively. The expansion takes place before the string is processed as a
    list, and the file that is searched could contain lines like this:

    192.168.3.4: domain1:domain2:...
    192.168.1.9: domain3:domain4:...

    When the lookup succeeds, the result of the expansion is a list of domains
    (and possibly other types of item that are allowed in domain lists). The
    result of the expansion is not tainted.

  * In the second example, the lookup is a single item in a domain list. It
    causes Exim to use a lookup to see if the domain that is being processed
    can be found in the file. The file could contains lines like this:

    domain1:
    domain2:

    Any data that follows the keys is not relevant when checking that the
    domain matches the list item.

    The key for a list-style lookup is implicit, from the lookup context, if
    the lookup is a single-key type (see below). For query-style lookup types
    the query must be given explicitly.

It is possible, though no doubt confusing, to use both kinds of lookup at once.
Consider a file containing lines like this:

192.168.5.6: lsearch;/another/file

If the value of $sender_host_address is 192.168.5.6, expansion of the first 
domains setting above generates the second setting, which therefore causes a
second lookup to occur.

The lookup type may optionally be followed by a comma and a comma-separated
list of options. Each option is a "name=value" pair. Whether an option is
meaningful depends on the lookup type.

All lookups support the option "cache=no_rd". If this is given then the cache
that Exim manages for lookup results is not checked before doing the lookup.
The result of the lookup is still written to the cache.

The rest of this chapter describes the different lookup types that are
available. Any of them can be used in any part of the configuration where a
lookup is permitted.


9.2 Lookup types
----------------

Two different types of data lookup are implemented:

  * The single-key type requires the specification of a file in which to look,
    and a single key to search for. The key must be a non-empty string for the
    lookup to succeed. The lookup type determines how the file is searched. The
    file string may not be tainted.

    All single-key lookups support the option "ret=key". If this is given and
    the lookup (either underlying implementation or cached value) returns data,
    the result is replaced with a non-tainted version of the lookup key.

  * The query-style type accepts a generalized database query. No particular
    key value is assumed by Exim for query-style lookups. You can use whichever
    Exim variables you need to construct the database query.

    For the string-expansion kind of lookups, the query is given in the first
    bracketed argument of the ${lookup ...} expansion. For the list-argument
    kind of lookup the quury is given by the remainder of the list item after
    the first semicolon.

    If tainted data is used in the query then it should be quuted by using the 
    ${quote_<lookup-type>:<string>} expansion operator appropriate for the
    lookup.

The code for each lookup type is in a separate source file that is included in
the binary of Exim only if the corresponding compile-time option is set. The
default settings in src/EDITME are:

LOOKUP_DBM=yes
LOOKUP_LSEARCH=yes

which means that only linear searching and DBM lookups are included by default.
For some types of lookup (e.g. SQL databases), you need to install appropriate
libraries and header files before building Exim.


9.3 Single-key lookup types
---------------------------

The following single-key lookup types are implemented:

9.3.1 cdb

The given file is searched as a Constant DataBase file, using the key string
without a terminating binary zero. The cdb format is designed for indexed files
that are read frequently and never updated, except by total re-creation. As
such, it is particularly suitable for large files containing aliases or other
indexed data referenced by an MTA. Information about cdb and tools for building
the files can be found in several places:

https://cr.yp.to/cdb.html
https://www.corpit.ru/mjt/tinycdb.html
https://packages.debian.org/stable/utils/freecdb
https://github.com/philpennock/cdbtools (in Go)

A cdb distribution is not needed in order to build Exim with cdb support,
because the code for reading cdb files is included directly in Exim itself.
However, no means of building or testing cdb files is provided with Exim, so
you need to obtain a cdb distribution in order to do this.

9.3.2 dbm

Calls to DBM library functions are used to extract data from the given DBM file
by looking up the record with the given key. A terminating binary zero is
included in the key that is passed to the DBM library. See section 4.4 for a
discussion of DBM libraries.

For all versions of Berkeley DB, Exim uses the DB_HASH style of database when
building DBM files using the exim_dbmbuild utility. However, when using
Berkeley DB versions 3 or 4, it opens existing databases for reading with the
DB_UNKNOWN option. This enables it to handle any of the types of database that
the library supports, and can be useful for accessing DBM files created by
other applications. (For earlier DB versions, DB_HASH is always used.)

9.3.3 dbmjz

This is the same as dbm, except that the lookup key is interpreted as an Exim
list; the elements of the list are joined together with ASCII NUL characters to
form the lookup key. An example usage would be to authenticate incoming SMTP
calls using the passwords from Cyrus SASL's /etc/sasldb2 file with the gsasl
authenticator or Exim's own cram_md5 authenticator.

9.3.4 dbmnz

This is the same as dbm, except that a terminating binary zero is not included
in the key that is passed to the DBM library. You may need this if you want to
look up data in files that are created by or shared with some other application
that does not use terminating zeros. For example, you need to use dbmnz rather
than dbm if you want to authenticate incoming SMTP calls using the passwords
from Courier's /etc/userdbshadow.dat file. Exim's utility program for creating
DBM files (exim_dbmbuild) includes the zeros by default, but has an option to
omit them (see section 54.9).

9.3.5 dsearch

The given file must be an absolute directory path; this is searched for an
entry whose name is the key by calling the lstat() function. The key may not
contain any forward slash characters. If lstat() succeeds then so does the
lookup. The result is regarded as untainted.

Options for the lookup can be given by appending them after the word "dsearch",
separated by a comma. Options, if present, are a comma-separated list having
each element starting with a tag name and an equals.

Two options are supported, for the return value and for filtering match
candidates. The "ret" option requests an alternate result value of the entire
path for the entry. Example:

${lookup {passwd} dsearch,ret=full {/etc}}

The default result is just the requested entry. The "filter" option requests
that only directory entries of a given type are matched. The match value is one
of "file", "dir" or "subdir" (the latter not matching "." or ".."). Example:

${lookup {passwd} dsearch,filter=file {/etc}}

The default matching is for any entry type, including directories and symlinks.

An example of how this lookup can be used to support virtual domains is given
in section 51.7.

9.3.6 iplsearch

The given file is a text file containing keys and data. A key is terminated by
a colon or white space or the end of the line. The keys in the file must be IP
addresses, or IP addresses with CIDR masks. Keys that involve IPv6 addresses
must be enclosed in quotes to prevent the first internal colon being
interpreted as a key terminator. For example:

1.2.3.4:           data for 1.2.3.4
192.168.0.0/16:    data for 192.168.0.0/16
"abcd::cdab":      data for abcd::cdab
"abcd:abcd::/32"   data for abcd:abcd::/32

The key for an iplsearch lookup must be an IP address (without a mask). The
file is searched linearly, using the CIDR masks where present, until a matching
key is found. The first key that matches is used; there is no attempt to find a
"best" match. Apart from the way the keys are matched, the processing for 
iplsearch is the same as for lsearch.

Warning 1: Unlike most other single-key lookup types, a file of data for 
iplsearch can not be turned into a DBM or cdb file, because those lookup types
support only literal keys.

Warning 2: In a host list, you must always use net-iplsearch so that the
implicit key is the host's IP address rather than its name (see section 10.4.3
).

Warning 3: Do not use an IPv4-mapped IPv6 address for a key; use the IPv4, in
dotted-quad form. (Exim converts IPv4-mapped IPv6 addresses to this notation
before executing the lookup.)

One option is supported, "ret=full", to request the return of the entire line
rather than omitting the key portion. Note however that the key portion will
have been de-quoted.

9.3.7 json

The given file is a text file with a JSON structure. An element of the
structure is extracted, defined by the search key. The key is a list of
subelement selectors (colon-separated by default but changeable in the usual
way) which are applied in turn to select smaller and smaller portions of the
JSON structure. If a selector is numeric, it must apply to a JSON array; the
(zero-based) nunbered array element is selected. Otherwise it must apply to a
JSON object; the named element is selected. The final resulting element can be
a simple JSON type or a JSON object or array; for the latter two a
string-representation of the JSON is returned. For elements of type string, the
returned value is de-quoted.

9.3.8 lmdb

The given file is an LMDB database. LMDB is a memory-mapped key-value store,
with API modeled loosely on that of BerkeleyDB. See https://symas.com/products/
lightning-memory-mapped-database/ for the feature set and operation modes.

Exim provides read-only access via the LMDB C library. The library can be
obtained from https://github.com/LMDB/lmdb or your operating system package
repository. To enable LMDB support in Exim set LOOKUP_LMDB=yes in Local/
Makefile.

You will need to separately create the LMDB database file, possibly using the
"mdb_load" utility.

9.3.9 lsearch

The given file is a text file that is searched linearly for a line beginning
with the search key, terminated by a colon or white space or the end of the
line. The search is case-insensitive; that is, upper and lower case letters are
treated as the same. The first occurrence of the key that is found in the file
is used.

White space between the key and the colon is permitted. The remainder of the
line, with leading and trailing white space removed, is the data. This can be
continued onto subsequent lines by starting them with any amount of white
space, but only a single space character is included in the data at such a
junction. If the data begins with a colon, the key must be terminated by a
colon, for example:

baduser:  :fail:

Empty lines and lines beginning with # are ignored, even if they occur in the
middle of an item. This is the traditional textual format of alias files. Note
that the keys in an lsearch file are literal strings. There is no wildcarding
of any kind.

In most lsearch files, keys are not required to contain colons or # characters,
or white space. However, if you need this feature, it is available. If a key
begins with a doublequote character, it is terminated only by a matching quote
(or end of line), and the normal escaping rules apply to its contents (see
section 6.17). An optional colon is permitted after quoted keys (exactly as for
unquoted keys). There is no special handling of quotes for the data part of an 
lsearch line.

9.3.10 nis

The given file is the name of a NIS map, and a NIS lookup is done with the
given key, without a terminating binary zero. There is a variant called nis0
which does include the terminating binary zero in the key. This is reportedly
needed for Sun-style alias files. Exim does not recognize NIS aliases; the full
map names must be used.

9.3.11 (n)wildlsearch

wildlsearch or nwildlsearch: These search a file linearly, like lsearch, but
instead of being interpreted as a literal string, each key in the file may be
wildcarded. The difference between these two lookup types is that for 
wildlsearch, each key in the file is string-expanded before being used, whereas
for nwildlsearch, no expansion takes place.

Like lsearch, the testing is done case-insensitively. However, keys in the file
that are regular expressions can be made case-sensitive by the use of "(-i)"
within the pattern. The following forms of wildcard are recognized:

 1. The string may begin with an asterisk to mean "ends with". For example:

    *.a.b.c       data for anything.a.b.c
    *fish         data for anythingfish

 2. The string may begin with a circumflex to indicate a regular expression.
    For example, for wildlsearch:

    ^\N\d+\.a\.b\N    data for <digits>.a.b

    Note the use of "\N" to disable expansion of the contents of the regular
    expression. If you are using nwildlsearch, where the keys are not
    string-expanded, the equivalent entry is:

    ^\d+\.a\.b        data for <digits>.a.b

    The case-insensitive flag is set at the start of compiling the regular
    expression, but it can be turned off by using "(-i)" at an appropriate
    point. For example, to make the entire pattern case-sensitive:

    ^(?-i)\d+\.a\.b        data for <digits>.a.b

    If the regular expression contains white space or colon characters, you
    must either quote it (see lsearch above), or represent these characters in
    other ways. For example, "\s" can be used for white space and "\x3A" for a
    colon. This may be easier than quoting, because if you quote, you have to
    escape all the backslashes inside the quotes.

    Note: It is not possible to capture substrings in a regular expression
    match for later use, because the results of all lookups are cached. If a
    lookup is repeated, the result is taken from the cache, and no actual
    pattern matching takes place. The values of all the numeric variables are
    unset after a (n)wildlsearch match.

 3. Although I cannot see it being of much use, the general matching function
    that is used to implement (n)wildlsearch means that the string may begin
    with a lookup name terminated by a semicolon, and followed by lookup data.
    For example:

    cdb;/some/file  data for keys that match the file

    The data that is obtained from the nested lookup is discarded.

Keys that do not match any of these patterns are interpreted literally. The
continuation rules for the data are the same as for lsearch, and keys may be
followed by optional colons.

Warning: Unlike most other single-key lookup types, a file of data for (n)
wildlsearch can not be turned into a DBM or cdb file, because those lookup
types support only literal keys.

9.3.12 spf

If Exim is built with SPF support, manual lookups can be done (as opposed to
the standard ACL condition method). For details see section 58.2.


9.4 Query-style lookup types
----------------------------

The supported query-style lookup types are listed below. Further details about
many of them are given in later sections.

9.4.1 dnsdb

This does a DNS search for one or more records whose domain names are given in
the supplied query. The resulting data is the contents of the records. See
section 9.10.

9.4.2 ibase

This does a lookup in an InterBase database.

9.4.3 ldap

This does an LDAP lookup using a query in the form of a URL, and returns
attributes from a single entry. There is a variant called ldapm that permits
values from multiple entries to be returned. A third variant called ldapdn
returns the Distinguished Name of a single entry instead of any attribute
values. See section 9.11.

9.4.4 mysql

The format of the query is an SQL statement that is passed to a MySQL database.
See section 9.13.

9.4.5 nisplus

This does a NIS+ lookup using a query that can specify the name of the field to
be returned. See section 9.12.

9.4.6 oracle

The format of the query is an SQL statement that is passed to an Oracle
database. See section 9.13.

9.4.7 passwd

This is a query-style lookup with queries that are just user names. The lookup
calls getpwnam() to interrogate the system password data, and on success, the
result string is the same as you would get from an lsearch lookup on a
traditional /etc/passwd file, though with "*" for the password value. For
example:

*:42:42:King Rat:/home/kr:/bin/bash

9.4.8 pgsql

The format of the query is an SQL statement that is passed to a PostgreSQL
database. See section 9.13.

9.4.9 redis

The format of the query is either a simple get or simple set, passed to a Redis
database. See section 9.13.

9.4.10 sqlite

The format of the query is an SQL statement that is passed to an SQLite
database. See section 9.13.5.

9.4.11 testdb

This is a lookup type that is used for testing Exim. It is not likely to be
useful in normal operation.

9.4.12 whoson

Whoson (http://whoson.sourceforge.net) is a protocol that allows a server to
check whether a particular (dynamically allocated) IP address is currently
allocated to a known (trusted) user and, optionally, to obtain the identity of
the said user. For SMTP servers, Whoson was popular at one time for "POP before
SMTP" authentication, but that approach has been superseded by SMTP
authentication. In Exim, Whoson can be used to implement "POP before SMTP"
checking using ACL statements such as

require condition = \
  ${lookup whoson {$sender_host_address}{yes}{no}}

The query consists of a single IP address. The value returned is the name of
the authenticated user, which is stored in the variable $value. However, in
this example, the data in $value is not used; the result of the lookup is one
of the fixed strings "yes" or "no".


9.5 Temporary errors in lookups
-------------------------------

Lookup functions can return temporary error codes if the lookup cannot be
completed. For example, an SQL or LDAP database might be unavailable. For this
reason, it is not advisable to use a lookup that might do this for critical
options such as a list of local domains.

When a lookup cannot be completed in a router or transport, delivery of the
message (to the relevant address) is deferred, as for any other temporary
error. In other circumstances Exim may assume the lookup has failed, or may
give up altogether.


9.6 Default values in single-key lookups
----------------------------------------

In this context, a "default value" is a value specified by the administrator
that is to be used if a lookup fails.

Note: This section applies only to single-key lookups. For query-style lookups,
the facilities of the query language must be used. An attempt to specify a
default for a query-style lookup provokes an error.

If "*" is added to a single-key lookup type (for example, lsearch*) and the
initial lookup fails, the key "*" is looked up in the file to provide a default
value. See also the section on partial matching below.

Alternatively, if "*@" is added to a single-key lookup type (for example dbm*@)
then, if the initial lookup fails and the key contains an @ character, a second
lookup is done with everything before the last @ replaced by *. This makes it
possible to provide per-domain defaults in alias files that include the domains
in the keys. If the second lookup fails (or doesn't take place because there is
no @ in the key), "*" is looked up. For example, a redirect router might
contain:

data = ${lookup{$local_part@$domain}lsearch*@{/etc/mix-aliases}}

Suppose the address that is being processed is jane@eyre.example. Exim looks up
these keys, in this order:

jane@eyre.example
*@eyre.example
*

The data is taken from whichever key it finds first. Note: In an lsearch file,
this does not mean the first of these keys in the file. A complete scan is done
for each key, and only if it is not found at all does Exim move on to try the
next key.


9.7 Partial matching in single-key lookups
------------------------------------------

The normal operation of a single-key lookup is to search the file for an exact
match with the given key. However, in a number of situations where domains are
being looked up, it is useful to be able to do partial matching. In this case,
information in the file that has a key starting with "*." is matched by any
domain that ends with the components that follow the full stop. For example, if
a key in a DBM file is

*.dates.fict.example

then when partial matching is enabled this is matched by (amongst others) 
2001.dates.fict.example and 1984.dates.fict.example. It is also matched by 
dates.fict.example, if that does not appear as a separate key in the file.

Note: Partial matching is not available for query-style lookups. It is also not
available for any lookup items in address lists (see section 10.5).

Partial matching is implemented by doing a series of separate lookups using
keys constructed by modifying the original subject key. This means that it can
be used with any of the single-key lookup types, provided that partial matching
keys beginning with a special prefix (default "*.") are included in the data
file. Keys in the file that do not begin with the prefix are matched only by
unmodified subject keys when partial matching is in use.

Partial matching is requested by adding the string "partial-" to the front of
the name of a single-key lookup type, for example, partial-dbm. When this is
done, the subject key is first looked up unmodified; if that fails, "*." is
added at the start of the subject key, and it is looked up again. If that
fails, further lookups are tried with dot-separated components removed from the
start of the subject key, one-by-one, and "*." added on the front of what
remains.

A minimum number of two non-* components are required. This can be adjusted by
including a number before the hyphen in the search type. For example, 
partial3-lsearch specifies a minimum of three non-* components in the modified
keys. Omitting the number is equivalent to "partial2-". If the subject key is 
2250.dates.fict.example then the following keys are looked up when the minimum
number of non-* components is two:

2250.dates.fict.example
*.2250.dates.fict.example
*.dates.fict.example
*.fict.example

As soon as one key in the sequence is successfully looked up, the lookup
finishes.

The use of "*." as the partial matching prefix is a default that can be
changed. The motivation for this feature is to allow Exim to operate with file
formats that are used by other MTAs. A different prefix can be supplied in
parentheses instead of the hyphen after "partial". For example:

domains = partial(.)lsearch;/some/file

In this example, if the domain is a.b.c, the sequence of lookups is "a.b.c",
".a.b.c", and ".b.c" (the default minimum of 2 non-wild components is
unchanged). The prefix may consist of any punctuation characters other than a
closing parenthesis. It may be empty, for example:

domains = partial1()cdb;/some/file

For this example, if the domain is a.b.c, the sequence of lookups is "a.b.c",
"b.c", and "c".

If "partial0" is specified, what happens at the end (when the lookup with just
one non-wild component has failed, and the original key is shortened right down
to the null string) depends on the prefix:

  * If the prefix has zero length, the whole lookup fails.

  * If the prefix has length 1, a lookup for just the prefix is done. For
    example, the final lookup for "partial0(.)" is for "." alone.

  * Otherwise, if the prefix ends in a dot, the dot is removed, and the
    remainder is looked up. With the default prefix, therefore, the final
    lookup is for "*" on its own.

  * Otherwise, the whole prefix is looked up.

If the search type ends in "*" or "*@" (see section 9.6 above), the search for
an ultimate default that this implies happens after all partial lookups have
failed. If "partial0" is specified, adding "*" to the search type has no effect
with the default prefix, because the "*" key is already included in the
sequence of partial lookups. However, there might be a use for lookup types
such as "partial0(.)lsearch*".

The use of "*" in lookup partial matching differs from its use as a wildcard in
domain lists and the like. Partial matching works only in terms of
dot-separated components; a key such as "*fict.example" in a database file is
useless, because the asterisk in a partial matching subject key is always
followed by a dot.

When the lookup is done from a string-expansion, the variables $1 and $2
contain the wild and non-wild parts of the key during the expansion of the
replacement text. They return to their previous values at the end of the lookup
item.


9.8 Lookup caching
------------------

Exim caches all lookup results in order to avoid needless repetition of
lookups. However, because (apart from the daemon) Exim operates as a collection
of independent, short-lived processes, this caching applies only within a
single Exim process. There is no inter-process lookup caching facility.

If an option "cache=no_rd" is used on the lookup then the cache is only written
to, cached data is not used for the operation and a real lookup is done.

For single-key lookups, Exim keeps the relevant files open in case there is
another lookup that needs them. In some types of configuration this can lead to
many files being kept open for messages with many recipients. To avoid hitting
the operating system limit on the number of simultaneously open files, Exim
closes the least recently used file when it needs to open more files than its
own internal limit, which can be changed via the lookup_open_max option.

The single-key lookup files are closed and the lookup caches are flushed at
strategic points during delivery - for example, after all routing is complete.


9.9 Quoting lookup data
-----------------------

When data from an incoming message is included in a query-style lookup, there
is the possibility of special characters in the data messing up the syntax of
the query. For example, a NIS+ query that contains

[name=$local_part]

will be broken if the local part happens to contain a closing square bracket.
For NIS+, data can be enclosed in double quotes like this:

[name="$local_part"]

but this still leaves the problem of a double quote in the data. The rule for
NIS+ is that double quotes must be doubled. Other lookup types have different
rules, and to cope with the differing requirements, an expansion operator of
the following form is provided:

${quote_<lookup-type>:<string>}

For example, the way to write the NIS+ query is

[name="${quote_nisplus:$local_part}"]

All tainted data used in a query-style lookup must be quoted using a mechanism
appropriate for the lookup type.

See chapter 11 for full coverage of string expansions. The quote operator can
be used for all lookup types, but has no effect for single-key lookups, since
no quoting is ever needed in their key strings.


9.10 More about dnsdb
---------------------

The dnsdb lookup type uses the DNS as its database. A simple query consists of
a record type and a domain name, separated by an equals sign. For example, an
expansion string could contain:

${lookup dnsdb{mx=a.b.example}{$value}fail}

If the lookup succeeds, the result is placed in $value, which in this case is
used on its own as the result. If the lookup does not succeed, the "fail"
keyword causes a forced expansion failure - see section 11.4 for an explanation
of what this means.

The supported DNS record types are A, CNAME, MX, NS, PTR, SOA, SPF, SRV, TLSA
and TXT, and, when Exim is compiled with IPv6 support, AAAA. If no type is
given, TXT is assumed.

For any record type, if multiple records are found, the data is returned as a
concatenation, with newline as the default separator. The order, of course,
depends on the DNS resolver. You can specify a different separator character
between multiple records by putting a right angle-bracket followed immediately
by the new separator at the start of the query. For example:

${lookup dnsdb{>: a=host1.example}}

It is permitted to specify a space as the separator character. Further white
space is ignored. For lookup types that return multiple fields per record, an
alternate field separator can be specified using a comma after the main
separator character, followed immediately by the field separator.

When the type is PTR, the data can be an IP address, written as normal;
inversion and the addition of in-addr.arpa or ip6.arpa happens automatically.
For example:

${lookup dnsdb{ptr=192.168.4.5}{$value}fail}

If the data for a PTR record is not a syntactically valid IP address, it is not
altered and nothing is added.

For an MX lookup, both the preference value and the host name are returned for
each record, separated by a space. For an SRV lookup, the priority, weight,
port, and host name are returned for each record, separated by spaces. The
field separator can be modified as above.

For TXT records with multiple items of data, only the first item is returned,
unless a field separator is specified. To concatenate items without a
separator, use a semicolon instead. For SPF records the default behaviour is to
concatenate multiple items without using a separator.

${lookup dnsdb{>\n,: txt=a.b.example}}
${lookup dnsdb{>\n; txt=a.b.example}}
${lookup dnsdb{spf=example.org}}

It is permitted to specify a space as the separator character. Further white
space is ignored.

For an SOA lookup, while no result is obtained the lookup is redone with
successively more leading components dropped from the given domain. Only the
primary-nameserver field is returned unless a field separator is specified.

${lookup dnsdb{>:,; soa=a.b.example.com}}

9.10.1 Dnsdb lookup modifiers

Modifiers for dnsdb lookups are given by optional keywords, each followed by a
comma, that may appear before the record type.

The dnsdb lookup fails only if all the DNS lookups fail. If there is a
temporary DNS error for any of them, the behaviour is controlled by a
defer-option modifier. The possible keywords are "defer_strict", "defer_never",
and "defer_lax". With "strict" behaviour, any temporary DNS error causes the
whole lookup to defer. With "never" behaviour, a temporary DNS error is
ignored, and the behaviour is as if the DNS lookup failed to find anything.
With "lax" behaviour, all the queries are attempted, but a temporary DNS error
causes the whole lookup to defer only if none of the other lookups succeed. The
default is "lax", so the following lookups are equivalent:

${lookup dnsdb{defer_lax,a=one.host.com:two.host.com}}
${lookup dnsdb{a=one.host.com:two.host.com}}

Thus, in the default case, as long as at least one of the DNS lookups yields
some data, the lookup succeeds.

Use of DNSSEC is controlled by a dnssec modifier. The possible keywords are
"dnssec_strict", "dnssec_lax", and "dnssec_never". With "strict" or "lax"
DNSSEC information is requested with the lookup. With "strict" a response from
the DNS resolver that is not labelled as authenticated data is treated as
equivalent to a temporary DNS error. The default is "lax".

See also the $lookup_dnssec_authenticated variable.

Timeout for the dnsdb lookup can be controlled by a retrans modifier. The form
is "retrans_VAL" where VAL is an Exim time specification (e.g. "5s"). The
default value is set by the main configuration option dns_retrans.

Retries for the dnsdb lookup can be controlled by a retry modifier. The form if
"retry_VAL" where VAL is an integer. The default count is set by the main
configuration option dns_retry.

Dnsdb lookup results are cached within a single process (and its children). The
cache entry lifetime is limited to the smallest time-to-live (TTL) value of the
set of returned DNS records.

9.10.2 Pseudo dnsdb record types

By default, both the preference value and the host name are returned for each
MX record, separated by a space. If you want only host names, you can use the
pseudo-type MXH:

${lookup dnsdb{mxh=a.b.example}}

In this case, the preference values are omitted, and just the host names are
returned.

Another pseudo-type is ZNS (for "zone NS"). It performs a lookup for NS records
on the given domain, but if none are found, it removes the first component of
the domain name, and tries again. This process continues until NS records are
found or there are no more components left (or there is a DNS error). In other
words, it may return the name servers for a top-level domain, but it never
returns the root name servers. If there are no NS records for the top-level
domain, the lookup fails. Consider these examples:

${lookup dnsdb{zns=xxx.quercite.com}}
${lookup dnsdb{zns=xxx.edu}}

Assuming that in each case there are no NS records for the full domain name,
the first returns the name servers for quercite.com, and the second returns the
name servers for edu.

You should be careful about how you use this lookup because, unless the
top-level domain does not exist, the lookup always returns some host names. The
sort of use to which this might be put is for seeing if the name servers for a
given domain are on a blacklist. You can probably assume that the name servers
for the high-level domains such as com or co.uk are not going to be on such a
list.

A third pseudo-type is CSA (Client SMTP Authorization). This looks up SRV
records according to the CSA rules, which are described in section 44.26.
Although dnsdb supports SRV lookups directly, this is not sufficient because of
the extra parent domain search behaviour of CSA. The result of a successful
lookup such as:

${lookup dnsdb {csa=$sender_helo_name}}

has two space-separated fields: an authorization code and a target host name.
The authorization code can be "Y" for yes, "N" for no, "X" for explicit
authorization required but absent, or "?" for unknown.

The pseudo-type A+ performs an AAAA and then an A lookup. All results are
returned; defer processing (see below) is handled separately for each lookup.
Example:

${lookup dnsdb {>; a+=$sender_helo_name}}

9.10.3 Multiple dnsdb lookups

In the previous sections, dnsdb lookups for a single domain are described.
However, you can specify a list of domains or IP addresses in a single dnsdb
lookup. The list is specified in the normal Exim way, with colon as the default
separator, but with the ability to change this. For example:

${lookup dnsdb{one.domain.com:two.domain.com}}
${lookup dnsdb{a=one.host.com:two.host.com}}
${lookup dnsdb{ptr = <; 1.2.3.4 ; 4.5.6.8}}

In order to retain backwards compatibility, there is one special case: if the
lookup type is PTR and no change of separator is specified, Exim looks to see
if the rest of the string is precisely one IPv6 address. In this case, it does
not treat it as a list.

The data from each lookup is concatenated, with newline separators by default,
in the same way that multiple DNS records for a single item are handled. A
different separator can be specified, as described above.


9.11 More about LDAP
--------------------

The original LDAP implementation came from the University of Michigan; this has
become "Open LDAP", and there are now two different releases. Another
implementation comes from Netscape, and Solaris 7 and subsequent releases
contain inbuilt LDAP support. Unfortunately, though these are all compatible at
the lookup function level, their error handling is different. For this reason
it is necessary to set a compile-time variable when building Exim with LDAP, to
indicate which LDAP library is in use. One of the following should appear in
your Local/Makefile:

LDAP_LIB_TYPE=UMICHIGAN
LDAP_LIB_TYPE=OPENLDAP1
LDAP_LIB_TYPE=OPENLDAP2
LDAP_LIB_TYPE=NETSCAPE
LDAP_LIB_TYPE=SOLARIS

If LDAP_LIB_TYPE is not set, Exim assumes "OPENLDAP1", which has the same
interface as the University of Michigan version.

There are three LDAP lookup types in Exim. These behave slightly differently in
the way they handle the results of a query:

  * ldap requires the result to contain just one entry; if there are more, it
    gives an error.

  * ldapdn also requires the result to contain just one entry, but it is the
    Distinguished Name that is returned rather than any attribute values.

  * ldapm permits the result to contain more than one entry; the attributes
    from all of them are returned.

For ldap and ldapm, if a query finds only entries with no attributes, Exim
behaves as if the entry did not exist, and the lookup fails. The format of the
data returned by a successful lookup is described in the next section. First we
explain how LDAP queries are coded.

9.11.1 Format of LDAP queries

An LDAP query takes the form of a URL as defined in RFC 2255. For example, in
the configuration of a redirect router one might have this setting:

data = ${lookup ldap \
  {ldap:///cn=$local_part,o=University%20of%20Cambridge,\
  c=UK?mailbox?base?}}

The URL may begin with "ldap" or "ldaps" if your LDAP library supports secure
(encrypted) LDAP connections. The second of these ensures that an encrypted TLS
connection is used.

With sufficiently modern LDAP libraries, Exim supports forcing TLS over regular
LDAP connections, rather than the SSL-on-connect "ldaps". See the 
ldap_start_tls option.

Starting with Exim 4.83, the initialization of LDAP with TLS is more tightly
controlled. Every part of the TLS configuration can be configured by settings
in exim.conf. Depending on the version of the client libraries installed on
your system, some of the initialization may have required setting options in /
etc/ldap.conf or ~/.ldaprc to get TLS working with self-signed certificates.
This revealed a nuance where the current UID that exim was running as could
affect which config files it read. With Exim 4.83, these methods become
optional, only taking effect if not specifically set in exim.conf.

9.11.2 LDAP quoting

Two levels of quoting are required in LDAP queries, the first for LDAP itself
and the second because the LDAP query is represented as a URL. Furthermore,
within an LDAP query, two different kinds of quoting are required. For this
reason, there are two different LDAP-specific quoting operators.

The quote_ldap operator is designed for use on strings that are part of filter
specifications. Conceptually, it first does the following conversions on the
string:

*   =>   \2A
(   =>   \28
)   =>   \29
\   =>   \5C

in accordance with RFC 2254. The resulting string is then quoted according to
the rules for URLs, that is, all non-alphanumeric characters except

! $ ' - . _ ( ) * +

are converted to their hex values, preceded by a percent sign. For example:

${quote_ldap: a(bc)*, a<yz>; }

yields

%20a%5C28bc%5C29%5C2A%2C%20a%3Cyz%3E%3B%20

Removing the URL quoting, this is (with a leading and a trailing space):

a\28bc\29\2A, a<yz>;

The quote_ldap_dn operator is designed for use on strings that are part of base
DN specifications in queries. Conceptually, it first converts the string by
inserting a backslash in front of any of the following characters:

, + " \ < > ;

It also inserts a backslash before any leading spaces or # characters, and
before any trailing spaces. (These rules are in RFC 2253.) The resulting string
is then quoted according to the rules for URLs. For example:

${quote_ldap_dn: a(bc)*, a<yz>; }

yields

%5C%20a(bc)*%5C%2C%20a%5C%3Cyz%5C%3E%5C%3B%5C%20

Removing the URL quoting, this is (with a trailing space):

\ a(bc)*\, a\<yz\>\;\

There are some further comments about quoting in the section on LDAP
authentication below.

9.11.3 LDAP connections

The connection to an LDAP server may either be over TCP/IP, or, when OpenLDAP
is in use, via a Unix domain socket. The example given above does not specify
an LDAP server. A server that is reached by TCP/IP can be specified in a query
by starting it with

ldap://<hostname>:<port>/...

If the port (and preceding colon) are omitted, the standard LDAP port (389) is
used. When no server is specified in a query, a list of default servers is
taken from the ldap_default_servers configuration option. This supplies a
colon-separated list of servers which are tried in turn until one successfully
handles a query, or there is a serious error. Successful handling either
returns the requested data, or indicates that it does not exist. Serious errors
are syntactical, or multiple values when only a single value is expected.
Errors which cause the next server to be tried are connection failures, bind
failures, and timeouts.

For each server name in the list, a port number can be given. The standard way
of specifying a host and port is to use a colon separator (RFC 1738). Because 
ldap_default_servers is a colon-separated list, such colons have to be doubled.
For example

ldap_default_servers = ldap1.example.com::145:ldap2.example.com

If ldap_default_servers is unset, a URL with no server name is passed to the
LDAP library with no server name, and the library's default (normally the local
host) is used.

If you are using the OpenLDAP library, you can connect to an LDAP server using
a Unix domain socket instead of a TCP/IP connection. This is specified by using
"ldapi" instead of "ldap" in LDAP queries. What follows here applies only to
OpenLDAP. If Exim is compiled with a different LDAP library, this feature is
not available.

For this type of connection, instead of a host name for the server, a pathname
for the socket is required, and the port number is not relevant. The pathname
can be specified either as an item in ldap_default_servers, or inline in the
query. In the former case, you can have settings such as

ldap_default_servers = /tmp/ldap.sock : backup.ldap.your.domain

When the pathname is given in the query, you have to escape the slashes as
"%2F" to fit in with the LDAP URL syntax. For example:

${lookup ldap {ldapi://%2Ftmp%2Fldap.sock/o=...

When Exim processes an LDAP lookup and finds that the "hostname" is really a
pathname, it uses the Unix domain socket code, even if the query actually
specifies "ldap" or "ldaps". In particular, no encryption is used for a socket
connection. This behaviour means that you can use a setting of 
ldap_default_servers such as in the example above with traditional "ldap" or
"ldaps" queries, and it will work. First, Exim tries a connection via the Unix
domain socket; if that fails, it tries a TCP/IP connection to the backup host.

If an explicit "ldapi" type is given in a query when a host name is specified,
an error is diagnosed. However, if there are more items in ldap_default_servers
, they are tried. In other words:

  * Using a pathname with "ldap" or "ldaps" forces the use of the Unix domain
    interface.

  * Using "ldapi" with a host name causes an error.

Using "ldapi" with no host or path in the query, and no setting of 
ldap_default_servers, does whatever the library does by default.

9.11.4 LDAP authentication and control information

The LDAP URL syntax provides no way of passing authentication and other control
information to the server. To make this possible, the URL in an LDAP query may
be preceded by any number of <name>=<value> settings, separated by spaces. If a
value contains spaces it must be enclosed in double quotes, and when double
quotes are used, backslash is interpreted in the usual way inside them. The
following names are recognized:

    DEREFERENCE set the dereferencing parameter
    NETTIME     set a timeout for a network operation
    USER        set the DN, for authenticating the LDAP bind
    PASS        set the password, likewise
    REFERRALS   set the referrals parameter
    SERVERS     set alternate server list for this query only
    SIZE        set the limit for the number of entries returned
    TIME        set the maximum waiting time for a query

The value of the DEREFERENCE parameter must be one of the words "never",
"searching", "finding", or "always". The value of the REFERRALS parameter must
be "follow" (the default) or "nofollow". The latter stops the LDAP library from
trying to follow referrals issued by the LDAP server.

The name CONNECT is an obsolete name for NETTIME, retained for backwards
compatibility. This timeout (specified as a number of seconds) is enforced from
the client end for operations that can be carried out over a network.
Specifically, it applies to network connections and calls to the ldap_result()
function. If the value is greater than zero, it is used if
LDAP_OPT_NETWORK_TIMEOUT is defined in the LDAP headers (OpenLDAP), or if
LDAP_X_OPT_CONNECT_TIMEOUT is defined in the LDAP headers (Netscape SDK 4.1). A
value of zero forces an explicit setting of "no timeout" for Netscape SDK; for
OpenLDAP no action is taken.

The TIME parameter (also a number of seconds) is passed to the server to set a
server-side limit on the time taken to complete a search.

The SERVERS parameter allows you to specify an alternate list of ldap servers
to use for an individual lookup. The global ldap_default_servers option
provides a default list of ldap servers, and a single lookup can specify a
single ldap server to use. But when you need to do a lookup with a list of
servers that is different than the default list (maybe different order, maybe a
completely different set of servers), the SERVERS parameter allows you to
specify this alternate list (colon-separated).

Here is an example of an LDAP query in an Exim lookup that uses some of these
values. This is a single line, folded to fit on the page:

${lookup ldap
  {user="cn=manager,o=University of Cambridge,c=UK" pass=secret
  ldap:///o=University%20of%20Cambridge,c=UK?sn?sub?(cn=foo)}
  {$value}fail}

The encoding of spaces as "%20" is a URL thing which should not be done for any
of the auxiliary data. Exim configuration settings that include lookups which
contain password information should be preceded by "hide" to prevent non-admin
users from using the -bP option to see their values.

The auxiliary data items may be given in any order. The default is no
connection timeout (the system timeout is used), no user or password, no limit
on the number of entries returned, and no time limit on queries.

When a DN is quoted in the USER= setting for LDAP authentication, Exim removes
any URL quoting that it may contain before passing it to the LDAP library.
Apparently some libraries do this for themselves, but some do not. Removing the
URL quoting has two advantages:

  * It makes it possible to use the same quote_ldap_dn expansion for USER= DNs
    as with DNs inside actual queries.

  * It permits spaces inside USER= DNs.

For example, a setting such as

USER=cn=${quote_ldap_dn:$1}

should work even if $1 contains spaces.

Expanded data for the PASS= value should be quoted using the quote expansion
operator, rather than the LDAP quote operators. The only reason this field
needs quoting is to ensure that it conforms to the Exim syntax, which does not
allow unquoted spaces. For example:

PASS=${quote:$3}

The LDAP authentication mechanism can be used to check passwords as part of
SMTP authentication. See the ldapauth expansion string condition in chapter 11.

9.11.5 Format of data returned by LDAP

The ldapdn lookup type returns the Distinguished Name from a single entry as a
sequence of values, for example

cn=manager,o=University of Cambridge,c=UK

The ldap lookup type generates an error if more than one entry matches the
search filter, whereas ldapm permits this case, and inserts a newline in the
result between the data from different entries. It is possible for multiple
values to be returned for both ldap and ldapm, but in the former case you know
that whatever values are returned all came from a single entry in the
directory.

In the common case where you specify a single attribute in your LDAP query, the
result is not quoted, and does not contain the attribute name. If the attribute
has multiple values, they are separated by commas. Any comma that is part of an
attribute's value is doubled.

If you specify multiple attributes, the result contains space-separated, quoted
strings, each preceded by the attribute name and an equals sign. Within the
quotes, the quote character, backslash, and newline are escaped with
backslashes, and commas are used to separate multiple values for the attribute.
Any commas in attribute values are doubled (permitting treatment of the values
as a comma-separated list). Apart from the escaping, the string within quotes
takes the same form as the output when a single attribute is requested.
Specifying no attributes is the same as specifying all of an entry's
attributes.

Here are some examples of the output format. The first line of each pair is an
LDAP query, and the second is the data that is returned. The attribute called 
attr1 has two values, one of them with an embedded comma, whereas attr2 has
only one value. Both attributes are derived from attr (they have SUP attr in
their schema definitions).

ldap:///o=base?attr1?sub?(uid=fred)
value1.1,value1,,2

ldap:///o=base?attr2?sub?(uid=fred)
value two

ldap:///o=base?attr?sub?(uid=fred)
value1.1,value1,,2,value two

ldap:///o=base?attr1,attr2?sub?(uid=fred)
attr1="value1.1,value1,,2" attr2="value two"

ldap:///o=base??sub?(uid=fred)
objectClass="top" attr1="value1.1,value1,,2" attr2="value two"

You can make use of Exim's -be option to run expansion tests and thereby check
the results of LDAP lookups. The extract operator in string expansions can be
used to pick out individual fields from data that consists of key=value pairs.
The listextract operator should be used to pick out individual values of
attributes, even when only a single value is expected. The doubling of embedded
commas allows you to use the returned data as a comma separated list (using the
"<," syntax for changing the input list separator).


9.12 More about NIS+
--------------------

NIS+ queries consist of a NIS+ indexed name followed by an optional colon and
field name. If this is given, the result of a successful query is the contents
of the named field; otherwise the result consists of a concatenation of 
field-name=field-value pairs, separated by spaces. Empty values and values
containing spaces are quoted. For example, the query

[name=mg1456],passwd.org_dir

might return the string

name=mg1456 passwd="" uid=999 gid=999 gcos="Martin Guerre"
home=/home/mg1456 shell=/bin/bash shadow=""

(split over two lines here to fit on the page), whereas

[name=mg1456],passwd.org_dir:gcos

would just return

Martin Guerre

with no quotes. A NIS+ lookup fails if NIS+ returns more than one table entry
for the given indexed key. The effect of the quote_nisplus expansion operator
is to double any quote characters within the text.


9.13 SQL lookups
----------------

Exim can support lookups in InterBase, MySQL, Oracle, PostgreSQL, Redis, and
SQLite databases. Queries for these databases contain SQL statements, so an
example might be

${lookup mysql{select mailbox from users where id='userx'}\
  {$value}fail}

If the result of the query contains more than one field, the data for each
field in the row is returned, preceded by its name, so the result of

${lookup pgsql{select home,name from users where id='userx'}\
  {$value}}

might be

home=/home/userx name="Mister X"

Empty values and values containing spaces are double quoted, with embedded
quotes escaped by a backslash. If the result of the query contains just one
field, the value is passed back verbatim, without a field name, for example:

Mister X

If the result of the query yields more than one row, it is all concatenated,
with a newline between the data for each row.

9.13.1 More about MySQL, PostgreSQL, Oracle, InterBase, and Redis

If any MySQL, PostgreSQL, Oracle, InterBase or Redis lookups are used, the 
mysql_servers, pgsql_servers, oracle_servers, ibase_servers, or redis_servers
option (as appropriate) must be set to a colon-separated list of server
information. (For MySQL and PostgreSQL, the global option need not be set if
all queries contain their own server information - see section 9.13.2.) For all
but Redis each item in the list is a slash-separated list of four items: host
name, database name, user name, and password. In the case of Oracle, the host
name field is used for the "service name", and the database name field is not
used and should be empty. For example:

hide oracle_servers = oracle.plc.example//userx/abcdwxyz

Because password data is sensitive, you should always precede the setting with
"hide", to prevent non-admin users from obtaining the setting via the -bP
option. Here is an example where two MySQL servers are listed:

hide mysql_servers = localhost/users/root/secret:\
                     otherhost/users/root/othersecret

For MySQL and PostgreSQL, a host may be specified as <name>:<port> but because
this is a colon-separated list, the colon has to be doubled. For each query,
these parameter groups are tried in order until a connection is made and a
query is successfully processed. The result of a query may be that no data is
found, but that is still a successful query. In other words, the list of
servers provides a backup facility, not a list of different places to look.

For Redis the global option need not be specified if all queries contain their
own server information - see section 9.13.2. If specified, the option must be
set to a colon-separated list of server information. Each item in the list is a
slash-separated list of three items: host, database number, and password.

 1. The host is required and may be either an IPv4 address and optional port
    number (separated by a colon, which needs doubling due to the higher-level
    list), or a Unix socket pathname enclosed in parentheses

 2. The database number is optional; if present that number is selected in the
    backend

 3. The password is optional; if present it is used to authenticate to the
    backend

The quote_mysql, quote_pgsql, and quote_oracle expansion operators convert
newline, tab, carriage return, and backspace to \n, \t, \r, and \b
respectively, and the characters single-quote, double-quote, and backslash
itself are escaped with backslashes.

The quote_redis expansion operator escapes whitespace and backslash characters
with a backslash.

9.13.2 Specifying the server in the query

For MySQL, PostgreSQL and Redis lookups (but not currently for Oracle and
InterBase), it is possible to specify a list of servers with an individual
query. This is done by appending a comma-separated option to the query type:

,servers=server1:server2:server3:...

Each item in the list may take one of two forms:

 1. If it contains no slashes it is assumed to be just a host name. The
    appropriate global option (mysql_servers or pgsql_servers) is searched for
    a host of the same name, and the remaining parameters (database, user,
    password) are taken from there.

 2. If it contains any slashes, it is taken as a complete parameter set.

The list of servers is used in exactly the same way as the global list. Once a
connection to a server has happened and a query has been successfully executed,
processing of the lookup ceases.

This feature is intended for use in master/slave situations where updates are
occurring and you want to update the master rather than a slave. If the master
is in the list as a backup for reading, you might have a global setting like
this:

mysql_servers = slave1/db/name/pw:\
                slave2/db/name/pw:\
                master/db/name/pw

In an updating lookup, you could then write:

${lookup mysql,servers=master {UPDATE ...} }

That query would then be sent only to the master server. If, on the other hand,
the master is not to be used for reading, and so is not present in the global
option, you can still update it by a query of this form:

${lookup pgsql,servers=master/db/name/pw {UPDATE ...} }

An older syntax places the servers specification before the query, semicolon
separated:

${lookup mysql{servers=master; UPDATE ...} }

The new version avoids potential issues with tainted arguments in the query,
for explicit expansion. Note: server specifications in list-style lookups are
still problematic.

9.13.3 Special MySQL features

For MySQL, an empty host name or the use of "localhost" in mysql_servers causes
a connection to the server on the local host by means of a Unix domain socket.
An alternate socket can be specified in parentheses. An option group name for
MySQL option files can be specified in square brackets; the default value is
"exim". The full syntax of each item in mysql_servers is:

<hostname>::<port>(<socket name>)[<option group>]/<database>/<user>/<password>

Any of the four sub-parts of the first field can be omitted. For normal use on
the local host it can be left blank or set to just "localhost".

No database need be supplied - but if it is absent here, it must be given in
the queries.

If a MySQL query is issued that does not request any data (an insert, update,
or delete command), the result of the lookup is the number of rows affected.

Warning: This can be misleading. If an update does not actually change anything
(for example, setting a field to the value it already has), the result is zero
because no rows are affected.

9.13.4 Special PostgreSQL features

PostgreSQL lookups can also use Unix domain socket connections to the database.
This is usually faster and costs less CPU time than a TCP/IP connection.
However it can be used only if the mail server runs on the same machine as the
database server. A configuration line for PostgreSQL via Unix domain sockets
looks like this:

hide pgsql_servers = (/tmp/.s.PGSQL.5432)/db/user/password : ...

In other words, instead of supplying a host name, a path to the socket is
given. The path name is enclosed in parentheses so that its slashes aren't
visually confused with the delimiters for the other server parameters.

If a PostgreSQL query is issued that does not request any data (an insert,
update, or delete command), the result of the lookup is the number of rows
affected.

9.13.5 More about SQLite

SQLite is different to the other SQL lookups because a filename is required in
addition to the SQL query. An SQLite database is a single file, and there is no
daemon as in the other SQL databases.

There are two ways of specifying the file. The first is by using the 
sqlite_dbfile main option. The second, which allows separate files for each
query, is to use an option appended, comma-separated, to the "sqlite" lookup
type word. The option is the word "file", then an equals, then the filename.
The filename in this case cannot contain whitespace or open-brace charachters.

A deprecated method is available, prefixing the query with the filename
separated by white space. This means that the query cannot use any tainted
values, as that taints the entire query including the filename - resulting in a
refusal to open the file.

In all the above cases the filename must be an absolute path.

Here is a lookup expansion example:

sqlite_dbfile = /some/thing/sqlitedb
...
${lookup sqlite {select name from aliases where id='userx';}}

In a list, the syntax is similar. For example:

domainlist relay_to_domains = sqlite;\
   select * from relays where ip='$sender_host_address';

The only character affected by the quote_sqlite operator is a single quote,
which it doubles.

The SQLite library handles multiple simultaneous accesses to the database
internally. Multiple readers are permitted, but only one process can update at
once. Attempts to access the database while it is being updated are rejected
after a timeout period, during which the SQLite library waits for the lock to
be released. In Exim, the default timeout is set to 5 seconds, but it can be
changed by means of the sqlite_lock_timeout option.

9.13.6 More about Redis

Redis is a non-SQL database. Commands are simple get and set. Examples:

${lookup redis{set keyname ${quote_redis:objvalue plus}}}
${lookup redis{get keyname}}

As of release 4.91, "lightweight" support for Redis Cluster is available.
Requires redis_servers list to contain all the servers in the cluster, all of
which must be reachable from the running exim instance. If the cluster has
master/slave replication, the list must contain all the master and slave
servers.

When the Redis Cluster returns a "MOVED" response to a query, Exim does not
immediately follow the redirection but treats the response as a DEFER, moving
on to the next server in the redis_servers list until the correct server is
reached.



===============================================================================
10. DOMAIN, HOST, ADDRESS, AND LOCAL PART LISTS

A number of Exim configuration options contain lists of domains, hosts, email
addresses, or local parts. For example, the hold_domains option contains a list
of domains whose delivery is currently suspended. These lists are also used as
data in ACL statements (see chapter 44), and as arguments to expansion
conditions such as match_domain.

Each item in one of these lists is a pattern to be matched against a domain,
host, email address, or local part, respectively. In the sections below, the
different types of pattern for each case are described, but first we cover some
general facilities that apply to all four kinds of list.

Note that other parts of Exim use a string list which does not support all the
complexity available in domain, host, address and local part lists.


10.1 Results of list checking
-----------------------------

The primary result of doing a list check is a truth value. In some contexts
additional information is stored about the list element that matched:

hosts

    A hosts ACL condition will store a result in the $host_data variable.

local_parts

    A local_parts router option or local_parts ACL condition will store a
    result in the $local_part_data variable.

domains

    A domains router option or domains ACL condition will store a result in the
    $domain_data variable.

senders

    A senders router option or senders ACL condition will store a result in the
    $sender_data variable.

recipients

    A recipients ACL condition will store a result in the $recipient_data
    variable.

The detail of the additional information depends on the type of match and is
given below as the value information.


10.2 Expansion of lists
-----------------------

Each list is expanded as a single string before it is used.

Exception: the router headers_remove option, where list-item splitting is done
before string-expansion.

The result of expansion must be a list, possibly containing empty items, which
is split up into separate items for matching. By default, colon is the
separator character, but this can be varied if necessary. See sections 6.20 and
6.22 for details of the list syntax; the second of these discusses the way to
specify empty list items.

If the string expansion is forced to fail, Exim behaves as if the item it is
testing (domain, host, address, or local part) is not in the list. Other
expansion failures cause temporary errors.

If an item in a list is a regular expression, backslashes, dollars and possibly
other special characters in the expression must be protected against
misinterpretation by the string expander. The easiest way to do this is to use
the "\N" expansion feature to indicate that the contents of the regular
expression should not be expanded. For example, in an ACL you might have:

deny senders = \N^\d{8}\w@.*\.baddomain\.example$\N : \
               ${lookup{$domain}lsearch{/badsenders/bydomain}}

The first item is a regular expression that is protected from expansion by "\
N", whereas the second uses the expansion to obtain a list of unwanted senders
based on the receiving domain.

10.2.1 Negated items in lists

Items in a list may be positive or negative. Negative items are indicated by a
leading exclamation mark, which may be followed by optional white space. A list
defines a set of items (domains, etc). When Exim processes one of these lists,
it is trying to find out whether a domain, host, address, or local part
(respectively) is in the set that is defined by the list. It works like this:

The list is scanned from left to right. If a positive item is matched, the
subject that is being checked is in the set; if a negative item is matched, the
subject is not in the set. If the end of the list is reached without the
subject having matched any of the patterns, it is in the set if the last item
was a negative one, but not if it was a positive one. For example, the list in

domainlist relay_to_domains = !a.b.c : *.b.c

matches any domain ending in .b.c except for a.b.c. Domains that match neither 
a.b.c nor *.b.c do not match, because the last item in the list is positive.
However, if the setting were

domainlist relay_to_domains = !a.b.c

then all domains other than a.b.c would match because the last item in the list
is negative. In other words, a list that ends with a negative item behaves as
if it had an extra item ":*" on the end.

Another way of thinking about positive and negative items in lists is to read
the connector as "or" after a positive item and as "and" after a negative item.

10.2.2 File names in lists

If an item in a domain, host, address, or local part list is an absolute
filename (beginning with a slash character), each line of the file is read and
processed as if it were an independent item in the list, except that further
filenames are not allowed, and no expansion of the data from the file takes
place. Empty lines in the file are ignored, and the file may also contain
comment lines:

  * For domain and host lists, if a # character appears anywhere in a line of
    the file, it and all following characters are ignored.

  * Because local parts may legitimately contain # characters, a comment in an
    address list or local part list file is recognized only if # is preceded by
    white space or the start of the line. For example:

    not#comment@x.y.z   # but this is a comment

Putting a filename in a list has the same effect as inserting each line of the
file as an item in the list (blank lines and comments excepted). However, there
is one important difference: the file is read each time the list is processed,
so if its contents vary over time, Exim's behaviour changes.

If a filename is preceded by an exclamation mark, the sense of any match within
the file is inverted. For example, if

hold_domains = !/etc/nohold-domains

and the file contains the lines

!a.b.c
*.b.c

then a.b.c is in the set of domains defined by hold_domains, whereas any domain
matching "*.b.c" is not.

10.2.3 An lsearch file is not an out-of-line list

As will be described in the sections that follow, lookups can be used in lists
to provide indexed methods of checking list membership. There has been some
confusion about the way lsearch lookups work in lists. Because an lsearch file
contains plain text and is scanned sequentially, it is sometimes thought that
it is allowed to contain wild cards and other kinds of non-constant pattern.
This is not the case. The keys in an lsearch file are always fixed strings,
just as for any other single-key lookup type.

If you want to use a file to contain wild-card patterns that form part of a
list, just give the filename on its own, without a search type, as described in
the previous section. You could also use the wildlsearch or nwildlsearch, but
there is no advantage in doing this.

10.2.4 Named lists

A list of domains, hosts, email addresses, or local parts can be given a name
which is then used to refer to the list elsewhere in the configuration. This is
particularly convenient if the same list is required in several different
places. It also allows lists to be given meaningful names, which can improve
the readability of the configuration. For example, it is conventional to define
a domain list called local_domains for all the domains that are handled locally
on a host, using a configuration line such as

domainlist local_domains = localhost:my.dom.example

Named lists are referenced by giving their name preceded by a plus sign, so,
for example, a router that is intended to handle local domains would be
configured with the line

domains = +local_domains

The first router in a configuration is often one that handles all domains
except the local ones, using a configuration with a negated item like this:

dnslookup:
  driver = dnslookup
  domains = ! +local_domains
  transport = remote_smtp
  no_more

The four kinds of named list are created by configuration lines starting with
the words domainlist, hostlist, addresslist, or localpartlist, respectively.
Then there follows the name that you are defining, followed by an equals sign
and the list itself. For example:

hostlist    relay_from_hosts = 192.168.23.0/24 : my.friend.example
addresslist bad_senders = cdb;/etc/badsenders

A named list may refer to other named lists:

domainlist  dom1 = first.example : second.example
domainlist  dom2 = +dom1 : third.example
domainlist  dom3 = fourth.example : +dom2 : fifth.example

Warning: If the last item in a referenced list is a negative one, the effect
may not be what you intended, because the negation does not propagate out to
the higher level. For example, consider:

domainlist  dom1 = !a.b
domainlist  dom2 = +dom1 : *.b

The second list specifies "either in the dom1 list or *.b". The first list
specifies just "not a.b", so the domain x.y matches it. That means it matches
the second list as well. The effect is not the same as

domainlist  dom2 = !a.b : *.b

where x.y does not match. It's best to avoid negation altogether in referenced
lists if you can.

Some named list definitions may contain sensitive data, for example, passwords
for accessing databases. To stop non-admin users from using the -bP command
line option to read these values, you can precede the definition with the word
"hide". For example:

hide domainlist filter_for_domains = ldap;PASS=secret ldap::/// ...

Named lists may have a performance advantage. When Exim is routing an address
or checking an incoming message, it caches the result of tests on named lists.
So, if you have a setting such as

domains = +local_domains

on several of your routers or in several ACL statements, the actual test is
done only for the first one. However, the caching works only if there are no
expansions within the list itself or any sublists that it references. In other
words, caching happens only for lists that are known to be the same each time
they are referenced.

By default, there may be up to 16 named lists of each type. This limit can be
extended by changing a compile-time variable. The use of domain and host lists
is recommended for concepts such as local domains, relay domains, and relay
hosts. The default configuration is set up like this.

10.2.5 Named lists compared with macros

At first sight, named lists might seem to be no different from macros in the
configuration file. However, macros are just textual substitutions. If you
write

ALIST = host1 : host2
auth_advertise_hosts = !ALIST

it probably won't do what you want, because that is exactly the same as

auth_advertise_hosts = !host1 : host2

Notice that the second host name is not negated. However, if you use a host
list, and write

hostlist alist = host1 : host2
auth_advertise_hosts = ! +alist

the negation applies to the whole list, and so that is equivalent to

auth_advertise_hosts = !host1 : !host2

10.2.6 Named list caching

While processing a message, Exim caches the result of checking a named list if
it is sure that the list is the same each time. In practice, this means that
the cache operates only if the list contains no $ characters, which guarantees
that it will not change when it is expanded. Sometimes, however, you may have
an expanded list that you know will be the same each time within a given
message. For example:

domainlist special_domains = \
           ${lookup{$sender_host_address}cdb{/some/file}}

This provides a list of domains that depends only on the sending host's IP
address. If this domain list is referenced a number of times (for example, in
several ACL lines, or in several routers) the result of the check is not cached
by default, because Exim does not know that it is going to be the same list
each time.

By appending "_cache" to "domainlist" you can tell Exim to go ahead and cache
the result anyway. For example:

domainlist_cache special_domains = ${lookup{...

If you do this, you should be absolutely sure that caching is going to do the
right thing in all cases. When in doubt, leave it out.


10.3 Domain lists
-----------------

Domain lists contain patterns that are to be matched against a mail domain. The
following types of item may appear in domain lists:

  * If a pattern consists of a single @ character, it matches the local host
    name, as set by the primary_hostname option (or defaulted). This makes it
    possible to use the same configuration file on several different hosts that
    differ only in their names.

    The value for a match will be the primary host name.

  * If a pattern consists of the string "@[]" it matches an IP address enclosed
    in square brackets (as in an email address that contains a domain literal),
    but only if that IP address is recognized as local for email routing
    purposes. The local_interfaces and extra_local_interfaces options can be
    used to control which of a host's several IP addresses are treated as
    local. In today's Internet, the use of domain literals is controversial;
    see the allow_domain_literals main option.

    The value for a match will be the string "@[]".

  * If a pattern consists of the string "@mx_any" it matches any domain that
    has an MX record pointing to the local host or to any host that is listed
    in hosts_treat_as_local. The items "@mx_primary" and "@mx_secondary" are
    similar, except that the first matches only when a primary MX target is the
    local host, and the second only when no primary MX target is the local
    host, but a secondary MX target is. "Primary" means an MX record with the
    lowest preference value - there may of course be more than one of them.

    The MX lookup that takes place when matching a pattern of this type is
    performed with the resolver options for widening names turned off. Thus,
    for example, a single-component domain will not be expanded by adding the
    resolver's default domain. See the qualify_single and search_parents
    options of the dnslookup router for a discussion of domain widening.

    Sometimes you may want to ignore certain IP addresses when using one of
    these patterns. You can specify this by following the pattern with "/ignore
    ="<ip list>, where <ip list> is a list of IP addresses. These addresses are
    ignored when processing the pattern (compare the ignore_target_hosts option
    on a router). For example:

    domains = @mx_any/ignore=127.0.0.1

    This example matches any domain that has an MX record pointing to one of
    the local host's IP addresses other than 127.0.0.1.

    The list of IP addresses is in fact processed by the same code that
    processes host lists, so it may contain CIDR-coded network specifications
    and it may also contain negative items.

    Because the list of IP addresses is a sublist within a domain list, you
    have to be careful about delimiters if there is more than one address. Like
    any other list, the default delimiter can be changed. Thus, you might have:

    domains = @mx_any/ignore=<;127.0.0.1;0.0.0.0 : \
              an.other.domain : ...

    so that the sublist uses semicolons for delimiters. When IPv6 addresses are
    involved, it is easiest to change the delimiter for the main list as well:

    domains = <? @mx_any/ignore=<;127.0.0.1;::1 ? \
              an.other.domain ? ...

    The value for a match will be the list element string (starting "@mx_").

  * If a pattern starts with an asterisk, the remaining characters of the
    pattern are compared with the terminating characters of the domain. The use
    of "*" in domain lists differs from its use in partial matching lookups. In
    a domain list, the character following the asterisk need not be a dot,
    whereas partial matching works only in terms of dot-separated components.
    For example, a domain list item such as "*key.ex" matches donkey.ex as well
    as cipher.key.ex.

    The value for a match will be the list element string (starting with the
    asterisk). Additionally, $0 will be set to the matched string and $1 to the
    variable portion which the asterisk matched.

  * If a pattern starts with a circumflex character, it is treated as a regular
    expression, and matched against the domain using a regular expression
    matching function. The circumflex is treated as part of the regular
    expression. Email domains are case-independent, so this regular expression
    match is by default case-independent, but you can make it case-dependent by
    starting it with "(?-i)". References to descriptions of the syntax of
    regular expressions are given in chapter 8.

    Warning: Because domain lists are expanded before being processed, you must
    escape any backslash and dollar characters in the regular expression, or
    use the special "\N" sequence (see chapter 11) to specify that it is not to
    be expanded (unless you really do want to build a regular expression by
    expansion, of course).

    The value for a match will be the list element string (starting with the
    circumflex). Additionally, $0 will be set to the string matching the
    regular expression, and $1 (onwards) to any submatches identified by
    parentheses.

  * If a pattern starts with the name of a single-key lookup type followed by a
    semicolon (for example, "dbm;" or "lsearch;"), the remainder of the pattern
    must be a filename in a suitable format for the lookup type. For example,
    for "cdb;" it must be an absolute path:

    domains = cdb;/etc/mail/local_domains.cdb

    The appropriate type of lookup is done on the file using the domain name as
    the key. In most cases, the value resulting from the lookup is not used;
    Exim is interested only in whether or not the key is present in the file.
    However, when a lookup is used for the domains option on a router or a 
    domains condition in an ACL statement, the value is preserved in the
    $domain_data variable and can be referred to in other router options or
    other statements in the same ACL. The value will be untainted.

    Note: If the data result of the lookup (as opposed to the key) is empty,
    then this empty value is stored in $domain_data. The option to return the
    key for the lookup, as the value, may be what is wanted.

  * Any of the single-key lookup type names may be preceded by "partial"<n>"-",
    where the <n> is optional, for example,

    domains = partial-dbm;/partial/domains

    This causes partial matching logic to be invoked; a description of how this
    works is given in section 9.7.

  * Any of the single-key lookup types may be followed by an asterisk. This
    causes a default lookup for a key consisting of a single asterisk to be
    done if the original lookup fails. This is not a useful feature when using
    a domain list to select particular domains (because any domain would
    match), but it might have value if the result of the lookup is being used
    via the $domain_data expansion variable.

  * If the pattern starts with the name of a query-style lookup type followed
    by a semicolon (for example, "nisplus;" or "ldap;"), the remainder of the
    pattern must be an appropriate query for the lookup type, as described in
    chapter 9. For example:

    hold_domains = mysql;select domain from holdlist \
      where domain = '${quote_mysql:$domain}';

    In most cases, the value resulting from the lookup is not used (so for an
    SQL query, for example, it doesn't matter what field you select). Exim is
    interested only in whether or not the query succeeds. However, when a
    lookup is used for the domains option on a router, the value is preserved
    in the $domain_data variable and can be referred to in other options. The
    value will be untainted.

  * If the pattern starts with the name of a lookup type of either kind
    (single-key or query-style) it may be followed by a comma and options, The
    options are lookup-type specific and consist of a comma-separated list.
    Each item starts with a tag and and equals "=" sign.

  * If none of the above cases apply, a caseless textual comparison is made
    between the pattern and the domain.

    The value for a match will be the list element string. Note that this is
    commonly untainted (depending on the way the list was created).
    Specifically, explicit text in the configuration file in not tainted. This
    is a useful way of obtaining an untainted equivalent to the domain, for
    later operations.

    However if the list (including one-element lists) is created by expanding a
    variable containing tainted data, it is tainted and so will the match value
    be.

Here is an example that uses several different kinds of pattern:

domainlist funny_domains = \
  @ : \
  lib.unseen.edu : \
  *.foundation.fict.example : \
  \N^[1-2]\d{3}\.fict\.example$\N : \
  partial-dbm;/opt/data/penguin/book : \
  nis;domains.byname : \
  nisplus;[name=$domain,status=local],domains.org_dir

There are obvious processing trade-offs among the various matching modes. Using
an asterisk is faster than a regular expression, and listing a few names
explicitly probably is too. The use of a file or database lookup is expensive,
but may be the only option if hundreds of names are required. Because the
patterns are tested in order, it makes sense to put the most commonly matched
patterns earlier.


10.4 Host lists
---------------

Host lists are used to control what remote hosts are allowed to do. For
example, some hosts may be allowed to use the local host as a relay, and some
may be permitted to use the SMTP ETRN command. Hosts can be identified in two
different ways, by name or by IP address. In a host list, some types of pattern
are matched to a host name, and some are matched to an IP address. You need to
be particularly careful with this when single-key lookups are involved, to
ensure that the right value is being used as the key.

10.4.1 Special host list patterns

If a host list item is the empty string, it matches only when no remote host is
involved. This is the case when a message is being received from a local
process using SMTP on the standard input, that is, when a TCP/IP connection is
not used.

The special pattern "*" in a host list matches any host or no host. Neither the
IP address nor the name is actually inspected.

10.4.2 Host list patterns that match by IP address

If an IPv4 host calls an IPv6 host and the call is accepted on an IPv6 socket,
the incoming address actually appears in the IPv6 host as "::ffff:"<v4address>.
When such an address is tested against a host list, it is converted into a
traditional IPv4 address first. (Not all operating systems accept IPv4 calls on
IPv6 sockets, as there have been some security concerns.)

The following types of pattern in a host list check the remote host by
inspecting its IP address:

  * If the pattern is a plain domain name (not a regular expression, not
    starting with *, not a lookup of any kind), Exim calls the operating system
    function to find the associated IP address(es). Exim uses the newer
    getipnodebyname() function when available, otherwise gethostbyname(). This
    typically causes a forward DNS lookup of the name. The result is compared
    with the IP address of the subject host.

    If there is a temporary problem (such as a DNS timeout) with the host name
    lookup, a temporary error occurs. For example, if the list is being used in
    an ACL condition, the ACL gives a "defer" response, usually leading to a
    temporary SMTP error code. If no IP address can be found for the host name,
    what happens is described in section 10.4.5 below.

  * If the pattern is "@", the primary host name is substituted and used as a
    domain name, as just described.

  * If the pattern is an IP address, it is matched against the IP address of
    the subject host. IPv4 addresses are given in the normal "dotted-quad"
    notation. IPv6 addresses can be given in colon-separated format, but the
    colons have to be doubled so as not to be taken as item separators when the
    default list separator is used. IPv6 addresses are recognized even when
    Exim is compiled without IPv6 support. This means that if they appear in a
    host list on an IPv4-only host, Exim will not treat them as host names.
    They are just addresses that can never match a client host.

  * If the pattern is "@[]", it matches the IP address of any IP interface on
    the local host. For example, if the local host is an IPv4 host with one
    interface address 10.45.23.56, these two ACL statements have the same
    effect:

    accept hosts = 127.0.0.1 : 10.45.23.56
    accept hosts = @[]

  * If the pattern is an IP address followed by a slash and a mask length, for
    example

    10.11.42.0/24

    , it is matched against the IP address of the subject host under the given
    mask. This allows an entire network of hosts to be included (or excluded)
    by a single item. The mask uses CIDR notation; it specifies the number of
    address bits that must match, starting from the most significant end of the
    address.

    Note: The mask is not a count of addresses, nor is it the high number of a
    range of addresses. It is the number of bits in the network portion of the
    address. The above example specifies a 24-bit netmask, so it matches all
    256 addresses in the 10.11.42.0 network. An item such as

    192.168.23.236/31

    matches just two addresses, 192.168.23.236 and 192.168.23.237. A mask value
    of 32 for an IPv4 address is the same as no mask at all; just a single
    address matches.

    Here is another example which shows an IPv4 and an IPv6 network:

    recipient_unqualified_hosts = 192.168.0.0/16: \
                                  3ffe::ffff::836f::::/48

    The doubling of list separator characters applies only when these items
    appear inline in a host list. It is not required when indirecting via a
    file. For example:

    recipient_unqualified_hosts = /opt/exim/unqualnets

    could make use of a file containing

    172.16.0.0/12
    3ffe:ffff:836f::/48

    to have exactly the same effect as the previous example. When listing IPv6
    addresses inline, it is usually more convenient to use the facility for
    changing separator characters. This list contains the same two networks:

    recipient_unqualified_hosts = <; 172.16.0.0/12; \
                                     3ffe:ffff:836f::/48

    The separator is changed to semicolon by the leading "<;" at the start of
    the list.

10.4.3 Host list patterns for single-key lookups by host address

When a host is to be identified by a single-key lookup of its complete IP
address, the pattern takes this form:

net-<single-key-search-type>;<search-data>

For example:

hosts_lookup = net-cdb;/hosts-by-ip.db

The text form of the IP address of the subject host is used as the lookup key.
IPv6 addresses are converted to an unabbreviated form, using lower case
letters, with dots as separators because colon is the key terminator in lsearch
files. [Colons can in fact be used in keys in lsearch files by quoting the
keys, but this is a facility that was added later.] The data returned by the
lookup is not used.

Single-key lookups can also be performed using masked IP addresses, using
patterns of this form:

net<number>-<single-key-search-type>;<search-data>

For example:

net24-dbm;/networks.db

The IP address of the subject host is masked using <number> as the mask length.
A textual string is constructed from the masked value, followed by the mask,
and this is used as the lookup key. For example, if the host's IP address is
192.168.34.6, the key that is looked up for the above example is "192.168.34.0/
24".

When an IPv6 address is converted to a string, dots are normally used instead
of colons, so that keys in lsearch files need not contain colons (which
terminate lsearch keys). This was implemented some time before the ability to
quote keys was made available in lsearch files. However, the more recently
implemented iplsearch files do require colons in IPv6 keys (notated using the
quoting facility) so as to distinguish them from IPv4 keys. For this reason,
when the lookup type is iplsearch, IPv6 addresses are converted using colons
and not dots. In all cases except IPv4-mapped IPv6, full, unabbreviated IPv6
addresses are always used. The latter are converted to IPv4 addresses, in
dotted-quad form.

Ideally, it would be nice to tidy up this anomalous situation by changing to
colons in all cases, given that quoting is now available for lsearch. However,
this would be an incompatible change that might break some existing
configurations.

Warning: Specifying net32- (for an IPv4 address) or net128- (for an IPv6
address) is not the same as specifying just net- without a number. In the
former case the key strings include the mask value, whereas in the latter case
the IP address is used on its own.

10.4.4 Host list patterns that match by host name

There are several types of pattern that require Exim to know the name of the
remote host. These are either wildcard patterns or lookups by name. (If a
complete hostname is given without any wildcarding, it is used to find an IP
address to match against, as described in section 10.4.2 above.)

If the remote host name is not already known when Exim encounters one of these
patterns, it has to be found from the IP address. Although many sites on the
Internet are conscientious about maintaining reverse DNS data for their hosts,
there are also many that do not do this. Consequently, a name cannot always be
found, and this may lead to unwanted effects. Take care when configuring host
lists with wildcarded name patterns. Consider what will happen if a name cannot
be found.

Because of the problems of determining host names from IP addresses, matching
against host names is not as common as matching against IP addresses.

By default, in order to find a host name, Exim first does a reverse DNS lookup;
if no name is found in the DNS, the system function (gethostbyaddr() or
getipnodebyaddr() if available) is tried. The order in which these lookups are
done can be changed by setting the host_lookup_order option. For security, once
Exim has found one or more names, it looks up the IP addresses for these names
and compares them with the IP address that it started with. Only those names
whose IP addresses match are accepted. Any other names are discarded. If no
names are left, Exim behaves as if the host name cannot be found. In the most
common case there is only one name and one IP address.

There are some options that control what happens if a host name cannot be
found. These are described in section 10.4.5 below.

As a result of aliasing, hosts may have more than one name. When processing any
of the following types of pattern, all the host's names are checked:

  * If a pattern starts with "*" the remainder of the item must match the end
    of the host name. For example, "*.b.c" matches all hosts whose names end in
    .b.c. This special simple form is provided because this is a very common
    requirement. Other kinds of wildcarding require the use of a regular
    expression.

  * If the item starts with "^" it is taken to be a regular expression which is
    matched against the host name. Host names are case-independent, so this
    regular expression match is by default case-independent, but you can make
    it case-dependent by starting it with "(?-i)". References to descriptions
    of the syntax of regular expressions are given in chapter 8. For example,

    ^(a|b)\.c\.d$

    is a regular expression that matches either of the two hosts a.c.d or b.c.d
    . When a regular expression is used in a host list, you must take care that
    backslash and dollar characters are not misinterpreted as part of the
    string expansion. The simplest way to do this is to use "\N" to mark that
    part of the string as non-expandable. For example:

    sender_unqualified_hosts = \N^(a|b)\.c\.d$\N : ....

    Warning: If you want to match a complete host name, you must include the
    "$" terminating metacharacter in the regular expression, as in the above
    example. Without it, a match at the start of the host name is all that is
    required.

10.4.5 Behaviour when an IP address or name cannot be found

While processing a host list, Exim may need to look up an IP address from a
name (see section 10.4.2), or it may need to look up a host name from an IP
address (see section 10.4.4). In either case, the behaviour when it fails to
find the information it is seeking is the same.

Note: This section applies to permanent lookup failures. It does not apply to
temporary DNS errors, whose handling is described in the next section.

Exim parses a host list from left to right. If it encounters a permanent lookup
failure in any item in the host list before it has found a match, Exim treats
it as a failure and the default behavior is as if the host does not match the
list. This may not always be what you want to happen. To change Exim's
behaviour, the special items "+include_unknown" or "+ignore_unknown" may appear
in the list (at top level - they are not recognized in an indirected file).

  * If any item that follows "+include_unknown" requires information that
    cannot found, Exim behaves as if the host does match the list. For example,

    host_reject_connection = +include_unknown:*.enemy.ex

    rejects connections from any host whose name matches "*.enemy.ex", and also
    any hosts whose name it cannot find.

  * If any item that follows "+ignore_unknown" requires information that cannot
    be found, Exim ignores that item and proceeds to the rest of the list. For
    example:

    accept hosts = +ignore_unknown : friend.example : \
                   192.168.4.5

    accepts from any host whose name is friend.example and from 192.168.4.5,
    whether or not its host name can be found. Without "+ignore_unknown", if no
    name can be found for 192.168.4.5, it is rejected.

Both "+include_unknown" and "+ignore_unknown" may appear in the same list. The
effect of each one lasts until the next, or until the end of the list.

10.4.6 Mixing wildcarded host names and addresses in host lists

This section explains the host/ip processing logic with the same concepts as
the previous section, but specifically addresses what happens when a wildcarded
hostname is one of the items in the hostlist.

  * If you have name lookups or wildcarded host names and IP addresses in the
    same host list, you should normally put the IP addresses first. For
    example, in an ACL you could have:

    accept hosts = 10.9.8.7 : *.friend.example

    The reason you normally would order it this way lies in the left-to-right
    way that Exim processes lists. It can test IP addresses without doing any
    DNS lookups, but when it reaches an item that requires a host name, it
    fails if it cannot find a host name to compare with the pattern. If the
    above list is given in the opposite order, the accept statement fails for a
    host whose name cannot be found, even if its IP address is 10.9.8.7.

  * If you really do want to do the name check first, and still recognize the
    IP address, you can rewrite the ACL like this:

    accept hosts = *.friend.example
    accept hosts = 10.9.8.7

    If the first accept fails, Exim goes on to try the second one. See chapter
    44 for details of ACLs. Alternatively, you can use "+ignore_unknown", which
    was discussed in depth in the first example in this section.

10.4.7 Temporary DNS errors when looking up host information

A temporary DNS lookup failure normally causes a defer action (except when 
dns_again_means_nonexist converts it into a permanent error). However, host
lists can include "+ignore_defer" and "+include_defer", analogous to
"+ignore_unknown" and "+include_unknown", as described in the previous section.
These options should be used with care, probably only in non-critical host
lists such as whitelists.

10.4.8 Host list patterns for single-key lookups by host name

If a pattern is of the form

<single-key-search-type>;<search-data>

for example

dbm;/host/accept/list

a single-key lookup is performed, using the host name as its key. If the lookup
succeeds, the host matches the item. The actual data that is looked up is not
used.

Reminder: With this kind of pattern, you must have host names as keys in the
file, not IP addresses. If you want to do lookups based on IP addresses, you
must precede the search type with "net-" (see section 10.4.3). There is,
however, no reason why you could not use two items in the same list, one doing
an address lookup and one doing a name lookup, both using the same file.

10.4.9 Host list patterns for query-style lookups

If a pattern is of the form

<query-style-search-type>;<query>

the query is obeyed, and if it succeeds, the host matches the item. The actual
data that is looked up is not used. The variables $sender_host_address and
$sender_host_name can be used in the query. For example:

hosts_lookup = pgsql;\
  select ip from hostlist where ip='$sender_host_address'

The value of $sender_host_address for an IPv6 address contains colons. You can
use the sg expansion item to change this if you need to. If you want to use
masked IP addresses in database queries, you can use the mask expansion
operator.

If the query contains a reference to $sender_host_name, Exim automatically
looks up the host name if it has not already done so. (See section 10.4.4 for
comments on finding host names.)

Historical note: prior to release 4.30, Exim would always attempt to find a
host name before running the query, unless the search type was preceded by
"net-". This is no longer the case. For backwards compatibility, "net-" is
still recognized for query-style lookups, but its presence or absence has no
effect. (Of course, for single-key lookups, "net-" is important. See section
10.4.3.)


10.5 Address lists
------------------

Address lists contain patterns that are matched against mail addresses. There
is one special case to be considered: the sender address of a bounce message is
always empty. You can test for this by providing an empty item in an address
list. For example, you can set up a router to process bounce messages by using
this option setting:

senders = :

The presence of the colon creates an empty item. If you do not provide any
data, the list is empty and matches nothing. The empty sender can also be
detected by a regular expression that matches an empty string, and by a
query-style lookup that succeeds when $sender_address is empty.

Non-empty items in an address list can be straightforward email addresses. For
example:

senders = jbc@askone.example : hs@anacreon.example

A certain amount of wildcarding is permitted. If a pattern contains an @
character, but is not a regular expression and does not begin with a
semicolon-terminated lookup type (described below), the local part of the
subject address is compared with the local part of the pattern, which may start
with an asterisk. If the local parts match, the domain is checked in exactly
the same way as for a pattern in a domain list. For example, the domain can be
wildcarded, refer to a named list, or be a lookup:

deny senders = *@*.spamming.site:\
               *@+hostile_domains:\
               bozo@partial-lsearch;/list/of/dodgy/sites:\
               *@dbm;/bad/domains.db

If a local part that begins with an exclamation mark is required, it has to be
specified using a regular expression, because otherwise the exclamation mark is
treated as a sign of negation, as is standard in lists.

If a non-empty pattern that is not a regular expression or a lookup does not
contain an @ character, it is matched against the domain part of the subject
address. The only two formats that are recognized this way are a literal
domain, or a domain pattern that starts with *. In both these cases, the effect
is the same as if "*@" preceded the pattern. For example:

deny senders = enemy.domain : *.enemy.domain

The following kinds of more complicated address list pattern can match any
address, including the empty address that is characteristic of bounce message
senders:

  * If (after expansion) a pattern starts with "^", a regular expression match
    is done against the complete address, with the pattern as the regular
    expression. You must take care that backslash and dollar characters are not
    misinterpreted as part of the string expansion. The simplest way to do this
    is to use "\N" to mark that part of the string as non-expandable. For
    example:

    deny senders = \N^.*this.*@example\.com$\N : \
                   \N^\d{8}.+@spamhaus.example$\N : ...

    The "\N" sequences are removed by the expansion, so these items do indeed
    start with "^" by the time they are being interpreted as address patterns.

  * Complete addresses can be looked up by using a pattern that starts with a
    lookup type terminated by a semicolon, followed by the data for the lookup.
    For example:

    deny senders = cdb;/etc/blocked.senders : \
      mysql;select address from blocked where \
      address='${quote_mysql:$sender_address}'

    Both query-style and single-key lookup types can be used. For a single-key
    lookup type, Exim uses the complete address as the key. However, empty keys
    are not supported for single-key lookups, so a match against the empty
    address always fails. This restriction does not apply to query-style
    lookups.

    Partial matching for single-key lookups (section 9.7) cannot be used, and
    is ignored if specified, with an entry being written to the panic log. 
    However, you can configure lookup defaults, as described in section 9.6,
    but this is useful only for the "*@" type of default. For example, with
    this lookup:

    accept senders = lsearch*@;/some/file

    the file could contains lines like this:

    user1@domain1.example
    *@domain2.example

    and for the sender address nimrod@jaeger.example, the sequence of keys that
    are tried is:

    nimrod@jaeger.example
    *@jaeger.example
    *

    Warning 1: Do not include a line keyed by "*" in the file, because that
    would mean that every address matches, thus rendering the test useless.

    Warning 2: Do not confuse these two kinds of item:

    deny recipients = dbm*@;/some/file
    deny recipients = *@dbm;/some/file

    The first does a whole address lookup, with defaulting, as just described,
    because it starts with a lookup type. The second matches the local part and
    domain independently, as described in a bullet point below.

The following kinds of address list pattern can match only non-empty addresses.
If the subject address is empty, a match against any of these pattern types
always fails.

  * If a pattern starts with "@@" followed by a single-key lookup item (for
    example, "@@lsearch;/some/file"), the address that is being checked is
    split into a local part and a domain. The domain is looked up in the file.
    If it is not found, there is no match. If it is found, the data that is
    looked up from the file is treated as a colon-separated list of local part
    patterns, each of which is matched against the subject local part in turn.

    The lookup may be a partial one, and/or one involving a search for a
    default keyed by "*" (see section 9.6). The local part patterns that are
    looked up can be regular expressions or begin with "*", or even be further
    lookups. They may also be independently negated. For example, with

    deny senders = @@dbm;/etc/reject-by-domain

    the data from which the DBM file is built could contain lines like

    baddomain.com:  !postmaster : *

    to reject all senders except postmaster from that domain.

    If a local part that actually begins with an exclamation mark is required,
    it has to be specified using a regular expression. In lsearch files, an
    entry may be split over several lines by indenting the second and
    subsequent lines, but the separating colon must still be included at line
    breaks. White space surrounding the colons is ignored. For example:

    aol.com:  spammer1 : spammer2 : ^[0-9]+$ :
      spammer3 : spammer4

    As in all colon-separated lists in Exim, a colon can be included in an item
    by doubling.

    If the last item in the list starts with a right angle-bracket, the
    remainder of the item is taken as a new key to look up in order to obtain a
    continuation list of local parts. The new key can be any sequence of
    characters. Thus one might have entries like

    aol.com: spammer1 : spammer 2 : >*
    xyz.com: spammer3 : >*
    *:       ^\d{8}$

    in a file that was searched with @@dbm*, to specify a match for 8-digit
    local parts for all domains, in addition to the specific local parts listed
    for each domain. Of course, using this feature costs another lookup each
    time a chain is followed, but the effort needed to maintain the data is
    reduced.

    It is possible to construct loops using this facility, and in order to
    catch them, the chains may be no more than fifty items long.

  * The @@<lookup> style of item can also be used with a query-style lookup,
    but in this case, the chaining facility is not available. The lookup can
    only return a single list of local parts.

Warning: There is an important difference between the address list items in
these two examples:

senders = +my_list
senders = *@+my_list

In the first one, "my_list" is a named address list, whereas in the second
example it is a named domain list.

10.5.1 Case of letters in address lists

Domains in email addresses are always handled caselessly, but for local parts
case may be significant on some systems (see caseful_local_part for how Exim
deals with this when routing addresses). However, RFC 2505 (Anti-Spam
Recommendations for SMTP MTAs) suggests that matching of addresses to blocking
lists should be done in a case-independent manner. Since most address lists in
Exim are used for this kind of control, Exim attempts to do this by default.

The domain portion of an address is always lowercased before matching it to an
address list. The local part is lowercased by default, and any string
comparisons that take place are done caselessly. This means that the data in
the address list itself, in files included as plain filenames, and in any file
that is looked up using the "@@" mechanism, can be in any case. However, the
keys in files that are looked up by a search type other than lsearch (which
works caselessly) must be in lower case, because these lookups are not
case-independent.

To allow for the possibility of caseful address list matching, if an item in an
address list is the string "+caseful", the original case of the local part is
restored for any comparisons that follow, and string comparisons are no longer
case-independent. This does not affect the domain, which remains in lower case.
However, although independent matches on the domain alone are still performed
caselessly, regular expressions that match against an entire address become
case-sensitive after "+caseful" has been seen.


10.6 Local part lists
---------------------

These behave in the same way as domain and host lists, with the following
changes:

Case-sensitivity in local part lists is handled in the same way as for address
lists, as just described. The "+caseful" item can be used if required. In a
setting of the local_parts option in a router with caseful_local_part set
false, the subject is lowercased and the matching is initially
case-insensitive. In this case, "+caseful" will restore case-sensitive matching
in the local part list, but not elsewhere in the router. If caseful_local_part
is set true in a router, matching in the local_parts option is case-sensitive
from the start.

If a local part list is indirected to a file (see section 10.2.2), comments are
handled in the same way as address lists - they are recognized only if the # is
preceded by white space or the start of the line. Otherwise, local part lists
are matched in the same way as domain lists, except that the special items that
refer to the local host ("@", "@[]", "@mx_any", "@mx_primary", and
"@mx_secondary") are not recognized. Refer to section 10.3 for details of the
other available item types.



===============================================================================
11. STRING EXPANSIONS

Many strings in Exim's runtime configuration are expanded before use. Some of
them are expanded every time they are used; others are expanded only once.

When a string is being expanded it is copied verbatim from left to right except
when a dollar or backslash character is encountered. A dollar specifies the
start of a portion of the string that is interpreted and replaced as described
below in section 11.5 onwards. Backslash is used as an escape character, as
described in the following section.

Whether a string is expanded depends upon the context. Usually this is solely
dependent upon the option for which a value is sought; in this documentation,
options for which string expansion is performed are marked with * after the
data type. ACL rules always expand strings. A couple of expansion conditions do
not expand some of the brace-delimited branches, for security reasons, and
expansion of data deriving from the sender ("tainted data") is not permitted
(including acessing a file using a tainted name).

Common ways of obtaining untainted equivalents of variables with tainted values
come down to using the tainted value as a lookup key in a trusted database.
This database could be the filesystem structure, or the password file, or
accessed via a DBMS. Specific methods are indexed under "de-tainting".


11.1 Literal text in expanded strings
-------------------------------------

An uninterpreted dollar can be included in an expanded string by putting a
backslash in front of it. A backslash can be used to prevent any special
character being treated specially in an expansion, including backslash itself.
If the string appears in quotes in the configuration file, two backslashes are
required because the quotes themselves cause interpretation of backslashes when
the string is read in (see section 6.17).

A portion of the string can specified as non-expandable by placing it between
two occurrences of "\N". This is particularly useful for protecting regular
expressions, which often contain backslashes and dollar signs. For example:

deny senders = \N^\d{8}[a-z]@some\.site\.example$\N

On encountering the first "\N", the expander copies subsequent characters
without interpretation until it reaches the next "\N" or the end of the string.


11.2 Character escape sequences in expanded strings
---------------------------------------------------

A backslash followed by one of the letters "n", "r", or "t" in an expanded
string is recognized as an escape sequence for the character newline, carriage
return, or tab, respectively. A backslash followed by up to three octal digits
is recognized as an octal encoding for a single character, and a backslash
followed by "x" and up to two hexadecimal digits is a hexadecimal encoding.

These escape sequences are also recognized in quoted strings when they are read
in. Their interpretation in expansions as well is useful for unquoted strings,
and for other cases such as looked-up strings that are then expanded.


11.3 Testing string expansions
------------------------------

Many expansions can be tested by calling Exim with the -be option. This takes
the command arguments, or lines from the standard input if there are no
arguments, runs them through the string expansion code, and writes the results
to the standard output. Variables based on configuration values are set up, but
since no message is being processed, variables such as $local_part have no
value. Nevertheless the -be option can be useful for checking out file and
database lookups, and the use of expansion operators such as sg, substr and 
nhash.

When reading lines from the standard input, macros can be defined and ACL
variables can be set. For example:

MY_MACRO = foo
set acl_m_myvar = bar

Such macros and variables can then be used in later input lines.

Exim gives up its root privilege when it is called with the -be option, and
instead runs under the uid and gid it was called with, to prevent users from
using -be for reading files to which they do not have access.

If you want to test expansions that include variables whose values are taken
from a message, there are two other options that can be used. The -bem option
is like -be except that it is followed by a filename. The file is read as a
message before doing the test expansions. For example:

exim -bem /tmp/test.message '$h_subject:'

The -Mset option is used in conjunction with -be and is followed by an Exim
message identifier. For example:

exim -be -Mset 1GrA8W-0004WS-LQ '$recipients'

This loads the message from Exim's spool before doing the test expansions, and
is therefore restricted to admin users.


11.4 Forced expansion failure
-----------------------------

A number of expansions that are described in the following section have
alternative "true" and "false" substrings, enclosed in brace characters (which
are sometimes called "curly brackets"). Which of the two strings is used
depends on some condition that is evaluated as part of the expansion. If,
instead of a "false" substring, the word "fail" is used (not in braces), the
entire string expansion fails in a way that can be detected by the code that
requested the expansion. This is called "forced expansion failure", and its
consequences depend on the circumstances. In some cases it is no different from
any other expansion failure, but in others a different action may be taken.
Such variations are mentioned in the documentation of the option that is being
expanded.


11.5 Expansion items
--------------------

The following items are recognized in expanded strings. White space may be used
between sub-items that are keywords or substrings enclosed in braces inside an
outer set of braces, to improve readability. Warning: Within braces, white
space is significant.

$<variable name> or ${<variable name>}

    Substitute the contents of the named variable, for example:

    $local_part
    ${domain}

    The second form can be used to separate the name from subsequent
    alphanumeric characters. This form (using braces) is available only for
    variables; it does not apply to message headers. The names of the variables
    are given in section 11.9 below. If the name of a non-existent variable is
    given, the expansion fails.

${<op>:<string>}

    The string is first itself expanded, and then the operation specified by <
    op> is applied to it. For example:

    ${lc:$local_part}

    The string starts with the first character after the colon, which may be
    leading white space. A list of operators is given in section 11.6 below.
    The operator notation is used for simple expansion items that have just one
    argument, because it reduces the number of braces and therefore makes the
    string easier to understand.

$bheader_<header name>: or $bh_<header name>:

    This item inserts "basic" header lines. It is described with the header
    expansion item below.

${acl{<name>}{<arg>}...}

    The name and zero to nine argument strings are first expanded separately.
    The expanded arguments are assigned to the variables $acl_arg1 to $acl_arg9
    in order. Any unused are made empty. The variable $acl_narg is set to the
    number of arguments. The named ACL (see chapter 44) is called and may use
    the variables; if another acl expansion is used the values are restored
    after it returns. If the ACL sets a value using a "message =" modifier and
    returns accept or deny, the value becomes the result of the expansion. If
    no message is set and the ACL returns accept or deny the expansion result
    is an empty string. If the ACL returns defer the result is a forced-fail.
    Otherwise the expansion fails.

${authresults{<authserv-id>}}

    This item returns a string suitable for insertion as an 
    Authentication-Results: header line. The given <authserv-id> is included in
    the result; typically this will be a domain name identifying the system
    performing the authentications. Methods that might be present in the result
    include:

    none
    iprev
    auth
    spf
    dkim

    Example use (as an ACL modifier):

          add_header = :at_start:${authresults {$primary_hostname}}

    This is safe even if no authentication results are available

    and would generally be placed in the DATA ACL.

${certextract{<field>}{<certificate>}{<string2>}{<string3>}}

    The <certificate> must be a variable of type certificate. The field name is
    expanded and used to retrieve the relevant field from the certificate.
    Supported fields are:

    version        
    serial_number  
    subject         RFC4514 DN
    issuer          RFC4514 DN
    notbefore       time
    notafter        time
    sig_algorithm  
    signature      
    subj_altname    tagged list
    ocsp_uri        list
    crl_uri         list

    If the field is found, <string2> is expanded, and replaces the whole item;
    otherwise <string3> is used. During the expansion of <string2> the variable
    $value contains the value that has been extracted. Afterwards, it is
    restored to any previous value it might have had.

    If {<string3>} is omitted, the item is replaced by an empty string if the
    key is not found. If {<string2>} is also omitted, the value that was
    extracted is used.

    Some field names take optional modifiers, appended and separated by commas.

    The field selectors marked as "RFC4514" above output a Distinguished Name
    string which is not quite parseable by Exim as a comma-separated tagged
    list (the exceptions being elements containing commas). RDN elements of a
    single type may be selected by a modifier of the type label; if so the
    expansion result is a list (newline-separated by default). The separator
    may be changed by another modifier of a right angle-bracket followed
    immediately by the new separator. Recognised RDN type labels include "CN",
    "O", "OU" and "DC".

    The field selectors marked as "time" above take an optional modifier of
    "int" for which the result is the number of seconds since epoch. Otherwise
    the result is a human-readable string in the timezone selected by the main
    "timezone" option.

    The field selectors marked as "list" above return a list, newline-separated
    by default, (embedded separator characters in elements are doubled). The
    separator may be changed by a modifier of a right angle-bracket followed
    immediately by the new separator.

    The field selectors marked as "tagged" above prefix each list element with
    a type string and an equals sign. Elements of only one type may be selected
    by a modifier which is one of "dns", "uri" or "mail"; if so the element
    tags are omitted.

    If not otherwise noted field values are presented in human-readable form.

${dlfunc{<file>}{<function>}{<arg>}{<arg>}...}

    This expansion dynamically loads and then calls a locally-written C
    function. This functionality is available only if Exim is compiled with

    EXPAND_DLFUNC=yes

    set in Local/Makefile. Once loaded, Exim remembers the dynamically loaded
    object so that it doesn't reload the same object file in the same Exim
    process (but of course Exim does start new processes frequently).

    There may be from zero to eight arguments to the function.

    When compiling a local function that is to be called in this way, first
    DLFUNC_IMPL should be defined, and second local_scan.h should be included.
    The Exim variables and functions that are defined by that API are also
    available for dynamically loaded functions. The function itself must have
    the following type:

    int dlfunction(uschar **yield, int argc, uschar *argv[])

    Where "uschar" is a typedef for "unsigned char" in local_scan.h. The
    function should return one of the following values:

    "OK": Success. The string that is placed in the variable yield is put into
    the expanded string that is being built.

    "FAIL": A non-forced expansion failure occurs, with the error message taken
    from yield, if it is set.

    "FAIL_FORCED": A forced expansion failure occurs, with the error message
    taken from yield if it is set.

    "ERROR": Same as "FAIL", except that a panic log entry is written.

    When compiling a function that is to be used in this way with gcc, you need
    to add -shared to the gcc command. Also, in the Exim build-time
    configuration, you must add -export-dynamic to EXTRALIBS.

${env{<key>}{<string1>}{<string2>}}

    The key is first expanded separately, and leading and trailing white space
    removed. This is then searched for as a name in the environment. If a
    variable is found then its value is placed in $value and <string1> is
    expanded, otherwise <string2> is expanded.

    Instead of {<string2>} the word "fail" (not in curly brackets) can appear,
    for example:

    ${env{USER}{$value} fail }

    This forces an expansion failure (see section 11.4); {<string1>} must be
    present for "fail" to be recognized.

    If {<string2>} is omitted an empty string is substituted on search failure.
    If {<string1>} is omitted the search result is substituted on search
    success.

    The environment is adjusted by the keep_environment and add_environment
    main section options.

${extract{<key>}{<string1>}{<string2>}{<string3>}}

    The key and <string1> are first expanded separately. Leading and trailing
    white space is removed from the key (but not from any of the strings). The
    key must not be empty and must not consist entirely of digits. The expanded
    <string1> must be of the form:

    <key1> = <value1>  <key2> = <value2> ...

    where the equals signs and spaces (but not both) are optional. If any of
    the values contain white space, they must be enclosed in double quotes, and
    any values that are enclosed in double quotes are subject to escape
    processing as described in section 6.17. The expanded <string1> is searched
    for the value that corresponds to the key. The search is case-insensitive.
    If the key is found, <string2> is expanded, and replaces the whole item;
    otherwise <string3> is used. During the expansion of <string2> the variable
    $value contains the value that has been extracted. Afterwards, it is
    restored to any previous value it might have had.

    If {<string3>} is omitted, the item is replaced by an empty string if the
    key is not found. If {<string2>} is also omitted, the value that was
    extracted is used. Thus, for example, these two expansions are identical,
    and yield "2001":

    ${extract{gid}{uid=1984 gid=2001}}
    ${extract{gid}{uid=1984 gid=2001}{$value}}

    Instead of {<string3>} the word "fail" (not in curly brackets) can appear,
    for example:

    ${extract{Z}{A=... B=...}{$value} fail }

    This forces an expansion failure (see section 11.4); {<string2>} must be
    present for "fail" to be recognized.

${extract json{<key>}{<string1>}{<string2>}{<string3>}}, ${extract jsons{<key>}
{<string1>}{<string2>}{<string3>}}

    The key and <string1> are first expanded separately. Leading and trailing
    white space is removed from the key (but not from any of the strings). The
    key must not be empty and must not consist entirely of digits. The expanded
    <string1> must be of the form:

    { <"key1"> : <value1> ,  <"key2"> , <value2> ... }

    The braces, commas and colons, and the quoting of the member name are
    required; the spaces are optional. Matching of the key against the member
    names is done case-sensitively. For the "json" variant, if a returned value
    is a JSON string, it retains its leading and trailing quotes. For the
    "jsons" variant, which is intended for use with JSON strings, the leading
    and trailing quotes are removed from the returned value.

    The results of matching are handled as above.

${extract{<number>}{<separators>}{<string1>}{<string2>}{<string3>}}

    The <number> argument must consist entirely of decimal digits, apart from
    leading and trailing white space, which is ignored. This is what
    distinguishes this form of extract from the previous kind. It behaves in
    the same way, except that, instead of extracting a named field, it extracts
    from <string1> the field whose number is given as the first argument. You
    can use $value in <string2> or "fail" instead of <string3> as before.

    The fields in the string are separated by any one of the characters in the
    separator string. These may include space or tab characters. The first
    field is numbered one. If the number is negative, the fields are counted
    from the end of the string, with the rightmost one numbered -1. If the
    number given is zero, the entire string is returned. If the modulus of the
    number is greater than the number of fields in the string, the result is
    the expansion of <string3>, or the empty string if <string3> is not
    provided. For example:

    ${extract{2}{:}{x:42:99:& Mailer::/bin/bash}}

    yields "42", and

    ${extract{-4}{:}{x:42:99:& Mailer::/bin/bash}}

    yields "99". Two successive separators mean that the field between them is
    empty (for example, the fifth field above).

${extract json {<number>}}{<string1>}{<string2>}{<string3>}}, ${extract jsons{<
number>}}{<string1>}{<string2>}{<string3>}}

    The <number> argument must consist entirely of decimal digits, apart from
    leading and trailing white space, which is ignored.

    Field selection and result handling is as above; there is no choice of
    field separator. For the "json" variant, if a returned value is a JSON
    string, it retains its leading and trailing quotes. For the "jsons"
    variant, which is intended for use with JSON strings, the leading and
    trailing quotes are removed from the returned value.

${filter{<string>}{<condition>}}

    After expansion, <string> is interpreted as a list, colon-separated by
    default, but the separator can be changed in the usual way (6.21). For each
    item in this list, its value is placed in $item, and then the condition is
    evaluated.

    Any modification of $value by this evaluation is discarded.

    If the condition is true, $item is added to the output as an item in a new
    list; if the condition is false, the item is discarded. The separator used
    for the output list is the same as the one used for the input, but a
    separator setting is not included in the output. For example:

    ${filter{a:b:c}{!eq{$item}{b}}}

    yields "a:c". At the end of the expansion, the value of $item is restored
    to what it was before. See also the map and reduce expansion items.

${hash{<string1>}{<string2>}{<string3>}}

    This is a textual hashing function, and was the first to be implemented in
    early versions of Exim. In current releases, there are other hashing
    functions (numeric, MD5, and SHA-1), which are described below.

    The first two strings, after expansion, must be numbers. Call them <m> and
    <n>. If you are using fixed values for these numbers, that is, if <string1>
    and <string2> do not change when they are expanded, you can use the simpler
    operator notation that avoids some of the braces:

    ${hash_<n>_<m>:<string>}

    The second number is optional (in both notations). If <n> is greater than
    or equal to the length of the string, the expansion item returns the
    string. Otherwise it computes a new string of length <n> by applying a
    hashing function to the string. The new string consists of characters taken
    from the first <m> characters of the string

    abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQWRSTUVWXYZ0123456789

    If <m> is not present the value 26 is used, so that only lower case letters
    appear. For example:

    $hash{3}{monty}}              yields  jmg
    $hash{5}{monty}}              yields  monty
    $hash{4}{62}{monty python}}   yields  fbWx

$header_<header name>: or $h_<header name>:, $bheader_<header name>: or $bh_<
header name>:, $lheader_<header name>: or $lh_<header name>:, $rheader_<
header name>: or $rh_<header name>:

    Substitute the contents of the named message header line, for example

    $header_reply-to:

    The newline that terminates a header line is not included in the expansion,
    but internal newlines (caused by splitting the header line over several
    physical lines) may be present.

    The difference between the four pairs of expansions is in the way the data
    in the header line is interpreted.

      o rheader gives the original "raw" content of the header line, with no
        processing at all, and without the removal of leading and trailing
        white space.

      o lheader gives a colon-separated list, one element per header when there
        are multiple headers with a given name. Any embedded colon characters
        within an element are doubled, so normal Exim list-processing
        facilities can be used. The terminating newline of each element is
        removed; in other respects the content is "raw".

      o bheader removes leading and trailing white space, and then decodes
        base64 or quoted-printable MIME "words" within the header text, but
        does no character set translation. If decoding of what looks
        superficially like a MIME "word" fails, the raw string is returned. If
        decoding produces a binary zero character, it is replaced by a question
        mark - this is what Exim does for binary zeros that are actually
        received in header lines.

      o header tries to translate the string as decoded by bheader to a
        standard character set. This is an attempt to produce the same string
        as would be displayed on a user's MUA. If translation fails, the 
        bheader string is returned. Translation is attempted only on operating
        systems that support the iconv() function. This is indicated by the
        compile-time macro HAVE_ICONV in a system Makefile or in Local/Makefile
        .

    In a filter file, the target character set for header can be specified by a
    command of the following form:

    headers charset "UTF-8"

    This command affects all references to $h_ (or $header_) expansions in
    subsequently obeyed filter commands. In the absence of this command, the
    target character set in a filter is taken from the setting of the 
    headers_charset option in the runtime configuration. The value of this
    option defaults to the value of HEADERS_CHARSET in Local/Makefile. The
    ultimate default is ISO-8859-1.

    Header names follow the syntax of RFC 2822, which states that they may
    contain any printing characters except space and colon. Consequently, curly
    brackets do not terminate header names, and should not be used to enclose
    them as if they were variables. Attempting to do so causes a syntax error.

    Only header lines that are common to all copies of a message are visible to
    this mechanism. These are the original header lines that are received with
    the message, and any that are added by an ACL statement or by a system
    filter. Header lines that are added to a particular copy of a message by a
    router or transport are not accessible.

    For incoming SMTP messages, no header lines are visible in ACLs that are
    obeyed before the data phase completes, because the header structure is not
    set up until the message is received. They are visible in DKIM, PRDR and
    DATA ACLs. Header lines that are added in a RCPT ACL (for example) are
    saved until the message's incoming header lines are available, at which
    point they are added. When any of the above ACLs are running, however,
    header lines added by earlier ACLs are visible.

    Upper case and lower case letters are synonymous in header names. If the
    following character is white space, the terminating colon may be omitted,
    but this is not recommended, because you may then forget it when it is
    needed. When white space terminates the header name, this white space is
    included in the expanded string. If the message does not contain the given
    header, the expansion item is replaced by an empty string. (See the def
    condition in section 11.7 for a means of testing for the existence of a
    header.)

    If there is more than one header with the same name, they are all
    concatenated to form the substitution string, up to a maximum length of
    64K. Unless rheader is being used, leading and trailing white space is
    removed from each header before concatenation, and a completely empty
    header is ignored. A newline character is then inserted between non-empty
    headers, but there is no newline at the very end. For the header and 
    bheader expansion, for those headers that contain lists of addresses, a
    comma is also inserted at the junctions between headers. This does not
    happen for the rheader expansion.

    When the headers are from an incoming message, the result of expanding any
    of these variables is tainted.

${hmac{<hashname>}{<secret>}{<string>}}

    This function uses cryptographic hashing (either MD5 or SHA-1) to convert a
    shared secret and some text into a message authentication code, as
    specified in RFC 2104. This differs from "${md5:secret_text...}" or "$
    {sha1:secret_text...}" in that the hmac step adds a signature to the
    cryptographic hash, allowing for authentication that is not possible with
    MD5 or SHA-1 alone. The hash name must expand to either "md5" or "sha1" at
    present. For example:

    ${hmac{md5}{somesecret}{$primary_hostname $tod_log}}

    For the hostname mail.example.com and time 2002-10-17 11:30:59, this
    produces:

    dd97e3ba5d1a61b5006108f8c8252953

    As an example of how this might be used, you might put in the main part of
    an Exim configuration:

    SPAMSCAN_SECRET=cohgheeLei2thahw

    In a router or a transport you could then have:

    headers_add = \
      X-Spam-Scanned: ${primary_hostname} ${message_exim_id} \
      ${hmac{md5}{SPAMSCAN_SECRET}\
      {${primary_hostname},${message_exim_id},$h_message-id:}}

    Then given a message, you can check where it was scanned by looking at the 
    X-Spam-Scanned: header line. If you know the secret, you can check that
    this header line is authentic by recomputing the authentication code from
    the host name, message ID and the Message-id: header line. This can be done
    using Exim's -be option, or by other means, for example, by using the 
    hmac_md5_hex() function in Perl.

${if <condition> {<string1>}{<string2>}}

    If <condition> is true, <string1> is expanded and replaces the whole item;
    otherwise <string2> is used. The available conditions are described in
    section 11.7 below. For example:

    ${if eq {$local_part}{postmaster} {yes}{no} }

    The second string need not be present; if it is not and the condition is
    not true, the item is replaced with nothing. Alternatively, the word "fail"
    may be present instead of the second string (without any curly brackets).
    In this case, the expansion is forced to fail if the condition is not true
    (see section 11.4).

    If both strings are omitted, the result is the string "true" if the
    condition is true, and the empty string if the condition is false. This
    makes it less cumbersome to write custom ACL and router conditions. For
    example, instead of

    condition = ${if >{$acl_m4}{3}{true}{false}}

    you can use

    condition = ${if >{$acl_m4}{3}}

${imapfolder{<foldername>}}

    This item converts a (possibly multilevel, or with non-ASCII characters)
    folder specification to a Maildir name for filesystem use. For information
    on internationalisation support see 60.2.

${length{<string1>}{<string2>}}

    The length item is used to extract the initial portion of a string. Both
    strings are expanded, and the first one must yield a number, <n>, say. If
    you are using a fixed value for the number, that is, if <string1> does not
    change when expanded, you can use the simpler operator notation that avoids
    some of the braces:

    ${length_<n>:<string>}

    The result of this item is either the first <n> bytes or the whole of <
    string2>, whichever is the shorter. Do not confuse length with strlen,
    which gives the length of a string. All measurement is done in bytes and is
    not UTF-8 aware.

${listextract{<number>}{<string1>}{<string2>}{<string3>}}

    The <number> argument must consist entirely of decimal digits, apart from
    an optional leading minus, and leading and trailing white space (which is
    ignored).

    After expansion, <string1> is interpreted as a list, colon-separated by
    default, but the separator can be changed in the usual way (6.21).

    The first field of the list is numbered one. If the number is negative, the
    fields are counted from the end of the list, with the rightmost one
    numbered -1. The numbered element of the list is extracted and placed in
    $value, then <string2> is expanded as the result.

    If the modulus of the number is zero or greater than the number of fields
    in the string, the result is the expansion of <string3>.

    For example:

    ${listextract{2}{x:42:99}}

    yields "42", and

    ${listextract{-3}{<, x,42,99,& Mailer,,/bin/bash}{result: $value}}

    yields "result: 42".

    If {<string3>} is omitted, an empty string is used for string3. If {<
    string2>} is also omitted, the value that was extracted is used. You can
    use "fail" instead of {<string3>} as in a string extract.

${listquote{<separator>}{<string>}}

    This item doubles any occurrence of the separator character in the given
    string. An empty string is replaced with a single space. This converts the
    string into a safe form for use as a list element, in a list using the
    given separator.

${lookup {<key>} <search type> {<file>} {<string1>} {<string2>}}, ${lookup <
search type> {<query>} {<string1>} {<string2>}}

    The two forms of lookup item specify data lookups in files and databases,
    as discussed in chapter 9. The first form is used for single-key lookups,
    and the second is used for query-style lookups. The <key>, <file>, and <
    query> strings are expanded before use.

    If there is any white space in a lookup item which is part of a filter
    command, a retry or rewrite rule, a routing rule for the manualroute
    router, or any other place where white space is significant, the lookup
    item must be enclosed in double quotes. The use of data lookups in users'
    filter files may be locked out by the system administrator.

    If the lookup succeeds, <string1> is expanded and replaces the entire item.
    During its expansion, the variable $value contains the data returned by the
    lookup. Afterwards it reverts to the value it had previously (at the outer
    level it is empty). If the lookup fails, <string2> is expanded and replaces
    the entire item. If {<string2>} is omitted, the replacement is the empty
    string on failure. If <string2> is provided, it can itself be a nested
    lookup, thus providing a mechanism for looking up a default value when the
    original lookup fails.

    If a nested lookup is used as part of <string1>, $value contains the data
    for the outer lookup while the parameters of the second lookup are
    expanded, and also while <string2> of the second lookup is expanded, should
    the second lookup fail. Instead of {<string2>} the word "fail" can appear,
    and in this case, if the lookup fails, the entire expansion is forced to
    fail (see section 11.4). If both {<string1>} and {<string2>} are omitted,
    the result is the looked up value in the case of a successful lookup, and
    nothing in the case of failure.

    For single-key lookups, the string "partial" is permitted to precede the
    search type in order to do partial matching, and * or *@ may follow a
    search type to request default lookups if the key does not match (see
    sections 9.6 and 9.7 for details).

    If a partial search is used, the variables $1 and $2 contain the wild and
    non-wild parts of the key during the expansion of the replacement text.
    They return to their previous values at the end of the lookup item.

    This example looks up the postmaster alias in the conventional alias file:

    ${lookup {postmaster} lsearch {/etc/aliases} {$value}}

    This example uses NIS+ to look up the full name of the user corresponding
    to the local part of an address, forcing the expansion to fail if it is not
    found:

    ${lookup nisplus {[name=$local_part],passwd.org_dir:gcos} \
      {$value}fail}

${map{<string1>}{<string2>}}

    After expansion, <string1> is interpreted as a list, colon-separated by
    default, but the separator can be changed in the usual way (6.21). For each
    item in this list, its value is place in $item, and then <string2> is
    expanded and added to the output as an item in a new list. The separator
    used for the output list is the same as the one used for the input, but a
    separator setting is not included in the output. For example:

    ${map{a:b:c}{[$item]}} ${map{<- x-y-z}{($item)}}

    expands to "[a]:[b]:[c] (x)-(y)-(z)". At the end of the expansion, the
    value of $item is restored to what it was before. See also the filter and 
    reduce expansion items.

${nhash{<string1>}{<string2>}{<string3>}}

    The three strings are expanded; the first two must yield numbers. Call them
    <n> and <m>. If you are using fixed values for these numbers, that is, if <
    string1> and <string2> do not change when they are expanded, you can use
    the simpler operator notation that avoids some of the braces:

    ${nhash_<n>_<m>:<string>}

    The second number is optional (in both notations). If there is only one
    number, the result is a number in the range 0-<n>-1. Otherwise, the string
    is processed by a div/mod hash function that returns two numbers, separated
    by a slash, in the ranges 0 to <n>-1 and 0 to <m>-1, respectively. For
    example,

    ${nhash{8}{64}{supercalifragilisticexpialidocious}}

    returns the string "6/33".

${perl{<subroutine>}{<arg>}{<arg>}...}

    This item is available only if Exim has been built to include an embedded
    Perl interpreter. The subroutine name and the arguments are first
    separately expanded, and then the Perl subroutine is called with those
    arguments. No additional arguments need be given; the maximum number
    permitted, including the name of the subroutine, is nine.

    The return value of the subroutine is inserted into the expanded string,
    unless the return value is undef. In that case, the entire expansion is
    forced to fail, in the same way as an explicit "fail" on a lookup item does
    (see section 11.4). Whatever you return is evaluated in a scalar context,
    thus the return value is a scalar. For example, if you return a Perl
    vector, the return value is the size of the vector, not its contents.

    If the subroutine exits by calling Perl's die function, the expansion fails
    with the error message that was passed to die. More details of the embedded
    Perl facility are given in chapter 12.

    The redirect router has an option called forbid_filter_perl which locks out
    the use of this expansion item in filter files.

${prvs{<address>}{<secret>}{<keynumber>}}

    The first argument is a complete email address and the second is secret
    keystring. The third argument, specifying a key number, is optional. If
    absent, it defaults to 0. The result of the expansion is a prvs-signed
    email address, to be typically used with the return_path option on an smtp
    transport as part of a bounce address tag validation (BATV) scheme. For
    more discussion and an example, see section 44.27.

${prvscheck{<address>}{<secret>}{<string>}}

    This expansion item is the complement of the prvs item. It is used for
    checking prvs-signed addresses. If the expansion of the first argument does
    not yield a syntactically valid prvs-signed address, the whole item expands
    to the empty string. When the first argument does expand to a syntactically
    valid prvs-signed address, the second argument is expanded, with the
    prvs-decoded version of the address and the key number extracted from the
    address in the variables $prvscheck_address and $prvscheck_keynum,
    respectively.

    These two variables can be used in the expansion of the second argument to
    retrieve the secret. The validity of the prvs-signed address is then
    checked against the secret. The result is stored in the variable
    $prvscheck_result, which is empty for failure or "1" for success.

    The third argument is optional; if it is missing, it defaults to an empty
    string. This argument is now expanded. If the result is an empty string,
    the result of the expansion is the decoded version of the address. This is
    the case whether or not the signature was valid. Otherwise, the result of
    the expansion is the expansion of the third argument.

    All three variables can be used in the expansion of the third argument.
    However, once the expansion is complete, only $prvscheck_result remains
    set. For more discussion and an example, see section 44.27.

${readfile{<file name>}{<eol string>}}

    The filename and end-of-line (eol) string are first expanded separately.
    The file is then read, and its contents replace the entire item. All
    newline characters in the file are replaced by the end-of-line string if it
    is present. Otherwise, newlines are left in the string. String expansion is
    not applied to the contents of the file. If you want this, you must wrap
    the item in an expand operator. If the file cannot be read, the string
    expansion fails.

    The redirect router has an option called forbid_filter_readfile which locks
    out the use of this expansion item in filter files.

${readsocket{<name>}{<request>}{<options>}{<eol string>}{<fail string>}}

    This item inserts data from a Unix domain or TCP socket into the expanded
    string. The minimal way of using it uses just two arguments, as in these
    examples:

    ${readsocket{/socket/name}{request string}}
    ${readsocket{inet:some.host:1234}{request string}}

    For a Unix domain socket, the first substring must be the path to the
    socket. For an Internet socket, the first substring must contain "inet:"
    followed by a host name or IP address, followed by a colon and a port,
    which can be a number or the name of a TCP port in /etc/services. An IP
    address may optionally be enclosed in square brackets. This is best for
    IPv6 addresses. For example:

    ${readsocket{inet:[::1]:1234}{request string}}

    Only a single host name may be given, but if looking it up yields more than
    one IP address, they are each tried in turn until a connection is made. For
    both kinds of socket, Exim makes a connection, writes the request string
    (unless it is an empty string; no terminating NUL is ever sent) and reads
    from the socket until an end-of-file is read. A timeout of 5 seconds is
    applied. Additional, optional arguments extend what can be done. Firstly,
    you can vary the timeout. For example:

    ${readsocket{/socket/name}{request string}{3s}}

    The third argument is a list of options, of which the first element is the
    timeout and must be present if any options are given. Further elements are
    options of form name=value. Example:

    ${readsocket{/socket/name}{request string}{3s:shutdown=no}}

    The following option names are recognised:

      o cache Defines if the result data can be cached for use by a later
        identical request in the same process. Values are "yes" or "no" (the
        default). If not, all cached results for this connection specification
        will be invalidated.

      o shutdown Defines whether or not a write-shutdown is done on the
        connection after sending the request. Values are "yes" (the default) or
        "no" (preferred, eg. by some webservers).

      o sni Controls the use of Server Name Identification on the connection.
        Any nonempty value will be the SNI sent; TLS will be forced.

      o tls Controls the use of TLS on the connection. Values are "yes" or "no"
        (the default). If it is enabled, a shutdown as described above is never
        done.

    A fourth argument allows you to change any newlines that are in the data
    that is read, in the same way as for readfile (see above). This example
    turns them into spaces:

    ${readsocket{inet:127.0.0.1:3294}{request string}{3s}{ }}

    As with all expansions, the substrings are expanded before the processing
    happens. Errors in these sub-expansions cause the expansion to fail. In
    addition, the following errors can occur:

      o Failure to create a socket file descriptor;

      o Failure to connect the socket;

      o Failure to write the request string;

      o Timeout on reading from the socket.

    By default, any of these errors causes the expansion to fail. However, if
    you supply a fifth substring, it is expanded and used when any of the above
    errors occurs. For example:

    ${readsocket{/socket/name}{request string}{3s}{\n}\
      {socket failure}}

    You can test for the existence of a Unix domain socket by wrapping this
    expansion in "${if exists", but there is a race condition between that test
    and the actual opening of the socket, so it is safer to use the fifth
    argument if you want to be absolutely sure of avoiding an expansion error
    for a non-existent Unix domain socket, or a failure to connect to an
    Internet socket.

    The redirect router has an option called forbid_filter_readsocket which
    locks out the use of this expansion item in filter files.

${reduce{<string1>}{<string2>}{<string3>}}

    This operation reduces a list to a single, scalar string. After expansion,
    <string1> is interpreted as a list, colon-separated by default, but the
    separator can be changed in the usual way (6.21). Then <string2> is
    expanded and assigned to the $value variable. After this, each item in the
    <string1> list is assigned to $item, in turn, and <string3> is expanded for
    each of them. The result of that expansion is assigned to $value before the
    next iteration. When the end of the list is reached, the final value of
    $value is added to the expansion output. The reduce expansion item can be
    used in a number of ways. For example, to add up a list of numbers:

    ${reduce {<, 1,2,3}{0}{${eval:$value+$item}}}

    The result of that expansion would be "6". The maximum of a list of numbers
    can be found:

    ${reduce {3:0:9:4:6}{0}{${if >{$item}{$value}{$item}{$value}}}}

    At the end of a reduce expansion, the values of $item and $value are
    restored to what they were before. See also the filter and map expansion
    items.

$rheader_<header name>: or $rh_<header name>:

    This item inserts "raw" header lines. It is described with the header
    expansion item in section 11.5 above.

${run<options> {<command string>}{<string1>}{<string2>}}

    This item runs an external command, as a subprocess. One option is
    supported after the word run, comma-separated and without whitespace.

    If the option preexpand is not used, the command string before expansion is
    split into individual arguments by spaces and then each argument is
    separately expanded. Then the command is run in a separate process, but
    under the same uid and gid. As in other command executions from Exim, a
    shell is not used by default. If the command requires a shell, you must
    explicitly code it. The command name may not be tainted, but the remaining
    arguments can be.

    Note: if tainted arguments are used, they are supplied by a potential
    attacker; a careful assessment for security vulnerabilities should be done.

    If the option preexpand is used, the command string is first expanded as a
    whole. The expansion result is split apart into individual arguments by
    spaces, and then the command is run as above. Since the arguments are split
    by spaces, when there is a variable expansion which has an empty result, it
    will cause the situation that the argument will simply be omitted when the
    program is actually executed by Exim. If the script/program requires a
    specific number of arguments and the expanded variable could possibly
    result in this empty expansion, the variable must be quoted. This is more
    difficult if the expanded variable itself could result in a string
    containing quotes, because it would interfere with the quotes around the
    command arguments. A possible guard against this is to wrap the variable in
    the sg operator to change any quote marks to some other character. Neither
    the command nor any argument may be tainted.

    The standard input for the command exists, but is empty. The standard
    output and standard error are set to the same file descriptor. If the
    command succeeds (gives a zero return code) <string1> is expanded and
    replaces the entire item; during this expansion, the standard output/error
    from the command is in the variable $value. If the command fails, <string2
    >, if present, is expanded and used. Once again, during the expansion, the
    standard output/error from the command is in the variable $value.

    If <string2> is absent, the result is empty. Alternatively, <string2> can
    be the word "fail" (not in braces) to force expansion failure if the
    command does not succeed. If both strings are omitted, the result is
    contents of the standard output/error on success, and nothing on failure.

    The standard output/error of the command is put in the variable $value. In
    this ACL example, the output of a command is logged for the admin to
    troubleshoot:

    warn  condition    = ${run{/usr/bin/id}{yes}{no}}
          log_message  = Output of id: $value

    If the command requires shell idioms, such as the > redirect operator, the
    shell must be invoked directly, such as with:

    ${run{/bin/bash -c "/usr/bin/id >/tmp/id"}{yes}{yes}}

    Note that $value will not persist beyond the reception of a single message.

    The return code from the command is put in the variable $runrc, and this
    remains set afterwards, so in a filter file you can do things like this:

    if "${run{x y z}{}}$runrc" is 1 then ...
      elif $runrc is 2 then ...
      ...
    endif

    If execution of the command fails (for example, the command does not
    exist), the return code is 127 - the same code that shells use for
    non-existent commands.

    Warning: In a router or transport, you cannot assume the order in which
    option values are expanded, except for those preconditions whose order of
    testing is documented. Therefore, you cannot reliably expect to set $runrc
    by the expansion of one option, and use it in another.

    The redirect router has an option called forbid_filter_run which locks out
    the use of this expansion item in filter files.

${sg{<subject>}{<regex>}{<replacement>}}

    This item works like Perl's substitution operator (s) with the global (/g)
    option; hence its name. However, unlike the Perl equivalent, Exim does not
    modify the subject string; instead it returns the modified string for
    insertion into the overall expansion. The item takes three arguments: the
    subject string, a regular expression, and a substitution string. For
    example:

    ${sg{abcdefabcdef}{abc}{xyz}}

    yields "xyzdefxyzdef". Because all three arguments are expanded before use,
    if any $, } or \ characters are required in the regular expression or in
    the substitution string, they have to be escaped. For example:

    ${sg{abcdef}{^(...)(...)\$}{\$2\$1}}

    yields "defabc", and

    ${sg{1=A 4=D 3=C}{\N(\d+)=\N}{K\$1=}}

    yields "K1=A K4=D K3=C". Note the use of "\N" to protect the contents of
    the regular expression from string expansion.

    The regular expression is compiled in 8-bit mode, working against bytes
    rather than any Unicode-aware character handling.

${sort{<string>}{<comparator>}{<extractor>}}

    After expansion, <string> is interpreted as a list, colon-separated by
    default, but the separator can be changed in the usual way (6.21). The <
    comparator> argument is interpreted as the operator of a two-argument
    expansion condition. The numeric operators plus ge, gt, le, lt (and ~i
    variants) are supported. The comparison should return true when applied to
    two values if the first value should sort before the second value. The <
    extractor> expansion is applied repeatedly to elements of the list, the
    element being placed in $item, to give values for comparison.

    The item result is a sorted list, with the original list separator, of the
    list elements (in full) of the original.

    Examples:

    ${sort{3:2:1:4}{<}{$item}}

    sorts a list of numbers, and

    ${sort {${lookup dnsdb{>:,,mx=example.com}}} {<} {${listextract{1}{<,$item}}}}

    will sort an MX lookup into priority order.

${srs_encode {<secret>}{<return path>}{<original domain>}}

    SRS encoding. See SECT 58.2.1 for details.

${substr{<start>}{<len>}{<subject>}}

    The three strings are expanded; the first two must yield numbers. Call them
    <n> and <m>. If you are using fixed values for these numbers, that is, if <
    start> and <len> do not change when they are expanded, you can use the
    simpler operator notation that avoids some of the braces:

    ${substr_<n>_<m>:<subject>}

    The second number is optional (in both notations). If it is absent in the
    simpler format, the preceding underscore must also be omitted.

    The substr item can be used to extract more general substrings than length.
    The first number, <n>, is a starting offset, and <m> is the length
    required. For example

    ${substr{3}{2}{$local_part}}

    If the starting offset is greater than the string length the result is the
    null string; if the length plus starting offset is greater than the string
    length, the result is the right-hand part of the string, starting from the
    given offset. The first byte (character) in the string has offset zero.

    The substr expansion item can take negative offset values to count from the
    right-hand end of its operand. The last byte (character) is offset -1, the
    second-last is offset -2, and so on. Thus, for example,

    ${substr{-5}{2}{1234567}}

    yields "34". If the absolute value of a negative offset is greater than the
    length of the string, the substring starts at the beginning of the string,
    and the length is reduced by the amount of overshoot. Thus, for example,

    ${substr{-5}{2}{12}}

    yields an empty string, but

    ${substr{-3}{2}{12}}

    yields "1".

    When the second number is omitted from substr, the remainder of the string
    is taken if the offset is positive. If it is negative, all bytes
    (characters) in the string preceding the offset point are taken. For
    example, an offset of -1 and no length, as in these semantically identical
    examples:

    ${substr_-1:abcde}
    ${substr{-1}{abcde}}

    yields all but the last character of the string, that is, "abcd".

    All measurement is done in bytes and is not UTF-8 aware.

${tr{<subject>}{<characters>}{<replacements>}}

    This item does single-character (in bytes) translation on its subject
    string. The second argument is a list of characters to be translated in the
    subject string. Each matching character is replaced by the corresponding
    character from the replacement list. For example

    ${tr{abcdea}{ac}{13}}

    yields "1b3de1". If there are duplicates in the second character string,
    the last occurrence is used. If the third string is shorter than the
    second, its last character is replicated. However, if it is empty, no
    translation takes place.

    All character handling is done in bytes and is not UTF-8 aware.


11.6 Expansion operators
------------------------

For expansion items that perform transformations on a single argument string,
the "operator" notation is used because it is simpler and uses fewer braces.
The substring is first expanded before the operation is applied to it. The
following operations can be performed:

${address:<string>}

    The string is interpreted as an RFC 2822 address, as it might appear in a
    header line, and the effective address is extracted from it. If the string
    does not parse successfully, the result is empty.

    The parsing correctly handles SMTPUTF8 Unicode in the string.

${addresses:<string>}

    The string (after expansion) is interpreted as a list of addresses in RFC
    2822 format, such as can be found in a To: or Cc: header line. The
    operative address (local-part@domain) is extracted from each item, and the
    result of the expansion is a colon-separated list, with appropriate
    doubling of colons should any happen to be present in the email addresses.
    Syntactically invalid RFC2822 address items are omitted from the output.

    It is possible to specify a character other than colon for the output
    separator by starting the string with > followed by the new separator
    character. For example:

    ${addresses:>& Chief <ceo@up.stairs>, sec@base.ment (dogsbody)}

    expands to "ceo@up.stairs&sec@base.ment". The string is expanded first, so
    if the expanded string starts with >, it may change the output separator
    unintentionally. This can be avoided by setting the output separator
    explicitly:

    ${addresses:>:$h_from:}

    Compare the address (singular) expansion item, which extracts the working
    address from a single RFC2822 address. See the filter, map, and reduce
    items for ways of processing lists.

    To clarify "list of addresses in RFC 2822 format" mentioned above, Exim
    follows a strict interpretation of header line formatting. Exim parses the
    bare, unquoted portion of an email address and if it finds a comma, treats
    it as an email address separator. For the example header line:

    From: =?iso-8859-2?Q?Last=2C_First?= <user@example.com>

    The first example below demonstrates that Q-encoded email addresses are
    parsed properly if it is given the raw header (in this example,
    "$rheader_from:"). It does not see the comma because it's still encoded as
    "=2C". The second example below is passed the contents of "$header_from:",
    meaning it gets de-mimed. Exim sees the decoded "," so it treats it as two
    email addresses. The third example shows that the presence of a comma is
    skipped when it is quoted. The fourth example shows SMTPUTF8 handling.

    # exim -be '${addresses:From: \
    =?iso-8859-2?Q?Last=2C_First?= <user@example.com>}'
    user@example.com
    # exim -be '${addresses:From: Last, First <user@example.com>}'
    Last:user@example.com
    # exim -be '${addresses:From: "Last, First" <user@example.com>}'
    user@example.com
    # exim -be '${addresses:?????? <??????????@example.jp>}'
    ??????????@example.jp

${base32:<digits>}

    The string must consist entirely of decimal digits. The number is converted
    to base 32 and output as a (empty, for zero) string of characters. Only
    lowercase letters are used.

${base32d:<base-32 digits>}

    The string must consist entirely of base-32 digits. The number is converted
    to decimal and output as a string.

${base62:<digits>}

    The string must consist entirely of decimal digits. The number is converted
    to base 62 and output as a string of six characters, including leading
    zeros. In the few operating environments where Exim uses base 36 instead of
    base 62 for its message identifiers (because those systems do not have
    case-sensitive filenames), base 36 is used by this operator, despite its
    name. Note: Just to be absolutely clear: this is not base64 encoding.

${base62d:<base-62 digits>}

    The string must consist entirely of base-62 digits, or, in operating
    environments where Exim uses base 36 instead of base 62 for its message
    identifiers, base-36 digits. The number is converted to decimal and output
    as a string.

${base64:<string>}

    This operator converts a string into one that is base64 encoded.

    If the string is a single variable of type certificate, returns the base64
    encoding of the DER form of the certificate.

${base64d:<string>}

    This operator converts a base64-encoded string into the un-coded form.

${domain:<string>}

    The string is interpreted as an RFC 2822 address and the domain is
    extracted from it. If the string does not parse successfully, the result is
    empty.

${escape:<string>}

    If the string contains any non-printing characters, they are converted to
    escape sequences starting with a backslash. Whether characters with the
    most significant bit set (so-called "8-bit characters") count as printing
    or not is controlled by the print_topbitchars option.

${escape8bit:<string>}

    If the string contains any characters with the most significant bit set,
    they are converted to escape sequences starting with a backslash.
    Backslashes and DEL characters are also converted.

${eval:<string>} and ${eval10:<string>}

    These items supports simple arithmetic and bitwise logical operations in
    expansion strings. The string (after expansion) must be a conventional
    arithmetic expression, but it is limited to basic arithmetic operators,
    bitwise logical operators, and parentheses. All operations are carried out
    using integer arithmetic. The operator priorities are as follows (the same
    as in the C programming language):

        highest: not (~), negate (-)
                 multiply (*), divide (/), remainder (%)
                 plus (+), minus (-)
                 shift-left (<<), shift-right (>>)
                 and (&)
                 xor (^)
        lowest:  or (|)

    Binary operators with the same priority are evaluated from left to right.
    White space is permitted before or after operators.

    For eval, numbers may be decimal, octal (starting with "0") or hexadecimal
    (starting with "0x"). For eval10, all numbers are taken as decimal, even if
    they start with a leading zero; hexadecimal numbers are not permitted. This
    can be useful when processing numbers extracted from dates or times, which
    often do have leading zeros.

    A number may be followed by "K", "M" or "G" to multiply it by 1024,
    1024*1024 or 1024*1024*1024, respectively. Negative numbers are supported.
    The result of the computation is a decimal representation of the answer
    (without "K", "M" or "G"). For example:

    ${eval:1+1}              yields 2
    ${eval:1+2*3}            yields 7
    ${eval:(1+2)*3}          yields 9
    ${eval:2+42%5}           yields 4
    ${eval:0xc&5}            yields 4
    ${eval:0xc|5}            yields 13
    ${eval:0xc^5}            yields 9
    ${eval:0xc>>1}           yields 6
    ${eval:0xc<<1}           yields 24
    ${eval:~255&0x1234}      yields 4608
    ${eval:-(~255&0x1234)}   yields -4608

    As a more realistic example, in an ACL you might have

    deny   condition =                    \
             ${if and {                   \
               {>{$rcpt_count}{10}}       \
               {                          \
               <                          \
                 {$recipients_count}      \
                 {${eval:$rcpt_count/2}}  \
               }                          \
             }{yes}{no}}
           message = Too many bad recipients

    The condition is true if there have been more than 10 RCPT commands and
    fewer than half of them have resulted in a valid recipient.

${expand:<string>}

    The expand operator causes a string to be expanded for a second time. For
    example,

    ${expand:${lookup{$domain}dbm{/some/file}{$value}}}

    first looks up a string in a file while expanding the operand for expand,
    and then re-expands what it has found.

${from_utf8:<string>}

    The world is slowly moving towards Unicode, although there are no standards
    for email yet. However, other applications (including some databases) are
    starting to store data in Unicode, using UTF-8 encoding. This operator
    converts from a UTF-8 string to an ISO-8859-1 string. UTF-8 code values
    greater than 255 are converted to underscores. The input must be a valid
    UTF-8 string. If it is not, the result is an undefined sequence of bytes.

    Unicode code points with values less than 256 are compatible with ASCII and
    ISO-8859-1 (also known as Latin-1). For example, character 169 is the
    copyright symbol in both cases, though the way it is encoded is different.
    In UTF-8, more than one byte is needed for characters with code values
    greater than 127, whereas ISO-8859-1 is a single-byte encoding (but thereby
    limited to 256 characters). This makes translation from UTF-8 to ISO-8859-1
    straightforward.

${hash_<n>_<m>:<string>}

    The hash operator is a simpler interface to the hashing function that can
    be used when the two parameters are fixed numbers (as opposed to strings
    that change when expanded). The effect is the same as

    ${hash{<n>}{<m>}{<string>}}

    See the description of the general hash item above for details. The
    abbreviation h can be used when hash is used as an operator.

${headerwrap_<cols>_<limit>:<string>}

    This operator line-wraps its argument in a way useful for headers. The cols
    value gives the column number to wrap after, the limit gives a limit number
    of result characters to truncate at. Either just the limit and the
    preceding underbar, or both, can be omitted; the defaults are 80 and 998.
    Wrapping will be inserted at a space if possible before the column number
    is reached. Whitespace at a chosen wrap point is removed. A line-wrap
    consists of a newline followed by a tab, and the tab is counted as 8
    columns.

${hex2b64:<hexstring>}

    This operator converts a hex string into one that is base64 encoded. This
    can be useful for processing the output of the various hashing functions.

${hexquote:<string>}

    This operator converts non-printable characters in a string into a hex
    escape form. Byte values between 33 (!) and 126 (~) inclusive are left as
    is, and other byte values are converted to "\xNN", for example, a byte
    value 127 is converted to "\x7f".

${ipv6denorm:<string>}

    This expands an IPv6 address to a full eight-element colon-separated set of
    hex digits including leading zeroes. A trailing ipv4-style dotted-decimal
    set is converted to hex. Pure IPv4 addresses are converted to IPv4-mapped
    IPv6.

${ipv6norm:<string>}

    This converts an IPv6 address to canonical form. Leading zeroes of groups
    are omitted, and the longest set of zero-valued groups is replaced with a
    double colon. A trailing ipv4-style dotted-decimal set is converted to hex.
    Pure IPv4 addresses are converted to IPv4-mapped IPv6.

${lc:<string>}

    This forces the letters in the string into lower-case, for example:

    ${lc:$local_part}

    Case is defined per the system C locale.

${length_<number>:<string>}

    The length operator is a simpler interface to the length function that can
    be used when the parameter is a fixed number (as opposed to a string that
    changes when expanded). The effect is the same as

    ${length{<number>}{<string>}}

    See the description of the general length item above for details. Note that
    length is not the same as strlen. The abbreviation l can be used when 
    length is used as an operator. All measurement is done in bytes and is not
    UTF-8 aware.

${listcount:<string>}

    The string is interpreted as a list and the number of items is returned.

${listnamed:<name>} and ${listnamed_<type>:<name>}

    The name is interpreted as a named list and the content of the list is
    returned, expanding any referenced lists, re-quoting as needed for
    colon-separation. If the optional type is given it must be one of "a", "d",
    "h" or "l" and selects address-, domain-, host- or localpart- lists to
    search among respectively. Otherwise all types are searched in an undefined
    order and the first matching list is returned. Note: Neither
    string-expansion of lists referenced by named-list syntax elements, nor
    expansion of lookup elements, is done by the listnamed operator.

${local_part:<string>}

    The string is interpreted as an RFC 2822 address and the local part is
    extracted from it. If the string does not parse successfully, the result is
    empty. The parsing correctly handles SMTPUTF8 Unicode in the string.

${mask:<IP address>/<bit count>}, ${mask_n:<IP address>/<bit count>}

    If the form of the string to be operated on is not an IP address followed
    by a slash and an integer (that is, a network address in CIDR notation),
    the expansion fails. Otherwise, this operator converts the IP address to
    binary, masks off the least significant bits according to the bit count,
    and converts the result back to text, with mask appended. For example,

    ${mask:10.111.131.206/28}

    returns the string "10.111.131.192/28".

    Since this operation is expected to be mostly used for looking up masked
    addresses in files, the normal result for an IPv6 address uses dots to
    separate components instead of colons, because colon terminates a key
    string in lsearch files. So, for example,

    ${mask:3ffe:ffff:836f:0a00:000a:0800:200a:c031/99}

    returns the string

    3ffe.ffff.836f.0a00.000a.0800.2000.0000/99

    If the optional form mask_n is used, IPv6 address result are instead
    returned in normailsed form, using colons and with zero-compression.
    Letters in IPv6 addresses are always output in lower case.

${md5:<string>}

    The md5 operator computes the MD5 hash value of the string, and returns it
    as a 32-digit hexadecimal number, in which any letters are in lower case.

    If the string is a single variable of type certificate, returns the MD5
    hash fingerprint of the certificate.

${nhash_<n>_<m>:<string>}

    The nhash operator is a simpler interface to the numeric hashing function
    that can be used when the two parameters are fixed numbers (as opposed to
    strings that change when expanded). The effect is the same as

    ${nhash{<n>}{<m>}{<string>}}

    See the description of the general nhash item above for details.

${quote:<string>}

    The quote operator puts its argument into double quotes if it is an empty
    string or contains anything other than letters, digits, underscores, dots,
    and hyphens. Any occurrences of double quotes and backslashes are escaped
    with a backslash. Newlines and carriage returns are converted to "\n" and "
    \r", respectively For example,

    ${quote:ab"*"cd}

    becomes

    "ab\"*\"cd"

    The place where this is useful is when the argument is a substitution from
    a variable or a message header.

${quote_local_part:<string>}

    This operator is like quote, except that it quotes the string only if
    required to do so by the rules of RFC 2822 for quoting local parts. For
    example, a plus sign would not cause quoting (but it would for quote). If
    you are creating a new email address from the contents of $local_part (or
    any other unknown data), you should always use this operator.

    This quoting determination is not SMTPUTF8-aware, thus quoting non-ASCII
    data will likely use the quoting form. Thus ${quote_local_part:??????} will
    always become "??????".

${quote_<lookup-type>:<string>}

    This operator applies lookup-specific quoting rules to the string. Each
    query-style lookup type has its own quoting rules which are described with
    the lookups in chapter 9. For example,

    ${quote_ldap:two * two}

    returns

    two%20%5C2A%20two

    For single-key lookup types, no quoting is ever necessary and this operator
    yields an unchanged string.

${randint:<n>}

    This operator returns a somewhat random number which is less than the
    supplied number and is at least 0. The quality of this randomness depends
    on how Exim was built; the values are not suitable for keying material. If
    Exim is linked against OpenSSL then RAND_pseudo_bytes() is used. If Exim is
    linked against GnuTLS then gnutls_rnd(GNUTLS_RND_NONCE) is used, for
    versions of GnuTLS with that function. Otherwise, the implementation may be
    arc4random(), random() seeded by srandomdev() or srandom(), or a custom
    implementation even weaker than random().

${reverse_ip:<ipaddr>}

    This operator reverses an IP address; for IPv4 addresses, the result is in
    dotted-quad decimal form, while for IPv6 addresses the result is in
    dotted-nibble hexadecimal form. In both cases, this is the "natural" form
    for DNS. For example,

    ${reverse_ip:192.0.2.4}
    ${reverse_ip:2001:0db8:c42:9:1:abcd:192.0.2.127}

    returns

    4.2.0.192
    f.7.2.0.0.0.0.c.d.c.b.a.1.0.0.0.9.0.0.0.2.4.c.0.8.b.d.0.1.0.0.2

${rfc2047:<string>}

    This operator encodes text according to the rules of RFC 2047. This is an
    encoding that is used in header lines to encode non-ASCII characters. It is
    assumed that the input string is in the encoding specified by the 
    headers_charset option, which gets its default at build time. If the string
    contains only characters in the range 33-126, and no instances of the
    characters

    ? = ( ) < > @ , ; : \ " . [ ] _

    it is not modified. Otherwise, the result is the RFC 2047 encoding of the
    string, using as many "encoded words" as necessary to encode all the
    characters.

${rfc2047d:<string>}

    This operator decodes strings that are encoded as per RFC 2047. Binary zero
    bytes are replaced by question marks. Characters are converted into the
    character set defined by headers_charset. Overlong RFC 2047 "words" are not
    recognized unless check_rfc2047_length is set false.

    Note: If you use $header_xxx: (or $h_xxx:) to access a header line, RFC
    2047 decoding is done automatically. You do not need to use this operator
    as well.

${rxquote:<string>}

    The rxquote operator inserts a backslash before any non-alphanumeric
    characters in its argument. This is useful when substituting the values of
    variables or headers inside regular expressions.

${sha1:<string>}

    The sha1 operator computes the SHA-1 hash value of the string, and returns
    it as a 40-digit hexadecimal number, in which any letters are in upper
    case.

    If the string is a single variable of type certificate, returns the SHA-1
    hash fingerprint of the certificate.

${sha256:<string>}, ${sha2:<string>}, ${sha2_<n>:<string>}

    The sha256 operator computes the SHA-256 hash value of the string and
    returns it as a 64-digit hexadecimal number, in which any letters are in
    upper case.

    If the string is a single variable of type certificate, returns the SHA-256
    hash fingerprint of the certificate.

    The operator can also be spelled sha2 and does the same as sha256 (except
    for certificates, which are not supported). Finally, if an underbar and a
    number is appended it specifies the output length, selecting a member of
    the SHA-2 family of hash functions. Values of 256, 384 and 512 are
    accepted, with 256 being the default.

${sha3:<string>}, ${sha3_<n>:<string>}

    The sha3 operator computes the SHA3-256 hash value of the string and
    returns it as a 64-digit hexadecimal number, in which any letters are in
    upper case.

    If a number is appended, separated by an underbar, it specifies the output
    length. Values of 224, 256, 384 and 512 are accepted; with 256 being the
    default.

    The sha3 expansion item is only supported if Exim has been compiled with
    GnuTLS 3.5.0 or later, or OpenSSL 1.1.1 or later. The macro
    "_CRYPTO_HASH_SHA3" will be defined if it is supported.

${stat:<string>}

    The string, after expansion, must be a file path. A call to the stat()
    function is made for this path. If stat() fails, an error occurs and the
    expansion fails. If it succeeds, the data from the stat replaces the item,
    as a series of <name>=<value> pairs, where the values are all numerical,
    except for the value of "smode". The names are: "mode" (giving the mode as
    a 4-digit octal number), "smode" (giving the mode in symbolic format as a
    10-character string, as for the ls command), "inode", "device", "links",
    "uid", "gid", "size", "atime", "mtime", and "ctime". You can extract
    individual fields using the extract expansion item.

    The use of the stat expansion in users' filter files can be locked out by
    the system administrator. Warning: The file size may be incorrect on 32-bit
    systems for files larger than 2GB.

${str2b64:<string>}

    Now deprecated, a synonym for the base64 expansion operator.

${strlen:<string>}

    The item is replaced by the length of the expanded string, expressed as a
    decimal number. Note: Do not confuse strlen with length. All measurement is
    done in bytes and is not UTF-8 aware.

${substr_<start>_<length>:<string>}

    The substr operator is a simpler interface to the substr function that can
    be used when the two parameters are fixed numbers (as opposed to strings
    that change when expanded). The effect is the same as

    ${substr{<start>}{<length>}{<string>}}

    See the description of the general substr item above for details. The
    abbreviation s can be used when substr is used as an operator. All
    measurement is done in bytes and is not UTF-8 aware.

${time_eval:<string>}

    This item converts an Exim time interval such as "2d4h5m" into a number of
    seconds.

${time_interval:<string>}

    The argument (after sub-expansion) must be a sequence of decimal digits
    that represents an interval of time as a number of seconds. It is converted
    into a number of larger units and output in Exim's normal time format, for
    example, "1w3d4h2m6s".

${uc:<string>}

    This forces the letters in the string into upper-case. Case is defined per
    the system C locale.

${utf8clean:<string>}

    This replaces any invalid utf-8 sequence in the string by the character "?
    ". In versions of Exim before 4.92, this did not correctly do so for a
    truncated final codepoint's encoding, and the character would be silently
    dropped. If you must handle detection of this scenario across both sets of
    Exim behavior, the complexity will depend upon the task. For instance, to
    detect if the first character is multibyte and a 1-byte extraction can be
    successfully used as a path component (as is common for dividing up
    delivery folders), you might use:

    condition = ${if inlist{${utf8clean:${length_1:$local_part}}}{:?}{yes}{no}}

    (which will false-positive if the first character of the local part is a
    literal question mark).

${utf8_domain_to_alabel:<string>}, ${utf8_domain_from_alabel:<string>}, $
{utf8_localpart_to_alabel:<string>}, ${utf8_localpart_from_alabel:<string>}

    These convert EAI mail name components between UTF-8 and a-label forms. For
    information on internationalisation support see 60.1.


11.7 Expansion conditions
-------------------------

The following conditions are available for testing by the ${if construct while
expanding strings:

!<condition>

    Preceding any condition with an exclamation mark negates the result of the
    condition.

<symbolic operator> {<string1>}{<string2>}

    There are a number of symbolic operators for doing numeric comparisons.
    They are:

        =  equal
        == equal
        >  greater
        >= greater or equal
        <  less
        <= less or equal

    For example:

    ${if >{$message_size}{10M} ...

    Note that the general negation operator provides for inequality testing.
    The two strings must take the form of optionally signed decimal integers,
    optionally followed by one of the letters "K", "M" or "G" (in either upper
    or lower case), signifying multiplication by 1024, 1024*1024 or
    1024*1024*1024, respectively. As a special case, the numerical value of an
    empty string is taken as zero.

    In all cases, a relative comparator OP is testing if <string1> OP <string2
    >; the above example is checking if $message_size is larger than 10M, not
    if 10M is larger than $message_size.

acl {{<name>}{<arg1>}{<arg2>}...}

    The name and zero to nine argument strings are first expanded separately.
    The expanded arguments are assigned to the variables $acl_arg1 to $acl_arg9
    in order. Any unused are made empty. The variable $acl_narg is set to the
    number of arguments. The named ACL (see chapter 44) is called and may use
    the variables; if another acl expansion is used the values are restored
    after it returns. If the ACL sets a value using a "message =" modifier the
    variable $value becomes the result of the expansion, otherwise it is empty.
    If the ACL returns accept the condition is true; if deny, false. If the ACL
    returns defer the result is a forced-fail.

bool {<string>}

    This condition turns a string holding a true or false representation into a
    boolean state. It parses "true", "false", "yes" and "no"
    (case-insensitively); also integer numbers map to true if non-zero, false
    if zero. An empty string is treated as false. Leading and trailing
    whitespace is ignored; thus a string consisting only of whitespace is
    false. All other string values will result in expansion failure.

    When combined with ACL variables, this expansion condition will let you
    make decisions in one place and act on those decisions in another place.
    For example:

    ${if bool{$acl_m_privileged_sender} ...

bool_lax {<string>}

    Like bool, this condition turns a string into a boolean state. But where 
    bool accepts a strict set of strings, bool_lax uses the same loose
    definition that the Router condition option uses. The empty string and the
    values "false", "no" and "0" map to false, all others map to true. Leading
    and trailing whitespace is ignored.

    Note that where "bool{00}" is false, "bool_lax{00}" is true.

crypteq {<string1>}{<string2>}

    This condition is included in the Exim binary if it is built to support any
    authentication mechanisms (see chapter 33). Otherwise, it is necessary to
    define SUPPORT_CRYPTEQ in Local/Makefile to get crypteq included in the
    binary.

    The crypteq condition has two arguments. The first is encrypted and
    compared against the second, which is already encrypted. The second string
    may be in the LDAP form for storing encrypted strings, which starts with
    the encryption type in curly brackets, followed by the data. If the second
    string does not begin with "{" it is assumed to be encrypted with crypt()
    or crypt16() (see below), since such strings cannot begin with "{".
    Typically this will be a field from a password file. An example of an
    encrypted string in LDAP form is:

    {md5}CY9rzUYh03PK3k6DJie09g==

    If such a string appears directly in an expansion, the curly brackets have
    to be quoted, because they are part of the expansion syntax. For example:

    ${if crypteq {test}{\{md5\}CY9rzUYh03PK3k6DJie09g==}{yes}{no}}

    The following encryption types (whose names are matched case-independently)
    are supported:

      o {md5} computes the MD5 digest of the first string, and expresses this
        as printable characters to compare with the remainder of the second
        string. If the length of the comparison string is 24, Exim assumes that
        it is base64 encoded (as in the above example). If the length is 32,
        Exim assumes that it is a hexadecimal encoding of the MD5 digest. If
        the length not 24 or 32, the comparison fails.

      o {sha1} computes the SHA-1 digest of the first string, and expresses
        this as printable characters to compare with the remainder of the
        second string. If the length of the comparison string is 28, Exim
        assumes that it is base64 encoded. If the length is 40, Exim assumes
        that it is a hexadecimal encoding of the SHA-1 digest. If the length is
        not 28 or 40, the comparison fails.

      o {crypt} calls the crypt() function, which traditionally used to use
        only the first eight characters of the password. However, in modern
        operating systems this is no longer true, and in many cases the entire
        password is used, whatever its length.

      o {crypt16} calls the crypt16() function, which was originally created to
        use up to 16 characters of the password in some operating systems.
        Again, in modern operating systems, more characters may be used.

    Exim has its own version of crypt16(), which is just a double call to crypt
    (). For operating systems that have their own version, setting HAVE_CRYPT16
    in Local/Makefile when building Exim causes it to use the operating system
    version instead of its own. This option is set by default in the
    OS-dependent Makefile for those operating systems that are known to support
    crypt16().

    Some years after Exim's crypt16() was implemented, a user discovered that
    it was not using the same algorithm as some operating systems' versions. It
    turns out that as well as crypt16() there is a function called bigcrypt()
    in some operating systems. This may or may not use the same algorithm, and
    both of them may be different to Exim's built-in crypt16().

    However, since there is now a move away from the traditional crypt()
    functions towards using SHA1 and other algorithms, tidying up this area of
    Exim is seen as very low priority.

    If you do not put a encryption type (in curly brackets) in a crypteq
    comparison, the default is usually either "{crypt}" or "{crypt16}", as
    determined by the setting of DEFAULT_CRYPT in Local/Makefile. The default
    default is "{crypt}". Whatever the default, you can always use either
    function by specifying it explicitly in curly brackets.

def:<variable name>

    The def condition must be followed by the name of one of the expansion
    variables defined in section 11.9. The condition is true if the variable
    does not contain the empty string. For example:

    ${if def:sender_ident {from $sender_ident}}

    Note that the variable name is given without a leading $ character. If the
    variable does not exist, the expansion fails.

def:header_<header name>:  or  def:h_<header name>:

    This condition is true if a message is being processed and the named header
    exists in the message. For example,

    ${if def:header_reply-to:{$h_reply-to:}{$h_from:}}

    Note: No $ appears before header_ or h_ in the condition, and the header
    name must be terminated by a colon if white space does not follow.

eq {<string1>}{<string2>}, eqi {<string1>}{<string2>}

    The two substrings are first expanded. The condition is true if the two
    resulting strings are identical. For eq the comparison includes the case of
    letters, whereas for eqi the comparison is case-independent, where case is
    defined per the system C locale.

exists {<file name>}

    The substring is first expanded and then interpreted as an absolute path.
    The condition is true if the named file (or directory) exists. The
    existence test is done by calling the stat() function. The use of the 
    exists test in users' filter files may be locked out by the system
    administrator.

    Note: Testing a path using this condition is not a sufficient way of
    de-tainting it. Consider using a dsearch lookup.

first_delivery

    This condition, which has no data, is true during a message's first
    delivery attempt. It is false during any subsequent delivery attempts.

forall{<a list>}{<a condition>}, forany{<a list>}{<a condition>}

    These conditions iterate over a list. The first argument is expanded to
    form the list. By default, the list separator is a colon, but it can be
    changed by the normal method (6.21). The second argument is interpreted as
    a condition that is to be applied to each item in the list in turn. During
    the interpretation of the condition, the current list item is placed in a
    variable called $item.

      o For forany, interpretation stops if the condition is true for any item,
        and the result of the whole condition is true. If the condition is
        false for all items in the list, the overall condition is false.

      o For forall, interpretation stops if the condition is false for any
        item, and the result of the whole condition is false. If the condition
        is true for all items in the list, the overall condition is true.

    Note that negation of forany means that the condition must be false for all
    items for the overall condition to succeed, and negation of forall means
    that the condition must be false for at least one item.

    Example:

    ${if forany{$recipients_list}{match{$item}{^user3@}}{yes}{no}}

    The value of $item is saved and restored while forany or forall is being
    processed, to enable these expansion items to be nested.

    To scan a named list, expand it with the listnamed operator.

forall_json{<a JSON array>}{<a condition>}, forany_json{<a JSON array>}{<a
condition>}, forall_jsons{<a JSON array>}{<a condition>}, forany_jsons{<a JSON
array>}{<a condition>}

    As for the above, except that the first argument must, after expansion, be
    a JSON array. The array separator is not changeable. For the "jsons"
    variants the elements are expected to be JSON strings and have their quotes
    removed before the evaluation of the condition.

ge {<string1>}{<string2>}, gei {<string1>}{<string2>}

    The two substrings are first expanded. The condition is true if the first
    string is lexically greater than or equal to the second string. For ge the
    comparison includes the case of letters, whereas for gei the comparison is
    case-independent. Case and collation order are defined per the system C
    locale.

gt {<string1>}{<string2>}, gti {<string1>}{<string2>}

    The two substrings are first expanded. The condition is true if the first
    string is lexically greater than the second string. For gt the comparison
    includes the case of letters, whereas for gti the comparison is
    case-independent. Case and collation order are defined per the system C
    locale.

inbound_srs {<local part>}{<secret>}

    SRS decode. See SECT 58.2.1 for details.

inlist {<subject>}{<list>}, inlisti {<subject>}{<list>}

    Both strings are expanded; the second string is treated as a list of simple
    strings; if the first string is a member of the second, then the condition
    is true. For the case-independent inlisti condition, case is defined per
    the system C locale.

    These are simpler to use versions of the more powerful forany condition.
    Examples, and the forany equivalents:

    ${if inlist{needle}{foo:needle:bar}}
      ${if forany{foo:needle:bar}{eq{$item}{needle}}}
    ${if inlisti{Needle}{fOo:NeeDLE:bAr}}
      ${if forany{fOo:NeeDLE:bAr}{eqi{$item}{Needle}}}

    The variable $value will be set for a successful match and can be used in
    the success clause of an if expansion item using the condition. It will
    have the same taint status as the list; expansions such as

    ${if inlist {$h_mycode:} {0 : 1 : 42} {$value}}

    can be used for de-tainting. Any previous $value is restored after the if.

isip {<string>}, isip4 {<string>}, isip6 {<string>}

    The substring is first expanded, and then tested to see if it has the form
    of an IP address. Both IPv4 and IPv6 addresses are valid for isip, whereas 
    isip4 and isip6 test specifically for IPv4 or IPv6 addresses.

    For an IPv4 address, the test is for four dot-separated components, each of
    which consists of from one to three digits. For an IPv6 address, up to
    eight colon-separated components are permitted, each containing from one to
    four hexadecimal digits. There may be fewer than eight components if an
    empty component (adjacent colons) is present. Only one empty component is
    permitted.

    Note: The checks used to be just on the form of the address; actual
    numerical values were not considered. Thus, for example, 999.999.999.999
    passed the IPv4 check. This is no longer the case.

    The main use of these tests is to distinguish between IP addresses and host
    names, or between IPv4 and IPv6 addresses. For example, you could use

    ${if isip4{$sender_host_address}...

    to test which IP version an incoming SMTP connection is using.

ldapauth {<ldap query>}

    This condition supports user authentication using LDAP. See section 9.11
    for details of how to use LDAP in lookups and the syntax of queries. For
    this use, the query must contain a user name and password. The query itself
    is not used, and can be empty. The condition is true if the password is not
    empty, and the user name and password are accepted by the LDAP server. An
    empty password is rejected without calling LDAP because LDAP binds with an
    empty password are considered anonymous regardless of the username, and
    will succeed in most configurations. See chapter 33 for details of SMTP
    authentication, and chapter 34 for an example of how this can be used.

le {<string1>}{<string2>}, lei {<string1>}{<string2>}

    The two substrings are first expanded. The condition is true if the first
    string is lexically less than or equal to the second string. For le the
    comparison includes the case of letters, whereas for lei the comparison is
    case-independent. Case and collation order are defined per the system C
    locale.

lt {<string1>}{<string2>}, lti {<string1>}{<string2>}

    The two substrings are first expanded. The condition is true if the first
    string is lexically less than the second string. For lt the comparison
    includes the case of letters, whereas for lti the comparison is
    case-independent. Case and collation order are defined per the system C
    locale.

match {<string1>}{<string2>}

    The two substrings are first expanded. The second is then treated as a
    regular expression and applied to the first. Because of the pre-expansion,
    if the regular expression contains dollar, or backslash characters, they
    must be escaped. Care must also be taken if the regular expression contains
    braces (curly brackets). A closing brace must be escaped so that it is not
    taken as a premature termination of <string2>. The easiest approach is to
    use the "\N" feature to disable expansion of the regular expression. For
    example,

    ${if match {$local_part}{\N^\d{3}\N} ...

    If the whole expansion string is in double quotes, further escaping of
    backslashes is also required.

    The condition is true if the regular expression match succeeds. The regular
    expression is not required to begin with a circumflex metacharacter, but if
    there is no circumflex, the expression is not anchored, and it may match
    anywhere in the subject, not just at the start. If you want the pattern to
    match at the end of the subject, you must include the "$" metacharacter at
    an appropriate point. All character handling is done in bytes and is not
    UTF-8 aware, but we might change this in a future Exim release.

    At the start of an if expansion the values of the numeric variable
    substitutions $1 etc. are remembered. Obeying a match condition that
    succeeds causes them to be reset to the substrings of that condition and
    they will have these values during the expansion of the success string. At
    the end of the if expansion, the previous values are restored. After
    testing a combination of conditions using or, the subsequent values of the
    numeric variables are those of the condition that succeeded.

match_address {<string1>}{<string2>}

    See match_local_part.

match_domain {<string1>}{<string2>}

    See match_local_part.

match_ip {<string1>}{<string2>}

    This condition matches an IP address to a list of IP address patterns. It
    must be followed by two argument strings. The first (after expansion) must
    be an IP address or an empty string. The second (not expanded) is a
    restricted host list that can match only an IP address, not a host name.
    For example:

    ${if match_ip{$sender_host_address}{1.2.3.4:5.6.7.8}{...}{...}}

    The specific types of host list item that are permitted in the list are:

      o An IP address, optionally with a CIDR mask.

      o A single asterisk, which matches any IP address.

      o An empty item, which matches only if the IP address is empty. This
        could be useful for testing for a locally submitted message or one from
        specific hosts in a single test such as

          ${if match_ip{$sender_host_address}{:4.3.2.1:...}{...}{...}}

        where the first item in the list is the empty string.

      o The item @[] matches any of the local host's interface addresses.

      o Single-key lookups are assumed to be like "net-" style lookups in host
        lists, even if "net-" is not specified. There is never any attempt to
        turn the IP address into a host name. The most common type of linear
        search for match_ip is likely to be iplsearch, in which the file can
        contain CIDR masks. For example:

          ${if match_ip{$sender_host_address}{iplsearch;/some/file}...

        It is of course possible to use other kinds of lookup, and in such a
        case, you do need to specify the "net-" prefix if you want to specify a
        specific address mask, for example:

          ${if match_ip{$sender_host_address}{net24-dbm;/some/file}...

        However, unless you are combining a match_ip condition with others, it
        is just as easy to use the fact that a lookup is itself a condition,
        and write:

          ${lookup{${mask:$sender_host_address/24}}dbm{/a/file}...

    Note that <string2> is not itself subject to string expansion, unless Exim
    was built with the EXPAND_LISTMATCH_RHS option.

    Consult section 10.4.2 for further details of these patterns.

    The variable $value will be set for a successful match and can be used in
    the success clause of an if expansion item using the condition. Any
    previous $value is restored after the if.

match_local_part {<string1>}{<string2>}

    This condition, together with match_address and match_domain, make it
    possible to test domain, address, and local part lists within expansions.
    Each condition requires two arguments: an item and a list to match. A
    trivial example is:

    ${if match_domain{a.b.c}{x.y.z:a.b.c:p.q.r}{yes}{no}}

    In each case, the second argument may contain any of the allowable items
    for a list of the appropriate type. Also, because the second argument is a
    standard form of list, it is possible to refer to a named list. Thus, you
    can use conditions like this:

    ${if match_domain{$domain}{+local_domains}{...

    For address lists, the matching starts off caselessly, but the "+caseful"
    item can be used, as in all address lists, to cause subsequent items to
    have their local parts matched casefully. Domains are always matched
    caselessly.

    The variable $value will be set for a successful match and can be used in
    the success clause of an if expansion item using the condition. It will
    have the same taint status as the list; expansions such as

    ${if match_local_part {$local_part} {alice : bill : charlotte : dave} {$value}}

    can be used for de-tainting. Any previous $value is restored after the if.

    Note that <string2> is not itself subject to string expansion, unless Exim
    was built with the EXPAND_LISTMATCH_RHS option.

    Note: Host lists are not supported in this way. This is because hosts have
    two identities: a name and an IP address, and it is not clear how to
    specify cleanly how such a test would work. However, IP addresses can be
    matched using match_ip.

pam {<string1>:<string2>:...}

    Pluggable Authentication Modules (https://mirrors.edge.kernel.org/pub/linux
    /libs/pam/) are a facility that is available in Solaris and in some GNU/
    Linux distributions. The Exim support, which is intended for use in
    conjunction with the SMTP AUTH command, is available only if Exim is
    compiled with

    SUPPORT_PAM=yes

    in Local/Makefile. You probably need to add -lpam to EXTRALIBS, and in some
    releases of GNU/Linux -ldl is also needed.

    The argument string is first expanded, and the result must be a
    colon-separated list of strings. Leading and trailing white space is
    ignored. The PAM module is initialized with the service name "exim" and the
    user name taken from the first item in the colon-separated data string (<
    string1>). The remaining items in the data string are passed over in
    response to requests from the authentication function. In the simple case
    there will only be one request, for a password, so the data consists of
    just two strings.

    There can be problems if any of the strings are permitted to contain colon
    characters. In the usual way, these have to be doubled to avoid being taken
    as separators. The listquote expansion item can be used for this. For
    example, the configuration of a LOGIN authenticator might contain this
    setting:

    server_condition = ${if pam{$auth1:${listquote{:}{$auth2}}}}

    In some operating systems, PAM authentication can be done only from a
    process running as root. Since Exim is running as the Exim user when
    receiving messages, this means that PAM cannot be used directly in those
    systems.

pwcheck {<string1>:<string2>}

    This condition supports user authentication using the Cyrus pwcheck daemon.
    This is one way of making it possible for passwords to be checked by a
    process that is not running as root. Note: The use of pwcheck is now
    deprecated. Its replacement is saslauthd (see below).

    The pwcheck support is not included in Exim by default. You need to specify
    the location of the pwcheck daemon's socket in Local/Makefile before
    building Exim. For example:

    CYRUS_PWCHECK_SOCKET=/var/pwcheck/pwcheck

    You do not need to install the full Cyrus software suite in order to use
    the pwcheck daemon. You can compile and install just the daemon alone from
    the Cyrus SASL library. Ensure that exim is the only user that has access
    to the /var/pwcheck directory.

    The pwcheck condition takes one argument, which must be the user name and
    password, separated by a colon. For example, in a LOGIN authenticator
    configuration, you might have this:

    server_condition = ${if pwcheck{$auth1:$auth2}}

    Again, for a PLAIN authenticator configuration, this would be:

    server_condition = ${if pwcheck{$auth2:$auth3}}

queue_running

    This condition, which has no data, is true during delivery attempts that
    are initiated by queue runner processes, and false otherwise.

radius {<authentication string>}

    Radius authentication (RFC 2865) is supported in a similar way to PAM. You
    must set RADIUS_CONFIG_FILE in Local/Makefile to specify the location of
    the Radius client configuration file in order to build Exim with Radius
    support.

    With just that one setting, Exim expects to be linked with the radiusclient
    library, using the original API. If you are using release 0.4.0 or later of
    this library, you need to set

    RADIUS_LIB_TYPE=RADIUSCLIENTNEW

    in Local/Makefile when building Exim. You can also link Exim with the 
    libradius library that comes with FreeBSD. To do this, set

    RADIUS_LIB_TYPE=RADLIB

    in Local/Makefile, in addition to setting RADIUS_CONFIGURE_FILE. You may
    also have to supply a suitable setting in EXTRALIBS so that the Radius
    library can be found when Exim is linked.

    The string specified by RADIUS_CONFIG_FILE is expanded and passed to the
    Radius client library, which calls the Radius server. The condition is true
    if the authentication is successful. For example:

    server_condition = ${if radius{<arguments>}}

saslauthd {{<user>}{<password>}{<service>}{<realm>}}

    This condition supports user authentication using the Cyrus saslauthd
    daemon. This replaces the older pwcheck daemon, which is now deprecated.
    Using this daemon is one way of making it possible for passwords to be
    checked by a process that is not running as root.

    The saslauthd support is not included in Exim by default. You need to
    specify the location of the saslauthd daemon's socket in Local/Makefile
    before building Exim. For example:

    CYRUS_SASLAUTHD_SOCKET=/var/state/saslauthd/mux

    You do not need to install the full Cyrus software suite in order to use
    the saslauthd daemon. You can compile and install just the daemon alone
    from the Cyrus SASL library.

    Up to four arguments can be supplied to the saslauthd condition, but only
    two are mandatory. For example:

    server_condition = ${if saslauthd{{$auth1}{$auth2}}}

    The service and the realm are optional (which is why the arguments are
    enclosed in their own set of braces). For details of the meaning of the
    service and realm, and how to run the daemon, consult the Cyrus
    documentation.


11.8 Combining expansion conditions
-----------------------------------

Several conditions can be tested at once by combining them using the and and or
combination conditions. Note that and and or are complete conditions on their
own, and precede their lists of sub-conditions. Each sub-condition must be
enclosed in braces within the overall braces that contain the list. No
repetition of if is used.

or {{<cond1>}{<cond2>}...}

    The sub-conditions are evaluated from left to right. The condition is true
    if any one of the sub-conditions is true. For example,

    ${if or {{eq{$local_part}{spqr}}{eq{$domain}{testing.com}}}...

    When a true sub-condition is found, the following ones are parsed but not
    evaluated. If there are several "match" sub-conditions the values of the
    numeric variables afterwards are taken from the first one that succeeds.

and {{<cond1>}{<cond2>}...}

    The sub-conditions are evaluated from left to right. The condition is true
    if all of the sub-conditions are true. If there are several "match"
    sub-conditions, the values of the numeric variables afterwards are taken
    from the last one. When a false sub-condition is found, the following ones
    are parsed but not evaluated.


11.9 Expansion variables
------------------------

This section contains an alphabetical list of all the expansion variables. Some
of them are available only when Exim is compiled with specific options such as
support for TLS or the content scanning extension. Variables marked as tainted
are likely to carry data supplied by a potential attacker. Variables without
such marking may also, depending on how their values are created. Such
variables should not be further expanded, used as filenames or used as
command-line arguments for external commands.

$0, $1, etc

    When a match expansion condition succeeds, these variables contain the
    captured substrings identified by the regular expression during subsequent
    processing of the success string of the containing if expansion item. In
    the expansion condition case they do not retain their values afterwards; in
    fact, their previous values are restored at the end of processing an if
    item. The numerical variables may also be set externally by some other
    matching process which precedes the expansion of the string. For example,
    the commands available in Exim filter files include an if command with its
    own regular expression matching condition. If the subject string was
    tainted then any captured substring will also be.

$acl_arg1, $acl_arg2, etc

    Within an acl condition, expansion condition or expansion item any
    arguments are copied to these variables, any unused variables being made
    empty.

$acl_c...

    Values can be placed in these variables by the set modifier in an ACL. They
    can be given any name that starts with $acl_c and is at least six
    characters long, but the sixth character must be either a digit or an
    underscore. For example: $acl_c5, $acl_c_mycount. The values of the
    $acl_c... variables persist throughout the lifetime of an SMTP connection.
    They can be used to pass information between ACLs and between different
    invocations of the same ACL. When a message is received, the values of
    these variables are saved with the message, and can be accessed by filters,
    routers, and transports during subsequent delivery.

$acl_m...

    These variables are like the $acl_c... variables, except that their values
    are reset after a message has been received. Thus, if several messages are
    received in one SMTP connection, $acl_m... values are not passed on from
    one message to the next, as $acl_c... values are. The $acl_m... variables
    are also reset by MAIL, RSET, EHLO, HELO, and after starting a TLS session.
    When a message is received, the values of these variables are saved with
    the message, and can be accessed by filters, routers, and transports during
    subsequent delivery.

$acl_narg

    Within an acl condition, expansion condition or expansion item this
    variable has the number of arguments.

$acl_verify_message

    After an address verification has failed, this variable contains the
    failure message. It retains its value for use in subsequent modifiers of
    the verb. The message can be preserved by coding like this:

    warn !verify = sender
         set acl_m0 = $acl_verify_message

    You can use $acl_verify_message during the expansion of the message or 
    log_message modifiers, to include information about the verification
    failure. Note: The variable is cleared at the end of processing the ACL
    verb.

$address_data

    This variable is set by means of the address_data option in routers. The
    value then remains with the address while it is processed by subsequent
    routers and eventually a transport. If the transport is handling multiple
    addresses, the value from the first address is used. See chapter 15 for
    more details. Note: The contents of $address_data are visible in user
    filter files.

    If $address_data is set when the routers are called from an ACL to verify a
    recipient address, the final value is still in the variable for subsequent
    conditions and modifiers of the ACL statement. If routing the address
    caused it to be redirected to just one address, the child address is also
    routed as part of the verification, and in this case the final value of
    $address_data is from the child's routing.

    If $address_data is set when the routers are called from an ACL to verify a
    sender address, the final value is also preserved, but this time in
    $sender_address_data, to distinguish it from data from a recipient address.

    In both cases (recipient and sender verification), the value does not
    persist after the end of the current ACL statement. If you want to preserve
    these values for longer, you can save them in ACL variables.

$address_file

    When, as a result of aliasing, forwarding, or filtering, a message is
    directed to a specific file, this variable holds the name of the file when
    the transport is running. At other times, the variable is empty. For
    example, using the default configuration, if user r2d2 has a .forward file
    containing

    /home/r2d2/savemail

    then when the address_file transport is running, $address_file contains the
    text string "/home/r2d2/savemail". For Sieve filters, the value may be
    "inbox" or a relative folder name. It is then up to the transport
    configuration to generate an appropriate absolute path to the relevant
    file.

$address_pipe

    When, as a result of aliasing or forwarding, a message is directed to a
    pipe, this variable holds the pipe command when the transport is running.

$auth1 - $auth4

    These variables are used in SMTP authenticators (see chapters 34-42).
    Elsewhere, they are empty.

$authenticated_id

    When a server successfully authenticates a client it may be configured to
    preserve some of the authentication information in the variable
    $authenticated_id (see chapter 33). For example, a user/password
    authenticator configuration might preserve the user name for use in the
    routers. Note that this is not the same information that is saved in
    $sender_host_authenticated.

    When a message is submitted locally (that is, not over a TCP connection)
    the value of $authenticated_id is normally the login name of the calling
    process. However, a trusted user can override this by means of the -oMai
    command line option. This second case also sets up information used by the
    $authresults expansion item.

$authenticated_fail_id

    When an authentication attempt fails, the variable $authenticated_fail_id
    will contain the failed authentication id. If more than one authentication
    id is attempted, it will contain only the last one. The variable is
    available for processing in the ACL's, generally the quit or notquit ACL. A
    message to a local recipient could still be accepted without requiring
    authentication, which means this variable could also be visible in all of
    the ACL's as well.

$authenticated_sender

    Tainted

    When acting as a server, Exim takes note of the AUTH= parameter on an
    incoming SMTP MAIL command if it believes the sender is sufficiently
    trusted, as described in section 33.2. Unless the data is the string "<>",
    it is set as the authenticated sender of the message, and the value is
    available during delivery in the $authenticated_sender variable. If the
    sender is not trusted, Exim accepts the syntax of AUTH=, but ignores the
    data.

    When a message is submitted locally (that is, not over a TCP connection),
    the value of $authenticated_sender is an address constructed from the login
    name of the calling process and $qualify_domain, except that a trusted user
    can override this by means of the -oMas command line option.

$authentication_failed

    This variable is set to "1" in an Exim server if a client issues an AUTH
    command that does not succeed. Otherwise it is set to "0". This makes it
    possible to distinguish between "did not try to authenticate" (
    $sender_host_authenticated is empty and $authentication_failed is set to
    "0") and "tried to authenticate but failed" ($sender_host_authenticated is
    empty and $authentication_failed is set to "1"). Failure includes
    cancellation of a authentication attempt, and any negative response to an
    AUTH command, (including, for example, an attempt to use an undefined
    mechanism).

$av_failed

    This variable is available when Exim is compiled with the content-scanning
    extension. It is set to "0" by default, but will be set to "1" if any
    problem occurs with the virus scanner (specified by av_scanner) during the
    ACL malware condition.

$body_linecount

    When a message is being received or delivered, this variable contains the
    number of lines in the message's body. See also $message_linecount.

$body_zerocount

    When a message is being received or delivered, this variable contains the
    number of binary zero bytes (ASCII NULs) in the message's body.

$bounce_recipient

    This is set to the recipient address of a bounce message while Exim is
    creating it. It is useful if a customized bounce message text file is in
    use (see chapter 50).

$bounce_return_size_limit

    This contains the value set in the bounce_return_size_limit option, rounded
    up to a multiple of 1000. It is useful when a customized error message text
    file is in use (see chapter 50).

$caller_gid

    The real group id under which the process that called Exim was running.
    This is not the same as the group id of the originator of a message (see
    $originator_gid). If Exim re-execs itself, this variable in the new
    incarnation normally contains the Exim gid.

$caller_uid

    The real user id under which the process that called Exim was running. This
    is not the same as the user id of the originator of a message (see
    $originator_uid). If Exim re-execs itself, this variable in the new
    incarnation normally contains the Exim uid.

$callout_address

    After a callout for verification, spamd or malware daemon service, the
    address that was connected to.

$compile_number

    The building process for Exim keeps a count of the number of times it has
    been compiled. This serves to distinguish different compilations of the
    same version of Exim.

$config_dir

    The directory name of the main configuration file. That is, the content of
    $config_file with the last component stripped. The value does not contain
    the trailing slash. If $config_file does not contain a slash, $config_dir
    is ".".

$config_file

    The name of the main configuration file Exim is using.

$dkim_verify_status

    Results of DKIM verification. For details see section 58.1.2.

$dkim_cur_signer, $dkim_verify_reason, $dkim_domain, $dkim_identity, 
$dkim_selector, $dkim_algo, $dkim_canon_body, $dkim_canon_headers, 
$dkim_copiedheaders, $dkim_bodylength, $dkim_created, $dkim_expires, 
$dkim_headernames, $dkim_key_testing, $dkim_key_nosubdomains, $dkim_key_srvtype
, $dkim_key_granularity, $dkim_key_notes, $dkim_key_length

    These variables are only available within the DKIM ACL. For details see
    section 58.1.2.

$dkim_signers

    When a message has been received this variable contains a colon-separated
    list of signer domains and identities for the message. For details see
    section 58.1.2.

$dmarc_domain_policy, $dmarc_status, $dmarc_status_text, $dmarc_used_domains

    Results of DMARC verification. For details see section 58.3.

$dnslist_domain, $dnslist_matched, $dnslist_text, $dnslist_value

    When a DNS (black) list lookup succeeds, these variables are set to contain
    the following data from the lookup: the list's domain name, the key that
    was looked up, the contents of any associated TXT record, and the value
    from the main A record. See section 44.18.5 for more details.

$domain

    Tainted

    When an address is being routed, or delivered on its own, this variable
    contains the domain. Uppercase letters in the domain are converted into
    lower case for $domain.

    Global address rewriting happens when a message is received, so the value
    of $domain during routing and delivery is the value after rewriting.
    $domain is set during user filtering, but not during system filtering,
    because a message may have many recipients and the system filter is called
    just once.

    When more than one address is being delivered at once (for example, several
    RCPT commands in one SMTP delivery), $domain is set only if they all have
    the same domain. Transports can be restricted to handling only one domain
    at a time if the value of $domain is required at transport time - this is
    the default for local transports. For further details of the environment in
    which local transports are run, see chapter 23.

    At the end of a delivery, if all deferred addresses have the same domain,
    it is set in $domain during the expansion of delay_warning_condition.

    The $domain variable is also used in some other circumstances:

      o When an ACL is running for a RCPT command, $domain contains the domain
        of the recipient address. The domain of the sender address is in
        $sender_address_domain at both MAIL time and at RCPT time. $domain is
        not normally set during the running of the MAIL ACL. However, if the
        sender address is verified with a callout during the MAIL ACL, the
        sender domain is placed in $domain during the expansions of hosts, 
        interface, and port in the smtp transport.

      o When a rewrite item is being processed (see chapter 31), $domain
        contains the domain portion of the address that is being rewritten; it
        can be used in the expansion of the replacement address, for example,
        to rewrite domains by file lookup.

      o With one important exception, whenever a domain list is being scanned,
        $domain contains the subject domain. Exception: When a domain list in a
        sender_domains condition in an ACL is being processed, the subject
        domain is in $sender_address_domain and not in $domain. It works this
        way so that, in a RCPT ACL, the sender domain list can be dependent on
        the recipient domain (which is what is in $domain at this time).

      o When the smtp_etrn_command option is being expanded, $domain contains
        the complete argument of the ETRN command (see section 49.2.5).

    If the origin of the data is an incoming message, the result of expanding
    this variable is tainted and may not be further expanded or used as a
    filename. When an untainted version is needed, one should be obtained from
    looking up the value in a local (therefore trusted) database. Often
    $domain_data is usable in this role.

$domain_data

    When the domains condition on a router or an ACL matches a domain against a
    list, the match value is copied to $domain_data. This is an enhancement
    over previous versions of Exim, when it only applied to the data read by a
    lookup. For details on match values see section 10.1 et. al.

    If the router routes the address to a transport, the value is available in
    that transport. If the transport is handling multiple addresses, the value
    from the first address is used.

    $domain_data set in an ACL is available during the rest of the ACL
    statement.

$exim_gid

    This variable contains the numerical value of the Exim group id.

$exim_path

    This variable contains the path to the Exim binary.

$exim_uid

    This variable contains the numerical value of the Exim user id.

$exim_version

    This variable contains the version string of the Exim build. The first
    character is a major version number, currently 4. Then after a dot, the
    next group of digits is a minor version number. There may be other
    characters following the minor version. This value may be overridden by the
    exim_version main config option.

$header_<name>

    Tainted

    This is not strictly an expansion variable. It is expansion syntax for
    inserting the message header line with the given name. Note that the name
    must be terminated by colon or white space, because it may contain a wide
    variety of characters. Note also that braces must not be used. See the full
    description in section 11.5 above.

$headers_added

    Within an ACL this variable contains the headers added so far by the ACL
    modifier add_header (section 44.15). The headers are a newline-separated
    list.

$home

    When the check_local_user option is set for a router, the user's home
    directory is placed in $home when the check succeeds. In particular, this
    means it is set during the running of users' filter files. A router may
    also explicitly set a home directory for use by a transport; this can be
    overridden by a setting on the transport itself.

    When running a filter test via the -bf option, $home is set to the value of
    the environment variable HOME, which is subject to the keep_environment and
    add_environment main config options.

$host

    If a router assigns an address to a transport (any transport), and passes a
    list of hosts with the address, the value of $host when the transport
    starts to run is the name of the first host on the list. Note that this
    applies both to local and remote transports.

    For the smtp transport, if there is more than one host, the value of $host
    changes as the transport works its way through the list. In particular,
    when the smtp transport is expanding its options for encryption using TLS,
    or for specifying a transport filter (see chapter 24), $host contains the
    name of the host to which it is connected.

    When used in the client part of an authenticator configuration (see chapter
    33), $host contains the name of the server to which the client is
    connected.

$host_address

    This variable is set to the remote host's IP address whenever $host is set
    for a remote connection. It is also set to the IP address that is being
    checked when the ignore_target_hosts option is being processed.

$host_data

    If a hosts condition in an ACL is satisfied by means of a lookup, the
    result of the lookup is made available in the $host_data variable. This
    allows you, for example, to do things like this:

    deny  hosts = net-lsearch;/some/file
          message = $host_data

$host_lookup_deferred

    This variable normally contains "0", as does $host_lookup_failed. When a
    message comes from a remote host and there is an attempt to look up the
    host's name from its IP address, and the attempt is not successful, one of
    these variables is set to "1".

      o If the lookup receives a definite negative response (for example, a DNS
        lookup succeeded, but no records were found), $host_lookup_failed is
        set to "1".

      o If there is any kind of problem during the lookup, such that Exim
        cannot tell whether or not the host name is defined (for example, a
        timeout for a DNS lookup), $host_lookup_deferred is set to "1".

    Looking up a host's name from its IP address consists of more than just a
    single reverse lookup. Exim checks that a forward lookup of at least one of
    the names it receives from a reverse lookup yields the original IP address.
    If this is not the case, Exim does not accept the looked up name(s), and
    $host_lookup_failed is set to "1". Thus, being able to find a name from an
    IP address (for example, the existence of a PTR record in the DNS) is not
    sufficient on its own for the success of a host name lookup. If the reverse
    lookup succeeds, but there is a lookup problem such as a timeout when
    checking the result, the name is not accepted, and $host_lookup_deferred is
    set to "1". See also $sender_host_name.

    Performing these checks sets up information used by the authresults
    expansion item.

$host_lookup_failed

    See $host_lookup_deferred.

$host_port

    This variable is set to the remote host's TCP port whenever $host is set
    for an outbound connection.

$initial_cwd

    This variable contains the full path name of the initial working directory
    of the current Exim process. This may differ from the current working
    directory, as Exim changes this to "/" during early startup, and to
    $spool_directory later.

$inode

    The only time this variable is set is while expanding the directory_file
    option in the appendfile transport. The variable contains the inode number
    of the temporary file which is about to be renamed. It can be used to
    construct a unique name for the file.

$interface_address, $interface_port

    These are obsolete names for $received_ip_address and $received_port.

$item

    This variable is used during the expansion of forall and forany conditions
    (see section 11.7), and filter, map, and reduce items (see section 11.7).
    In other circumstances, it is empty.

$ldap_dn

    This variable, which is available only when Exim is compiled with LDAP
    support, contains the DN from the last entry in the most recently
    successful LDAP lookup.

$load_average

    This variable contains the system load average, multiplied by 1000 so that
    it is an integer. For example, if the load average is 0.21, the value of
    the variable is 210. The value is recomputed every time the variable is
    referenced.

$local_part

    Tainted

    When an address is being routed, or delivered on its own, this variable
    contains the local part. When a number of addresses are being delivered
    together (for example, multiple RCPT commands in an SMTP session),
    $local_part is not set.

    Global address rewriting happens when a message is received, so the value
    of $local_part during routing and delivery is the value after rewriting.
    $local_part is set during user filtering, but not during system filtering,
    because a message may have many recipients and the system filter is called
    just once.

    If the origin of the data is an incoming message, the result of expanding
    this variable is tainted and may not be further expanded or used as a
    filename.

    Warning: the content of this variable is usually provided by a potential
    attacker. Consider carefully the implications of using it unvalidated as a
    name for file access. This presents issues for users' .forward and filter
    files. For traditional full user accounts, use check_local_users and the
    $local_part_data variable rather than this one. For virtual users, store a
    suitable pathname component in the database which is used for account name
    validation, and use that retrieved value rather than this variable. Often
    $local_part_data is usable in this role. If needed, use a router 
    address_data or set option for the retrieved data.

    When a message is being delivered to a file, pipe, or autoreply transport
    as a result of aliasing or forwarding, $local_part is set to the local part
    of the parent address, not to the filename or command (see $address_file
    and $address_pipe).

    When an ACL is running for a RCPT command, $local_part contains the local
    part of the recipient address.

    When a rewrite item is being processed (see chapter 31), $local_part
    contains the local part of the address that is being rewritten; it can be
    used in the expansion of the replacement address, for example.

    In all cases, all quoting is removed from the local part. For example, for
    both the addresses

    "abc:xyz"@test.example
    abc\:xyz@test.example

    the value of $local_part is

    abc:xyz

    If you use $local_part to create another address, you should always wrap it
    inside a quoting operator. For example, in a redirect router you could
    have:

    data = ${quote_local_part:$local_part}@new.domain.example

    Note: The value of $local_part is normally lower cased. If you want to
    process local parts in a case-dependent manner in a router, you can set the
    caseful_local_part option (see chapter 15).

$local_part_data

    When the local_parts condition on a router or ACL matches a local part list
    the match value is copied to $local_part_data. This is an enhancement over
    previous versions of Exim, when it only applied to the data read by a
    lookup. For details on match values see section 10.1 et. al.

    The check_local_user router option also sets this variable.

    If a local part prefix or suffix has been recognized, it is not included in
    the value of $local_part during routing and subsequent delivery. The values
    of any prefix or suffix are in $local_part_prefix and $local_part_suffix,
    respectively. If the specification did not include a wildcard then the
    affix variable value is not tainted.

    If the affix specification included a wildcard then the portion of the
    affix matched by the wildcard is in $local_part_prefix_v or
    $local_part_suffix_v as appropriate, and both the whole and varying values
    are tainted.

$local_scan_data

    This variable contains the text returned by the local_scan() function when
    a message is received. See chapter 46 for more details.

$local_user_gid

    See $local_user_uid.

$local_user_uid

    This variable and $local_user_gid are set to the uid and gid after the 
    check_local_user router precondition succeeds. This means that their values
    are available for the remaining preconditions (senders, require_files, and 
    condition), for the address_data expansion, and for any router-specific
    expansions. At all other times, the values in these variables are "(uid_t)
    (-1)" and "(gid_t)(-1)", respectively.

$localhost_number

    This contains the expanded value of the localhost_number option. The
    expansion happens after the main options have been read.

$log_inodes

    The number of free inodes in the disk partition where Exim's log files are
    being written. The value is recalculated whenever the variable is
    referenced. If the relevant file system does not have the concept of
    inodes, the value of is -1. See also the check_log_inodes option.

$log_space

    The amount of free space (as a number of kilobytes) in the disk partition
    where Exim's log files are being written. The value is recalculated
    whenever the variable is referenced. If the operating system does not have
    the ability to find the amount of free space (only true for experimental
    systems), the space value is -1. See also the check_log_space option.

$lookup_dnssec_authenticated

    This variable is set after a DNS lookup done by a dnsdb lookup expansion,
    dnslookup router or smtp transport. It will be empty if DNSSEC was not
    requested, "no" if the result was not labelled as authenticated data and
    "yes" if it was. Results that are labelled as authoritative answer that
    match the dns_trust_aa configuration variable count also as authenticated
    data.

$mailstore_basename

    This variable is set only when doing deliveries in "mailstore" format in
    the appendfile transport. During the expansion of the mailstore_prefix, 
    mailstore_suffix, message_prefix, and message_suffix options, it contains
    the basename of the files that are being written, that is, the name without
    the ".tmp", ".env", or ".msg" suffix. At all other times, this variable is
    empty.

$malware_name

    This variable is available when Exim is compiled with the content-scanning
    extension. It is set to the name of the virus that was found when the ACL 
    malware condition is true (see section 45.1).

$max_received_linelength

    This variable contains the number of bytes in the longest line that was
    received as part of the message, not counting the line termination
    character(s). It is not valid if the spool_wireformat option is used.

$message_age

    This variable is set at the start of a delivery attempt to contain the
    number of seconds since the message was received. It does not change during
    a single delivery attempt.

$message_body

    Tainted

    This variable contains the initial portion of a message's body while it is
    being delivered, and is intended mainly for use in filter files. The
    maximum number of characters of the body that are put into the variable is
    set by the message_body_visible configuration option; the default is 500.

    By default, newlines are converted into spaces in $message_body, to make it
    easier to search for phrases that might be split over a line break.
    However, this can be disabled by setting message_body_newlines to be true.
    Binary zeros are always converted into spaces.

$message_body_end

    Tainted

    This variable contains the final portion of a message's body while it is
    being delivered. The format and maximum size are as for $message_body.

$message_body_size

    When a message is being delivered, this variable contains the size of the
    body in bytes. The count starts from the character after the blank line
    that separates the body from the header. Newlines are included in the
    count. See also $message_size, $body_linecount, and $body_zerocount.

    If the spool file is wireformat (see the spool_wireformat main option) the
    CRLF line-terminators are included in the count.

$message_exim_id

    When a message is being received or delivered, this variable contains the
    unique message id that is generated and used by Exim to identify the
    message. An id is not created for a message until after its header has been
    successfully received. Note: This is not the contents of the Message-ID:
    header line; it is the local id that Exim assigns to the message, for
    example: "1BXTIK-0001yO-VA".

$message_headers

    Tainted

    This variable contains a concatenation of all the header lines when a
    message is being processed, except for lines added by routers or
    transports. The header lines are separated by newline characters. Their
    contents are decoded in the same way as a header line that is inserted by 
    bheader.

$message_headers_raw

    Tainted

    This variable is like $message_headers except that no processing of the
    contents of header lines is done.

$message_id

    This is an old name for $message_exim_id. It is now deprecated.

$message_linecount

    This variable contains the total number of lines in the header and body of
    the message. Compare $body_linecount, which is the count for the body only.
    During the DATA and content-scanning ACLs, $message_linecount contains the
    number of lines received. Before delivery happens (that is, before filters,
    routers, and transports run) the count is increased to include the 
    Received: header line that Exim standardly adds, and also any other header
    lines that are added by ACLs. The blank line that separates the message
    header from the body is not counted.

    As with the special case of $message_size, during the expansion of the
    appendfile transport's maildir_tag option in maildir format, the value of
    $message_linecount is the precise size of the number of newlines in the
    file that has been written (minus one for the blank line between the header
    and the body).

    Here is an example of the use of this variable in a DATA ACL:

    deny condition = \
          ${if <{250}{${eval:$message_linecount - $body_linecount}}}
         message   = Too many lines in message header

    In the MAIL and RCPT ACLs, the value is zero because at that stage the
    message has not yet been received.

    This variable is not valid if the spool_wireformat option is used.

$message_size

    When a message is being processed, this variable contains its size in
    bytes. In most cases, the size includes those headers that were received
    with the message, but not those (such as Envelope-to:) that are added to
    individual deliveries as they are written. However, there is one special
    case: during the expansion of the maildir_tag option in the appendfile
    transport while doing a delivery in maildir format, the value of
    $message_size is the precise size of the file that has been written. See
    also $message_body_size, $body_linecount, and $body_zerocount.

    While running a per message ACL (mail/rcpt/predata), $message_size contains
    the size supplied on the MAIL command, or -1 if no size was given. The
    value may not, of course, be truthful.

$mime_anomaly_level, $mime_anomaly_text, $mime_boundary, $mime_charset, 
$mime_content_description, $mime_content_disposition, $mime_content_id, 
$mime_content_size, $mime_content_transfer_encoding, $mime_content_type, 
$mime_decoded_filename, $mime_filename, $mime_is_coverletter, 
$mime_is_multipart, $mime_is_rfc822, $mime_part_count

    A number of variables whose names start with $mime are available when Exim
    is compiled with the content-scanning extension. For details, see section
    45.4.

$n0 - $n9

    These variables are counters that can be incremented by means of the add
    command in filter files.

$original_domain

    Tainted

    When a top-level address is being processed for delivery, this contains the
    same value as $domain. However, if a "child" address (for example,
    generated by an alias, forward, or filter file) is being processed, this
    variable contains the domain of the original address (lower cased). This
    differs from $parent_domain only when there is more than one level of
    aliasing or forwarding. When more than one address is being delivered in a
    single transport run, $original_domain is not set.

    If a new address is created by means of a deliver command in a system
    filter, it is set up with an artificial "parent" address. This has the
    local part system-filter and the default qualify domain.

$original_local_part

    Tainted

    When a top-level address is being processed for delivery, this contains the
    same value as $local_part, unless a prefix or suffix was removed from the
    local part, because $original_local_part always contains the full local
    part. When a "child" address (for example, generated by an alias, forward,
    or filter file) is being processed, this variable contains the full local
    part of the original address.

    If the router that did the redirection processed the local part
    case-insensitively, the value in $original_local_part is in lower case.
    This variable differs from $parent_local_part only when there is more than
    one level of aliasing or forwarding. When more than one address is being
    delivered in a single transport run, $original_local_part is not set.

    If a new address is created by means of a deliver command in a system
    filter, it is set up with an artificial "parent" address. This has the
    local part system-filter and the default qualify domain.

$originator_gid

    This variable contains the value of $caller_gid that was set when the
    message was received. For messages received via the command line, this is
    the gid of the sending user. For messages received by SMTP over TCP/IP,
    this is normally the gid of the Exim user.

$originator_uid

    The value of $caller_uid that was set when the message was received. For
    messages received via the command line, this is the uid of the sending
    user. For messages received by SMTP over TCP/IP, this is normally the uid
    of the Exim user.

$parent_domain

    Tainted

    This variable is similar to $original_domain (see above), except that it
    refers to the immediately preceding parent address.

$parent_local_part

    Tainted

    This variable is similar to $original_local_part (see above), except that
    it refers to the immediately preceding parent address.

$pid

    This variable contains the current process id.

$pipe_addresses

    This is not an expansion variable, but is mentioned here because the string
    "$pipe_addresses" is handled specially in the command specification for the
    pipe transport (chapter 29) and in transport filters (described under 
    transport_filter in chapter 24). It cannot be used in general expansion
    strings, and provokes an "unknown variable" error if encountered. Note:
    This value permits data supplied by a potential attacker to be used in the
    command for a pipe transport. Such configurations should be carefully
    assessed for security vulnerbilities.

$primary_hostname

    This variable contains the value set by primary_hostname in the
    configuration file, or read by the uname() function. If uname() returns a
    single-component name, Exim calls gethostbyname() (or getipnodebyname()
    where available) in an attempt to acquire a fully qualified host name. See
    also $smtp_active_hostname.

$proxy_external_address, $proxy_external_port, $proxy_local_address, 
$proxy_local_port, $proxy_session

    These variables are only available when built with Proxy Protocol or SOCKS5
    support. For details see chapter 59.1.

$prdr_requested

    This variable is set to "yes" if PRDR was requested by the client for the
    current message, otherwise "no".

$prvscheck_address, $prvscheck_keynum, $prvscheck_result

    These variables are used in conjunction with the prvscheck expansion item,
    which is described in sections 11.5 and 44.27.

$qualify_domain

    The value set for the qualify_domain option in the configuration file.

$qualify_recipient

    The value set for the qualify_recipient option in the configuration file,
    or if not set, the value of $qualify_domain.

$queue_name

    The name of the spool queue in use; empty for the default queue.

$queue_size

    This variable contains the number of messages queued. It is evaluated on
    demand, but no more often than once every minute. If there is no daemon
    notifier socket open, the value will be an empty string.

$r_...

    Values can be placed in these variables by the set option of a router. They
    can be given any name that starts with $r_. The values persist for the
    address being handled through subsequent routers and the eventual
    transport.

$rcpt_count

    When a message is being received by SMTP, this variable contains the number
    of RCPT commands received for the current message. If this variable is used
    in a RCPT ACL, its value includes the current command.

$rcpt_defer_count

    When a message is being received by SMTP, this variable contains the number
    of RCPT commands in the current message that have previously been rejected
    with a temporary (4xx) response.

$rcpt_fail_count

    When a message is being received by SMTP, this variable contains the number
    of RCPT commands in the current message that have previously been rejected
    with a permanent (5xx) response.

$received_count

    This variable contains the number of Received: header lines in the message,
    including the one added by Exim (so its value is always greater than zero).
    It is available in the DATA ACL, the non-SMTP ACL, and while routing and
    delivering.

$received_for

    Tainted

    If there is only a single recipient address in an incoming message, this
    variable contains that address when the Received: header line is being
    built. The value is copied after recipient rewriting has happened, but
    before the local_scan() function is run.

$received_ip_address, $received_port

    As soon as an Exim server starts processing an incoming TCP/IP connection,
    these variables are set to the address and port on the local IP interface.
    (The remote IP address and port are in $sender_host_address and
    $sender_host_port.) When testing with -bh, the port value is -1 unless it
    has been set using the -oMi command line option.

    As well as being useful in ACLs (including the "connect" ACL), these
    variable could be used, for example, to make the filename for a TLS
    certificate depend on which interface and/or port is being used for the
    incoming connection. The values of $received_ip_address and $received_port
    are saved with any messages that are received, thus making these variables
    available at delivery time. For outbound connections see
    $sending_ip_address.

$received_protocol

    When a message is being processed, this variable contains the name of the
    protocol by which it was received. Most of the names used by Exim are
    defined by RFCs 821, 2821, and 3848. They start with "smtp" (the client
    used HELO) or "esmtp" (the client used EHLO). This can be followed by "s"
    for secure (encrypted) and/or "a" for authenticated. Thus, for example, if
    the protocol is set to "esmtpsa", the message was received over an
    encrypted SMTP connection and the client was successfully authenticated.

    Exim uses the protocol name "smtps" for the case when encryption is
    automatically set up on connection without the use of STARTTLS (see 
    tls_on_connect_ports), and the client uses HELO to initiate the encrypted
    SMTP session. The name "smtps" is also used for the rare situation where
    the client initially uses EHLO, sets up an encrypted connection using
    STARTTLS, and then uses HELO afterwards.

    The -oMr option provides a way of specifying a custom protocol name for
    messages that are injected locally by trusted callers. This is commonly
    used to identify messages that are being re-injected after some kind of
    scanning.

$received_time

    This variable contains the date and time when the current message was
    received, as a number of seconds since the start of the Unix epoch.

$recipient_data

    This variable is set after an indexing lookup success in an ACL recipients
    condition. It contains the data from the lookup, and the value remains set
    until the next recipients test. Thus, you can do things like this:

    require recipients  = cdb*@;/some/file
    deny    some further test involving $recipient_data

    Warning: This variable is set only when a lookup is used as an indexing
    method in the address list, using the semicolon syntax as in the example
    above. The variable is not set for a lookup that is used as part of the
    string expansion that all such lists undergo before being interpreted.

$recipient_verify_failure

    In an ACL, when a recipient verification fails, this variable contains
    information about the failure. It is set to one of the following words:

      o "qualify": The address was unqualified (no domain), and the message was
        neither local nor came from an exempted host.

      o "route": Routing failed.

      o "mail": Routing succeeded, and a callout was attempted; rejection
        occurred at or before the MAIL command (that is, on initial connection,
        HELO, or MAIL).

      o "recipient": The RCPT command in a callout was rejected.

      o "postmaster": The postmaster check in a callout was rejected.

    The main use of this variable is expected to be to distinguish between
    rejections of MAIL and rejections of RCPT.

$recipients

    Tainted

$recipients_list

    Tainted

    These variables both contain the envelope recipients for a message.

    The first uses a comma and a space separate the addresses in the
    replacement text. Note: an address can legitimately contain a comma; this
    variable is not intended for further processing.

    The second is a proper Exim list; colon-separated.

    However, the variables are not generally available, to prevent exposure of
    Bcc recipients in unprivileged users' filter files. You can use either of
    them only in these cases:

     1. In a system filter file.

     2. In the ACLs associated with the DATA command and with non-SMTP
        messages, that is, the ACLs defined by acl_smtp_predata, acl_smtp_data,
        acl_smtp_mime, acl_not_smtp_start, acl_not_smtp, and acl_not_smtp_mime.

     3. From within a local_scan() function.

$recipients_count

    When a message is being processed, this variable contains the number of
    envelope recipients that came with the message. Duplicates are not excluded
    from the count. While a message is being received over SMTP, the number
    increases for each accepted recipient. It can be referenced in an ACL.

$regex_match_string

    This variable is set to contain the matching regular expression after a 
    regex ACL condition has matched (see section 45.5).

$regex1, $regex2, etc

    When a regex or mime_regex ACL condition succeeds, these variables contain
    the captured substrings identified by the regular expression. If the
    subject string was tainted then so will any captured substring.

$reply_address

    Tainted

    When a message is being processed, this variable contains the contents of
    the Reply-To: header line if one exists and it is not empty, or otherwise
    the contents of the From: header line. Apart from the removal of leading
    white space, the value is not processed in any way. In particular, no RFC
    2047 decoding or character code translation takes place.

$return_path

    When a message is being delivered, this variable contains the return path -
    the sender field that will be sent as part of the envelope. It is not
    enclosed in <> characters. At the start of routing an address, $return_path
    has the same value as $sender_address, but if, for example, an incoming
    message to a mailing list has been expanded by a router which specifies a
    different address for bounce messages, $return_path subsequently contains
    the new bounce address, whereas $sender_address always contains the
    original sender address that was received with the message. In other words,
    $sender_address contains the incoming envelope sender, and $return_path
    contains the outgoing envelope sender.

$return_size_limit

    This is an obsolete name for $bounce_return_size_limit.

$router_name

    During the running of a router, or a transport called, this variable
    contains the router name.

$runrc

    This variable contains the return code from a command that is run by the $
    {run...} expansion item. Warning: In a router or transport, you cannot
    assume the order in which option values are expanded, except for those
    preconditions whose order of testing is documented. Therefore, you cannot
    reliably expect to set $runrc by the expansion of one option, and use it in
    another.

$self_hostname

    When an address is routed to a supposedly remote host that turns out to be
    the local host, what happens is controlled by the self generic router
    option. One of its values causes the address to be passed to another
    router. When this happens, $self_hostname is set to the name of the local
    host that the original router encountered. In other circumstances its
    contents are null.

$sender_address

    Tainted

    When a message is being processed, this variable contains the sender's
    address that was received in the message's envelope. The case of letters in
    the address is retained, in both the local part and the domain. For bounce
    messages, the value of this variable is the empty string. See also
    $return_path.

$sender_address_data

    If $address_data is set when the routers are called from an ACL to verify a
    sender address, the final value is preserved in $sender_address_data, to
    distinguish it from data from a recipient address. The value does not
    persist after the end of the current ACL statement. If you want to preserve
    it for longer, you can save it in an ACL variable.

$sender_address_domain

    Tainted

    The domain portion of $sender_address.

$sender_address_local_part

    Tainted

    The local part portion of $sender_address.

$sender_data

    This variable is set after a lookup success in an ACL senders condition or
    in a router senders option. It contains the data from the lookup, and the
    value remains set until the next senders test. Thus, you can do things like
    this:

    require senders      = cdb*@;/some/file
    deny    some further test involving $sender_data

    Warning: This variable is set only when a lookup is used as an indexing
    method in the address list, using the semicolon syntax as in the example
    above. The variable is not set for a lookup that is used as part of the
    string expansion that all such lists undergo before being interpreted.

$sender_fullhost

    When a message is received from a remote host, this variable contains the
    host name and IP address in a single string. It ends with the IP address in
    square brackets, followed by a colon and a port number if the logging of
    ports is enabled. The format of the rest of the string depends on whether
    the host issued a HELO or EHLO SMTP command, and whether the host name was
    verified by looking up its IP address. (Looking up the IP address can be
    forced by the host_lookup option, independent of verification.) A plain
    host name at the start of the string is a verified host name; if this is
    not present, verification either failed or was not requested. A host name
    in parentheses is the argument of a HELO or EHLO command. This is omitted
    if it is identical to the verified host name or to the host's IP address in
    square brackets.

$sender_helo_dnssec

    This boolean variable is true if a successful HELO verification was done
    using DNS information the resolver library stated was authenticated data.

$sender_helo_name

    Tainted

    When a message is received from a remote host that has issued a HELO or
    EHLO command, the argument of that command is placed in this variable. It
    is also set if HELO or EHLO is used when a message is received using SMTP
    locally via the -bs or -bS options.

$sender_host_address

    When a message is received from a remote host using SMTP, this variable
    contains that host's IP address. For locally non-SMTP submitted messages,
    it is empty.

$sender_host_authenticated

    This variable contains the name (not the public name) of the authenticator
    driver that successfully authenticated the client from which the message
    was received. It is empty if there was no successful authentication. See
    also $authenticated_id.

$sender_host_dnssec

    If an attempt to populate $sender_host_name has been made (by reference, 
    hosts_lookup or otherwise) then this boolean will have been set true if,
    and only if, the resolver library states that both the reverse and forward
    DNS were authenticated data. At all other times, this variable is false.

    It is likely that you will need to coerce DNSSEC support on in the resolver
    library, by setting:

    dns_dnssec_ok = 1

    In addition, on Linux with glibc 2.31 or newer the resolver library will
    default to stripping out a successful validation status. This will break a
    previously working Exim installation. Provided that you do trust the
    resolver (ie, is on localhost) you can tell glibc to pass through any
    successful validation with a new option in /etc/resolv.conf:

    options trust-ad

    Exim does not perform DNSSEC validation itself, instead leaving that to a
    validating resolver (e.g. unbound, or bind with suitable configuration).

    If you have changed host_lookup_order so that "bydns" is not the first
    mechanism in the list, then this variable will be false.

    This requires that your system resolver library support EDNS0 (and that
    DNSSEC flags exist in the system headers). If the resolver silently drops
    all EDNS0 options, then this will have no effect. OpenBSD's asr resolver is
    known to currently ignore EDNS0, documented in CAVEATS of asr_run(3).

$sender_host_name

    Tainted

    When a message is received from a remote host, this variable contains the
    host's name as obtained by looking up its IP address. For messages received
    by other means, this variable is empty.

    If the host name has not previously been looked up, a reference to
    $sender_host_name triggers a lookup (for messages from remote hosts). A
    looked up name is accepted only if it leads back to the original IP address
    via a forward lookup. If either the reverse or the forward lookup fails to
    find any data, or if the forward lookup does not yield the original IP
    address, $sender_host_name remains empty, and $host_lookup_failed is set to
    "1".

    However, if either of the lookups cannot be completed (for example, there
    is a DNS timeout), $host_lookup_deferred is set to "1", and
    $host_lookup_failed remains set to "0".

    Once $host_lookup_failed is set to "1", Exim does not try to look up the
    host name again if there is a subsequent reference to $sender_host_name in
    the same Exim process, but it does try again if $host_lookup_deferred is
    set to "1".

    Exim does not automatically look up every calling host's name. If you want
    maximum efficiency, you should arrange your configuration so that it avoids
    these lookups altogether. The lookup happens only if one or more of the
    following are true:

      o A string containing $sender_host_name is expanded.

      o The calling host matches the list in host_lookup. In the default
        configuration, this option is set to *, so it must be changed if
        lookups are to be avoided. (In the code, the default for host_lookup is
        unset.)

      o Exim needs the host name in order to test an item in a host list. The
        items that require this are described in sections 10.4.4 and 10.4.8.

      o The calling host matches helo_try_verify_hosts or helo_verify_hosts. In
        this case, the host name is required to compare with the name quoted in
        any EHLO or HELO commands that the client issues.

      o The remote host issues a EHLO or HELO command that quotes one of the
        domains in helo_lookup_domains. The default value of this option is

          helo_lookup_domains = @ : @[]

        which causes a lookup if a remote host (incorrectly) gives the server's
        name or IP address in an EHLO or HELO command.

$sender_host_port

    When a message is received from a remote host, this variable contains the
    port number that was used on the remote host.

$sender_ident

    When a message is received from a remote host, this variable contains the
    identification received in response to an RFC 1413 request. When a message
    has been received locally, this variable contains the login name of the
    user that called Exim.

$sender_rate_xxx

    A number of variables whose names begin $sender_rate_ are set as part of
    the ratelimit ACL condition. Details are given in section 44.20.

$sender_rcvhost

    This is provided specifically for use in Received: headers. It starts with
    either the verified host name (as obtained from a reverse DNS lookup) or,
    if there is no verified host name, the IP address in square brackets. After
    that there may be text in parentheses. When the first item is a verified
    host name, the first thing in the parentheses is the IP address in square
    brackets, followed by a colon and a port number if port logging is enabled.
    When the first item is an IP address, the port is recorded as "port=xxxx"
    inside the parentheses.

    There may also be items of the form "helo=xxxx" if HELO or EHLO was used
    and its argument was not identical to the real host name or IP address, and
    "ident=xxxx" if an RFC 1413 ident string is available. If all three items
    are present in the parentheses, a newline and tab are inserted into the
    string, to improve the formatting of the Received: header.

$sender_verify_failure

    In an ACL, when a sender verification fails, this variable contains
    information about the failure. The details are the same as for
    $recipient_verify_failure.

$sending_ip_address

    This variable is set whenever an outgoing SMTP connection to another host
    has been set up. It contains the IP address of the local interface that is
    being used. This is useful if a host that has more than one IP address
    wants to take on different personalities depending on which one is being
    used. For incoming connections, see $received_ip_address.

$sending_port

    This variable is set whenever an outgoing SMTP connection to another host
    has been set up. It contains the local port that is being used. For
    incoming connections, see $received_port.

$smtp_active_hostname

    During an incoming SMTP session, this variable contains the value of the
    active host name, as specified by the smtp_active_hostname option. The
    value of $smtp_active_hostname is saved with any message that is received,
    so its value can be consulted during routing and delivery.

$smtp_command

    Tainted

    During the processing of an incoming SMTP command, this variable contains
    the entire command. This makes it possible to distinguish between HELO and
    EHLO in the HELO ACL, and also to distinguish between commands such as
    these:

    MAIL FROM:<>
    MAIL FROM: <>

    For a MAIL command, extra parameters such as SIZE can be inspected. For a
    RCPT command, the address in $smtp_command is the original address before
    any rewriting, whereas the values in $local_part and $domain are taken from
    the address after SMTP-time rewriting.

$smtp_command_argument

    Tainted

    While an ACL is running to check an SMTP command, this variable contains
    the argument, that is, the text that follows the command name, with leading
    white space removed. Following the introduction of $smtp_command, this
    variable is somewhat redundant, but is retained for backwards
    compatibility.

$smtp_command_history

    A comma-separated list (with no whitespace) of the most-recent SMTP
    commands received, in time-order left to right. Only a limited number of
    commands are remembered.

$smtp_count_at_connection_start

    This variable is set greater than zero only in processes spawned by the
    Exim daemon for handling incoming SMTP connections. The name is
    deliberately long, in order to emphasize what the contents are. When the
    daemon accepts a new connection, it increments this variable. A copy of the
    variable is passed to the child process that handles the connection, but
    its value is fixed, and never changes. It is only an approximation of how
    many incoming connections there actually are, because many other
    connections may come and go while a single connection is being processed.
    When a child process terminates, the daemon decrements its copy of the
    variable.

$smtp_notquit_reason

    When the not-QUIT ACL is running, this variable is set to a string that
    indicates the reason for the termination of the SMTP connection.

$sn0 - $sn9

    These variables are copies of the values of the $n0 - $n9 accumulators that
    were current at the end of the system filter file. This allows a system
    filter file to set values that can be tested in users' filter files. For
    example, a system filter could set a value indicating how likely it is that
    a message is junk mail.

$spam_score, $spam_score_int, $spam_bar, $spam_report, $spam_action

    A number of variables whose names start with $spam are available when Exim
    is compiled with the content-scanning extension. For details, see section
    45.2.

$spf_header_comment, $spf_received, $spf_result, $spf_result_guessed, 
$spf_smtp_comment

    These variables are only available if Exim is built with SPF support. For
    details see section 58.2.

$spool_directory

    The name of Exim's spool directory.

$spool_inodes

    The number of free inodes in the disk partition where Exim's spool files
    are being written. The value is recalculated whenever the variable is
    referenced. If the relevant file system does not have the concept of
    inodes, the value of is -1. See also the check_spool_inodes option.

$spool_space

    The amount of free space (as a number of kilobytes) in the disk partition
    where Exim's spool files are being written. The value is recalculated
    whenever the variable is referenced. If the operating system does not have
    the ability to find the amount of free space (only true for experimental
    systems), the space value is -1. For example, to check in an ACL that there
    is at least 50 megabytes free on the spool, you could write:

    condition = ${if > {$spool_space}{50000}}

    See also the check_spool_space option.

$thisaddress

    This variable is set only during the processing of the foranyaddress
    command in a filter file. Its use is explained in the description of that
    command, which can be found in the separate document entitled Exim's
    interfaces to mail filtering.

$tls_in_bits

    Contains an approximation of the TLS cipher's bit-strength on the inbound
    connection; the meaning of this depends upon the TLS implementation used.
    If TLS has not been negotiated, the value will be 0. The value of this is
    automatically fed into the Cyrus SASL authenticator when acting as a
    server, to specify the "external SSF" (a SASL term).

    The deprecated $tls_bits variable refers to the inbound side except when
    used in the context of an outbound SMTP delivery, when it refers to the
    outbound.

$tls_out_bits

    Contains an approximation of the TLS cipher's bit-strength on an outbound
    SMTP connection; the meaning of this depends upon the TLS implementation
    used. If TLS has not been negotiated, the value will be 0.

$tls_in_ourcert

    This variable refers to the certificate presented to the peer of an inbound
    connection when the message was received. It is only useful as the argument
    of a certextract expansion item, md5, sha1 or sha256 operator, or a def
    condition.

    Note: Under versions of OpenSSL preceding 1.1.1, when a list of more than
    one file is used for tls_certificate, this variable is not reliable. The
    macro "_TLS_BAD_MULTICERT_IN_OURCERT" will be defined for those versions.

$tls_in_peercert

    This variable refers to the certificate presented by the peer of an inbound
    connection when the message was received. It is only useful as the argument
    of a certextract expansion item, md5, sha1 or sha256 operator, or a def
    condition. If certificate verification fails it may refer to a failing
    chain element which is not the leaf.

$tls_out_ourcert

    This variable refers to the certificate presented to the peer of an
    outbound connection. It is only useful as the argument of a certextract
    expansion item, md5, sha1 or sha256 operator, or a def condition.

$tls_out_peercert

    This variable refers to the certificate presented by the peer of an
    outbound connection. It is only useful as the argument of a certextract
    expansion item, md5, sha1 or sha256 operator, or a def condition. If
    certificate verification fails it may refer to a failing chain element
    which is not the leaf.

$tls_in_certificate_verified

    This variable is set to "1" if a TLS certificate was verified when the
    message was received, and "0" otherwise.

    The deprecated $tls_certificate_verified variable refers to the inbound
    side except when used in the context of an outbound SMTP delivery, when it
    refers to the outbound.

$tls_out_certificate_verified

    This variable is set to "1" if a TLS certificate was verified when an
    outbound SMTP connection was made, and "0" otherwise.

$tls_in_cipher

    When a message is received from a remote host over an encrypted SMTP
    connection, this variable is set to the cipher suite that was negotiated,
    for example DES-CBC3-SHA. In other circumstances, in particular, for
    message received over unencrypted connections, the variable is empty.
    Testing $tls_in_cipher for emptiness is one way of distinguishing between
    encrypted and non-encrypted connections during ACL processing.

    The deprecated $tls_cipher variable is the same as $tls_in_cipher during
    message reception, but in the context of an outward SMTP delivery taking
    place via the smtp transport becomes the same as $tls_out_cipher.

$tls_in_cipher_std

    As above, but returning the RFC standard name for the cipher suite.

$tls_out_cipher

    This variable is cleared before any outgoing SMTP connection is made, and
    then set to the outgoing cipher suite if one is negotiated. See chapter 43
    for details of TLS support and chapter 30 for details of the smtp
    transport.

$tls_out_cipher_std

    As above, but returning the RFC standard name for the cipher suite.

$tls_out_dane

    DANE active status. See section 43.12.

$tls_in_ocsp

    When a message is received from a remote client connection the result of
    any OCSP request from the client is encoded in this variable:

    0 OCSP proof was not requested (default value)
    1 No response to request
    2 Response not verified
    3 Verification failed
    4 Verification succeeded

$tls_out_ocsp

    When a message is sent to a remote host connection the result of any OCSP
    request made is encoded in this variable. See $tls_in_ocsp for values.

$tls_in_peerdn

    When a message is received from a remote host over an encrypted SMTP
    connection, and Exim is configured to request a certificate from the
    client, the value of the Distinguished Name of the certificate is made
    available in the $tls_in_peerdn during subsequent processing. If
    certificate verification fails it may refer to a failing chain element
    which is not the leaf.

    The deprecated $tls_peerdn variable refers to the inbound side except when
    used in the context of an outbound SMTP delivery, when it refers to the
    outbound.

$tls_out_peerdn

    When a message is being delivered to a remote host over an encrypted SMTP
    connection, and Exim is configured to request a certificate from the
    server, the value of the Distinguished Name of the certificate is made
    available in the $tls_out_peerdn during subsequent processing. If
    certificate verification fails it may refer to a failing chain element
    which is not the leaf.

$tls_in_resumption, $tls_out_resumption

    Observability for TLS session resumption. See 43.11 for details.

$tls_in_sni

    Tainted

    When a TLS session is being established, if the client sends the Server
    Name Indication extension, the value will be placed in this variable. If
    the variable appears in tls_certificate then this option and some others,
    described in 43.8, will be re-expanded early in the TLS session, to permit
    a different certificate to be presented (and optionally a different key to
    be used) to the client, based upon the value of the SNI extension.

    The deprecated $tls_sni variable refers to the inbound side except when
    used in the context of an outbound SMTP delivery, when it refers to the
    outbound.

$tls_out_sni

    During outbound SMTP deliveries, this variable reflects the value of the 
    tls_sni option on the transport.

$tls_out_tlsa_usage

    Bitfield of TLSA record types found. See section 43.12.

$tls_in_ver

    When a message is received from a remote host over an encrypted SMTP
    connection this variable is set to the protocol version, eg TLS1.2.

$tls_out_ver

    When a message is being delivered to a remote host over an encrypted SMTP
    connection this variable is set to the protocol version.

$tod_bsdinbox

    The time of day and the date, in the format required for BSD-style mailbox
    files, for example: Thu Oct 17 17:14:09 1995.

$tod_epoch

    The time and date as a number of seconds since the start of the Unix epoch.

$tod_epoch_l

    The time and date as a number of microseconds since the start of the Unix
    epoch.

$tod_full

    A full version of the time and date, for example: Wed, 16 Oct 1995 09:51:40
    +0100. The timezone is always given as a numerical offset from UTC, with
    positive values used for timezones that are ahead (east) of UTC, and
    negative values for those that are behind (west).

$tod_log

    The time and date in the format used for writing Exim's log files, for
    example: 1995-10-12 15:32:29, but without a timezone.

$tod_logfile

    This variable contains the date in the format yyyymmdd. This is the format
    that is used for datestamping log files when log_file_path contains the
    "%D" flag.

$tod_zone

    This variable contains the numerical value of the local timezone, for
    example: -0500.

$tod_zulu

    This variable contains the UTC date and time in "Zulu" format, as specified
    by ISO 8601, for example: 20030221154023Z.

$transport_name

    During the running of a transport, this variable contains its name.

$value

    This variable contains the result of an expansion lookup, extraction
    operation, or external command, as described above. It is also used during
    a reduce expansion.

$verify_mode

    While a router or transport is being run in verify mode or for cutthrough
    delivery, contains "S" for sender-verification or "R" for
    recipient-verification. Otherwise, empty.

$version_number

    The version number of Exim. Same as $exim_version, may be overridden by the
    exim_version main config option.

$warn_message_delay

    This variable is set only during the creation of a message warning about a
    delivery delay. Details of its use are explained in section 50.2.

$warn_message_recipients

    This variable is set only during the creation of a message warning about a
    delivery delay. Details of its use are explained in section 50.2.



===============================================================================
12. EMBEDDED PERL

Exim can be built to include an embedded Perl interpreter. When this is done,
Perl subroutines can be called as part of the string expansion process. To make
use of the Perl support, you need version 5.004 or later of Perl installed on
your system. To include the embedded interpreter in the Exim binary, include
the line

EXIM_PERL = perl.o

in your Local/Makefile and then build Exim in the normal way.


12.1 Setting up so Perl can be used
-----------------------------------

Access to Perl subroutines is via a global configuration option called 
perl_startup and an expansion string operator ${perl ...}. If there is no 
perl_startup option in the Exim configuration file then no Perl interpreter is
started and there is almost no overhead for Exim (since none of the Perl
library will be paged in unless used). If there is a perl_startup option then
the associated value is taken to be Perl code which is executed in a newly
created Perl interpreter.

The value of perl_startup is not expanded in the Exim sense, so you do not need
backslashes before any characters to escape special meanings. The option should
usually be something like

perl_startup = do '/etc/exim.pl'

where /etc/exim.pl is Perl code which defines any subroutines you want to use
from Exim. Exim can be configured either to start up a Perl interpreter as soon
as it is entered, or to wait until the first time it is needed. Starting the
interpreter at the beginning ensures that it is done while Exim still has its
setuid privilege, but can impose an unnecessary overhead if Perl is not in fact
used in a particular run. Also, note that this does not mean that Exim is
necessarily running as root when Perl is called at a later time. By default,
the interpreter is started only when it is needed, but this can be changed in
two ways:

  * Setting perl_at_start (a boolean option) in the configuration requests a
    startup when Exim is entered.

  * The command line option -ps also requests a startup when Exim is entered,
    overriding the setting of perl_at_start.

There is also a command line option -pd (for delay) which suppresses the
initial startup, even if perl_at_start is set.

  * To provide more security executing Perl code via the embedded Perl
    interpreter, the perl_taintmode option can be set. This enables the taint
    mode of the Perl interpreter. You are encouraged to set this option to a
    true value. To avoid breaking existing installations, it defaults to false.

    Note: This is entirely separate from Exim's tainted-data tracking.


12.2 Calling Perl subroutines
-----------------------------

When the configuration file includes a perl_startup option you can make use of
the string expansion item to call the Perl subroutines that are defined by the 
perl_startup code. The operator is used in any of the following forms:

${perl{foo}}
${perl{foo}{argument}}
${perl{foo}{argument1}{argument2} ... }

which calls the subroutine foo with the given arguments. A maximum of eight
arguments may be passed. Passing more than this results in an expansion failure
with an error message of the form

Too many arguments passed to Perl subroutine "foo" (max is 8)

The return value of the Perl subroutine is evaluated in a scalar context before
it is passed back to Exim to be inserted into the expanded string. If the
return value is undef, the expansion is forced to fail in the same way as an
explicit "fail" on an if or lookup item. If the subroutine aborts by obeying
Perl's die function, the expansion fails with the error message that was passed
to die.


12.3 Calling Exim functions from Perl
-------------------------------------

Within any Perl code called from Exim, the function Exim::expand_string() is
available to call back into Exim's string expansion function. For example, the
Perl code

my $lp = Exim::expand_string('$local_part');

makes the current Exim $local_part available in the Perl variable $lp. Note
those are single quotes and not double quotes to protect against $local_part
being interpolated as a Perl variable.

If the string expansion is forced to fail by a "fail" item, the result of 
Exim::expand_string() is undef. If there is a syntax error in the expansion
string, the Perl call from the original expansion string fails with an
appropriate error message, in the same way as if die were used.

Two other Exim functions are available for use from within Perl code. 
Exim::debug_write() writes a string to the standard error stream if Exim's
debugging is enabled. If you want a newline at the end, you must supply it. 
Exim::log_write() writes a string to Exim's main log, adding a leading
timestamp. In this case, you should not supply a terminating newline.


12.4 Use of standard output and error by Perl
---------------------------------------------

You should not write to the standard error or output streams from within your
Perl code, as it is not defined how these are set up. In versions of Exim
before 4.50, it is possible for the standard output or error to refer to the
SMTP connection during message reception via the daemon. Writing to this stream
is certain to cause chaos. From Exim 4.50 onwards, the standard output and
error streams are connected to /dev/null in the daemon. The chaos is avoided,
but the output is lost.

The Perl warn statement writes to the standard error stream by default. Calls
to warn may be embedded in Perl modules that you use, but over which you have
no control. When Exim starts up the Perl interpreter, it arranges for output
from the warn statement to be written to the Exim main log. You can change this
by including appropriate Perl magic somewhere in your Perl code. For example,
to discard warn output completely, you need this:

$SIG{__WARN__} = sub { };

Whenever a warn is obeyed, the anonymous subroutine is called. In this example,
the code for the subroutine is empty, so it does nothing, but you can include
any Perl code that you like. The text of the warn message is passed as the
first subroutine argument.



===============================================================================
13. STARTING THE DAEMON AND THE USE OF NETWORK INTERFACES

A host that is connected to a TCP/IP network may have one or more physical
hardware network interfaces. Each of these interfaces may be configured as one
or more "logical" interfaces, which are the entities that a program actually
works with. Each of these logical interfaces is associated with an IP address.
In addition, TCP/IP software supports "loopback" interfaces (127.0.0.1 in IPv4
and ::1 in IPv6), which do not use any physical hardware. Exim requires
knowledge about the host's interfaces for use in three different circumstances:

 1. When a listening daemon is started, Exim needs to know which interfaces and
    ports to listen on.

 2. When Exim is routing an address, it needs to know which IP addresses are
    associated with local interfaces. This is required for the correct
    processing of MX lists by removing the local host and others with the same
    or higher priority values. Also, Exim needs to detect cases when an address
    is routed to an IP address that in fact belongs to the local host. Unless
    the self router option or the allow_localhost option of the smtp transport
    is set (as appropriate), this is treated as an error situation.

 3. When Exim connects to a remote host, it may need to know which interface to
    use for the outgoing connection.

Exim's default behaviour is likely to be appropriate in the vast majority of
cases. If your host has only one interface, and you want all its IP addresses
to be treated in the same way, and you are using only the standard SMTP port,
you should not need to take any special action. The rest of this chapter does
not apply to you.

In a more complicated situation you may want to listen only on certain
interfaces, or on different ports, and for this reason there are a number of
options that can be used to influence Exim's behaviour. The rest of this
chapter describes how they operate.

When a message is received over TCP/IP, the interface and port that were
actually used are set in $received_ip_address and $received_port.


13.1 Starting a listening daemon
--------------------------------

When a listening daemon is started (by means of the -bd command line option),
the interfaces and ports on which it listens are controlled by the following
options:

  * daemon_smtp_ports contains a list of default ports or service names. (For
    backward compatibility, this option can also be specified in the singular.)

  * local_interfaces contains list of interface IP addresses on which to
    listen. Each item may optionally also specify a port.

The default list separator in both cases is a colon, but this can be changed as
described in section 6.21. When IPv6 addresses are involved, it is usually best
to change the separator to avoid having to double all the colons. For example:

local_interfaces = <; 127.0.0.1 ; \
                      192.168.23.65 ; \
                      ::1 ; \
                      3ffe:ffff:836f::fe86:a061

There are two different formats for specifying a port along with an IP address
in local_interfaces:

 1. The port is added onto the address with a dot separator. For example, to
    listen on port 1234 on two different IP addresses:

    local_interfaces = <; 192.168.23.65.1234 ; \
                          3ffe:ffff:836f::fe86:a061.1234

 2. The IP address is enclosed in square brackets, and the port is added with a
    colon separator, for example:

    local_interfaces = <; [192.168.23.65]:1234 ; \
                          [3ffe:ffff:836f::fe86:a061]:1234

When a port is not specified, the value of daemon_smtp_ports is used. The
default setting contains just one port:

daemon_smtp_ports = smtp

If more than one port is listed, each interface that does not have its own port
specified listens on all of them. Ports that are listed in daemon_smtp_ports
can be identified either by name (defined in /etc/services) or by number.
However, when ports are given with individual IP addresses in local_interfaces,
only numbers (not names) can be used.


13.2 Special IP listening addresses
-----------------------------------

The addresses 0.0.0.0 and ::0 are treated specially. They are interpreted as
"all IPv4 interfaces" and "all IPv6 interfaces", respectively. In each case,
Exim tells the TCP/IP stack to "listen on all IPvx interfaces" instead of
setting up separate listening sockets for each interface. The default value of 
local_interfaces is

local_interfaces = 0.0.0.0

when Exim is built without IPv6 support; otherwise it is:

local_interfaces = <; ::0 ; 0.0.0.0

Thus, by default, Exim listens on all available interfaces, on the SMTP port.


13.3 Overriding local_interfaces and daemon_smtp_ports
------------------------------------------------------

The -oX command line option can be used to override the values of 
daemon_smtp_ports and/or local_interfaces for a particular daemon instance.
Another way of doing this would be to use macros and the -D option. However, 
-oX can be used by any admin user, whereas modification of the runtime
configuration by -D is allowed only when the caller is root or exim.

The value of -oX is a list of items. The default colon separator can be changed
in the usual way (6.21) if required. If there are any items that do not contain
dots or colons (that is, are not IP addresses), the value of daemon_smtp_ports
is replaced by the list of those items. If there are any items that do contain
dots or colons, the value of local_interfaces is replaced by those items. Thus,
for example,

-oX 1225

overrides daemon_smtp_ports, but leaves local_interfaces unchanged, whereas

-oX 192.168.34.5.1125

overrides local_interfaces, leaving daemon_smtp_ports unchanged. (However,
since local_interfaces now contains no items without ports, the value of 
daemon_smtp_ports is no longer relevant in this example.)


13.4 Support for the submissions (aka SSMTP or SMTPS) protocol
--------------------------------------------------------------

Exim supports the use of TLS-on-connect, used by mail clients in the
"submissions" protocol, historically also known as SMTPS or SSMTP. For some
years, IETF Standards Track documents only blessed the STARTTLS-based
Submission service (port 587) while common practice was to support the same
feature set on port 465, but using TLS-on-connect. If your installation needs
to provide service to mail clients (Mail User Agents, MUAs) then you should
provide service on both the 587 and the 465 TCP ports.

If the tls_on_connect_ports option is set to a list of port numbers or service
names, connections to those ports must first establish TLS, before proceeding
to the application layer use of the SMTP protocol.

The common use of this option is expected to be

tls_on_connect_ports = 465

per RFC 8314. There is also a command line option -tls-on-connect, which forces
all ports to behave in this way when a daemon is started.

Warning: Setting tls_on_connect_ports does not of itself cause the daemon to
listen on those ports. You must still specify them in daemon_smtp_ports, 
local_interfaces, or the -oX option. (This is because tls_on_connect_ports
applies to inetd connections as well as to connections via the daemon.)


13.5 IPv6 address scopes
------------------------

IPv6 addresses have "scopes", and a host with multiple hardware interfaces can,
in principle, have the same link-local IPv6 address on different interfaces.
Thus, additional information is needed, over and above the IP address, to
distinguish individual interfaces. A convention of using a percent sign
followed by something (often the interface name) has been adopted in some
cases, leading to addresses like this:

fe80::202:b3ff:fe03:45c1%eth0

To accommodate this usage, a percent sign followed by an arbitrary string is
allowed at the end of an IPv6 address. By default, Exim calls getaddrinfo() to
convert a textual IPv6 address for actual use. This function recognizes the
percent convention in operating systems that support it, and it processes the
address appropriately. Unfortunately, some older libraries have problems with
getaddrinfo(). If

IPV6_USE_INET_PTON=yes

is set in Local/Makefile (or an OS-dependent Makefile) when Exim is built, Exim
uses inet_pton() to convert a textual IPv6 address for actual use, instead of
getaddrinfo(). (Before version 4.14, it always used this function.) Of course,
this means that the additional functionality of getaddrinfo() - recognizing
scoped addresses - is lost.


13.6 Disabling IPv6
-------------------

Sometimes it happens that an Exim binary that was compiled with IPv6 support is
run on a host whose kernel does not support IPv6. The binary will fall back to
using IPv4, but it may waste resources looking up AAAA records, and trying to
connect to IPv6 addresses, causing delays to mail delivery. If you set the 
disable_ipv6 option true, even if the Exim binary has IPv6 support, no IPv6
activities take place. AAAA records are never looked up, and any IPv6 addresses
that are listed in local_interfaces, data for the manualroute router, etc. are
ignored. If IP literals are enabled, the ipliteral router declines to handle
IPv6 literal addresses.

On the other hand, when IPv6 is in use, there may be times when you want to
disable it for certain hosts or domains. You can use the dns_ipv4_lookup option
to globally suppress the lookup of AAAA records for specified domains, and you
can use the ignore_target_hosts generic router option to ignore IPv6 addresses
in an individual router.


13.7 Examples of starting a listening daemon
--------------------------------------------

The default case in an IPv6 environment is

daemon_smtp_ports = smtp
local_interfaces = <; ::0 ; 0.0.0.0

This specifies listening on the smtp port on all IPv6 and IPv4 interfaces.
Either one or two sockets may be used, depending on the characteristics of the
TCP/IP stack. (This is complicated and messy; for more information, read the
comments in the daemon.c source file.)

To specify listening on ports 25 and 26 on all interfaces:

daemon_smtp_ports = 25 : 26

(leaving local_interfaces at the default setting) or, more explicitly:

local_interfaces = <; ::0.25     ; ::0.26 \
                      0.0.0.0.25 ; 0.0.0.0.26

To listen on the default port on all IPv4 interfaces, and on port 26 on the
IPv4 loopback address only:

local_interfaces = 0.0.0.0 : 127.0.0.1.26

To specify listening on the default port on specific interfaces only:

local_interfaces = 10.0.0.67 : 192.168.34.67

Warning: Such a setting excludes listening on the loopback interfaces.


13.8 Recognizing the local host
-------------------------------

The local_interfaces option is also used when Exim needs to determine whether
or not an IP address refers to the local host. That is, the IP addresses of all
the interfaces on which a daemon is listening are always treated as local.

For this usage, port numbers in local_interfaces are ignored. If either of the
items 0.0.0.0 or ::0 are encountered, Exim gets a complete list of available
interfaces from the operating system, and extracts the relevant (that is, IPv4
or IPv6) addresses to use for checking.

Some systems set up large numbers of virtual interfaces in order to provide
many virtual web servers. In this situation, you may want to listen for email
on only a few of the available interfaces, but nevertheless treat all
interfaces as local when routing. You can do this by setting 
extra_local_interfaces to a list of IP addresses, possibly including the "all"
wildcard values. These addresses are recognized as local, but are not used for
listening. Consider this example:

local_interfaces = <; 127.0.0.1 ; ::1 ; \
                      192.168.53.235 ; \
                      3ffe:2101:12:1:a00:20ff:fe86:a061

extra_local_interfaces = <; ::0 ; 0.0.0.0

The daemon listens on the loopback interfaces and just one IPv4 and one IPv6
address, but all available interface addresses are treated as local when Exim
is routing.

In some environments the local host name may be in an MX list, but with an IP
address that is not assigned to any local interface. In other cases it may be
desirable to treat other host names as if they referred to the local host. Both
these cases can be handled by setting the hosts_treat_as_local option. This
contains host names rather than IP addresses. When a host is referenced during
routing, either via an MX record or directly, it is treated as the local host
if its name matches hosts_treat_as_local, or if any of its IP addresses match 
local_interfaces or extra_local_interfaces.


13.9 Delivering to a remote host
--------------------------------

Delivery to a remote host is handled by the smtp transport. By default, it
allows the system's TCP/IP functions to choose which interface to use (if there
is more than one) when connecting to a remote host. However, the interface
option can be set to specify which interface is used. See the description of
the smtp transport in chapter 30 for more details.



===============================================================================
14. MAIN CONFIGURATION

The first part of the runtime configuration file contains three types of item:

  * Macro definitions: These lines start with an upper case letter. See section
    6.4 for details of macro processing.

  * Named list definitions: These lines start with one of the words
    "domainlist", "hostlist", "addresslist", or "localpartlist". Their use is
    described in section 10.2.4.

  * Main configuration settings: Each setting occupies one line of the file
    (with possible continuations). If any setting is preceded by the word
    "hide", the -bP command line option displays its value to admin users only.
    See section 6.11 for a description of the syntax of these option settings.

This chapter specifies all the main configuration options, along with their
types and default values. For ease of finding a particular option, they appear
in alphabetical order in section 14.23 below. However, because there are now so
many options, they are first listed briefly in functional groups, as an aid to
finding the name of the option you are looking for. Some options are listed in
more than one group.


14.1 Miscellaneous
------------------

add_environment       environment variables
bi_command            to run for -bi command line option
debug_store           do extra internal checks
disable_ipv6          do no IPv6 processing
keep_environment      environment variables
keep_malformed        for broken files - should not happen
localhost_number      for unique message ids in clusters
message_body_newlines retain newlines in $message_body
message_body_visible  how much to show in $message_body
mua_wrapper           run in "MUA wrapper" mode
print_topbitchars     top-bit characters are printing
spool_wireformat      use wire-format spool data files when possible
timezone              force time zone


14.2 Exim parameters
--------------------

exim_group            override compiled-in value
exim_path             override compiled-in value
exim_user             override compiled-in value
primary_hostname      default from uname()
split_spool_directory use multiple directories
spool_directory       override compiled-in value


14.3 Privilege controls
-----------------------

admin_groups                     groups that are Exim admin users
commandline_checks_require_admin require admin for various checks
deliver_drop_privilege           drop root for delivery processes
local_from_check                 insert Sender: if necessary
local_from_prefix                for testing From: for local sender
local_from_suffix                for testing From: for local sender
local_sender_retain              keep Sender: from untrusted user
never_users                      do not run deliveries as these
prod_requires_admin              forced delivery requires admin user
queue_list_requires_admin        queue listing requires admin user
trusted_groups                   groups that are trusted
trusted_users                    users that are trusted


14.4 Logging
------------

event_action           custom logging
hosts_connection_nolog exemption from connect logging
log_file_path          override compiled-in value
log_selector           set/unset optional logging
log_timezone           add timezone to log lines
message_logs           create per-message logs
preserve_message_logs  after message completion
panic_coredump         request coredump on fatal errors
process_log_path       for SIGUSR1 and exiwhat
slow_lookup_log        control logging of slow DNS lookups
syslog_duplication     controls duplicate log lines on syslog
syslog_facility        set syslog "facility" field
syslog_pid             pid in syslog lines
syslog_processname     set syslog "ident" field
syslog_timestamp       timestamp syslog lines
write_rejectlog        control use of message log


14.5 Frozen messages
--------------------

auto_thaw            sets time for retrying frozen messages
freeze_tell          send message when freezing
move_frozen_messages to another directory
timeout_frozen_after keep frozen messages only so long


14.6 Data lookups
-----------------

ibase_servers        InterBase servers
ldap_ca_cert_dir     dir of CA certs to verify LDAP server's
ldap_ca_cert_file    file of CA certs to verify LDAP server's
ldap_cert_file       client cert file for LDAP
ldap_cert_key        client key file for LDAP
ldap_cipher_suite    TLS negotiation preference control
ldap_default_servers used if no server in query
ldap_require_cert    action to take without LDAP server cert
ldap_start_tls       require TLS within LDAP
ldap_version         set protocol version
lookup_open_max      lookup files held open
mysql_servers        default MySQL servers
oracle_servers       Oracle servers
pgsql_servers        default PostgreSQL servers
sqlite_lock_timeout  as it says


14.7 Message ids
----------------

message_id_header_domain used to build Message-ID: header
message_id_header_text   ditto


14.8 Embedded Perl Startup
--------------------------

perl_at_start  always start the interpreter
perl_startup   code to obey when starting Perl
perl_taintmode enable taint mode in Perl


14.9 Daemon
-----------

daemon_smtp_ports      default ports
daemon_startup_retries number of times to retry
daemon_startup_sleep   time to sleep between tries
extra_local_interfaces not necessarily listened on
local_interfaces       on which to listen, with optional ports
notifier_socket        override compiled-in value
pid_file_path          override compiled-in value
queue_run_max          maximum simultaneous queue runners
smtp_backlog_monitor   level to log listen backlog


14.10 Resource control
----------------------

check_log_inodes                 before accepting a message
check_log_space                  before accepting a message
check_spool_inodes               before accepting a message
check_spool_space                before accepting a message
deliver_queue_load_max           no queue deliveries if load high
queue_only_load                  queue incoming if load high
queue_only_load_latch            don't re-evaluate load for each message
queue_run_max                    maximum simultaneous queue runners
remote_max_parallel              parallel SMTP delivery per message
smtp_accept_max                  simultaneous incoming connections
smtp_accept_max_nonmail          non-mail commands
smtp_accept_max_nonmail_hosts    hosts to which the limit applies
smtp_accept_max_per_connection   messages per connection
smtp_accept_max_per_host         connections from one host
smtp_accept_queue                queue mail if more connections
smtp_accept_queue_per_connection queue if more messages per connection
smtp_accept_reserve              only reserve hosts if more connections
smtp_check_spool_space           from SIZE on MAIL command
smtp_connect_backlog             passed to TCP/IP stack
smtp_load_reserve                SMTP from reserved hosts if load high
smtp_reserve_hosts               these are the reserve hosts


14.11 Policy controls
---------------------

acl_not_smtp              ACL for non-SMTP messages
acl_not_smtp_mime         ACL for non-SMTP MIME parts
acl_not_smtp_start        ACL for start of non-SMTP message
acl_smtp_auth             ACL for AUTH
acl_smtp_connect          ACL for connection
acl_smtp_data             ACL for DATA
acl_smtp_data_prdr        ACL for DATA, per-recipient
acl_smtp_dkim             ACL for DKIM verification
acl_smtp_etrn             ACL for ETRN
acl_smtp_expn             ACL for EXPN
acl_smtp_helo             ACL for EHLO or HELO
acl_smtp_mail             ACL for MAIL
acl_smtp_mailauth         ACL for AUTH on MAIL command
acl_smtp_mime             ACL for MIME parts
acl_smtp_notquit          ACL for non-QUIT terminations
acl_smtp_predata          ACL for start of data
acl_smtp_quit             ACL for QUIT
acl_smtp_rcpt             ACL for RCPT
acl_smtp_starttls         ACL for STARTTLS
acl_smtp_vrfy             ACL for VRFY
av_scanner                specify virus scanner
check_rfc2047_length      check length of RFC 2047 "encoded words"
dns_cname_loops           follow CNAMEs returned by resolver
dns_csa_search_limit      control CSA parent search depth
dns_csa_use_reverse       en/disable CSA IP reverse search
header_maxsize            total size of message header
header_line_maxsize       individual header line limit
helo_accept_junk_hosts    allow syntactic junk from these hosts
helo_allow_chars          allow illegal chars in HELO names
helo_lookup_domains       lookup hostname for these HELO names
helo_try_verify_hosts     HELO soft-checked for these hosts
helo_verify_hosts         HELO hard-checked for these hosts
host_lookup               host name looked up for these hosts
host_lookup_order         order of DNS and local name lookups
hosts_proxy               use proxy protocol for these hosts
host_reject_connection    reject connection from these hosts
hosts_treat_as_local      useful in some cluster configurations
local_scan_timeout        timeout for local_scan()
message_size_limit        for all messages
percent_hack_domains      recognize %-hack for these domains
proxy_protocol_timeout    timeout for proxy protocol negotiation
spamd_address             set interface to SpamAssassin
strict_acl_vars           object to unset ACL variables
spf_smtp_comment_template template for $spf_smtp_comment


14.12 Callout cache
-------------------

callout_domain_negative_expire timeout for negative domain cache item
callout_domain_positive_expire timeout for positive domain cache item
callout_negative_expire        timeout for negative address cache item
callout_positive_expire        timeout for positive address cache item
callout_random_local_part      string to use for "random" testing


14.13 TLS
---------

gnutls_compat_mode       use GnuTLS compatibility mode
gnutls_allow_auto_pkcs11 allow GnuTLS to autoload PKCS11 modules
hosts_require_alpn       mandatory ALPN
hosts_require_helo       mandatory HELO/EHLO
openssl_options          adjust OpenSSL compatibility options
tls_advertise_hosts      advertise TLS to these hosts
tls_alpn                 acceptable protocol names
tls_certificate          location of server certificate
tls_crl                  certificate revocation list
tls_dh_max_bits          clamp D-H bit count suggestion
tls_dhparam              DH parameters for server
tls_eccurve              EC curve selection for server
tls_ocsp_file            location of server certificate status proof
tls_on_connect_ports     specify SSMTP (SMTPS) ports
tls_privatekey           location of server private key
tls_remember_esmtp       don't reset after starting TLS
tls_require_ciphers      specify acceptable ciphers
tls_try_verify_hosts     try to verify client certificate
tls_verify_certificates  expected client certificates
tls_verify_hosts         insist on client certificate verify


14.14 Local user handling
-------------------------

finduser_retries    useful in NIS environments
gecos_name          used when creating Sender:
gecos_pattern       ditto
max_username_length for systems that truncate
unknown_login       used when no login name found
unknown_username    ditto
uucp_from_pattern   for recognizing "From " lines
uucp_from_sender    ditto


14.15 All incoming messages (SMTP and non-SMTP)
-----------------------------------------------

header_maxsize        total size of message header
header_line_maxsize   individual header line limit
message_size_limit    applies to all messages
percent_hack_domains  recognize %-hack for these domains
received_header_text  expanded to make Received:
received_headers_max  for mail loop detection
recipients_max        limit per message
recipients_max_reject permanently reject excess recipients


14.16 Non-SMTP incoming messages
--------------------------------

receive_timeout for non-SMTP messages


14.17 Incoming SMTP messages
----------------------------

See also the Policy controls section above.

dkim_verify_hashes               DKIM hash methods accepted for signatures
dkim_verify_keytypes             DKIM key types accepted for signatures
dkim_verify_min_keysizes         DKIM key sizes accepted for signatures
dkim_verify_signers              DKIM domains for which DKIM ACL is run
dmarc_forensic_sender            DMARC sender for report messages
dmarc_history_file               DMARC results log
dmarc_tld_file                   DMARC toplevel domains file
host_lookup                      host name looked up for these hosts
host_lookup_order                order of DNS and local name lookups
recipient_unqualified_hosts      may send unqualified recipients
rfc1413_hosts                    make ident calls to these hosts
rfc1413_query_timeout            zero disables ident calls
sender_unqualified_hosts         may send unqualified senders
smtp_accept_keepalive            some TCP/IP magic
smtp_accept_max                  simultaneous incoming connections
smtp_accept_max_nonmail          non-mail commands
smtp_accept_max_nonmail_hosts    hosts to which the limit applies
smtp_accept_max_per_connection   messages per connection
smtp_accept_max_per_host         connections from one host
smtp_accept_queue                queue mail if more connections
smtp_accept_queue_per_connection queue if more messages per connection
smtp_accept_reserve              only reserve hosts if more connections
smtp_active_hostname             host name to use in messages
smtp_banner                      text for welcome banner
smtp_check_spool_space           from SIZE on MAIL command
smtp_connect_backlog             passed to TCP/IP stack
smtp_enforce_sync                of SMTP command/responses
smtp_etrn_command                what to run for ETRN
smtp_etrn_serialize              only one at once
smtp_load_reserve                only reserve hosts if this load
smtp_max_unknown_commands        before dropping connection
smtp_ratelimit_hosts             apply ratelimiting to these hosts
smtp_ratelimit_mail              ratelimit for MAIL commands
smtp_ratelimit_rcpt              ratelimit for RCPT commands
smtp_receive_timeout             per command or data line
smtp_reserve_hosts               these are the reserve hosts
smtp_return_error_details        give detail on rejections


14.18 SMTP extensions
---------------------

accept_8bitmime                    advertise 8BITMIME
auth_advertise_hosts               advertise AUTH to these hosts
chunking_advertise_hosts           advertise CHUNKING to these hosts
dsn_advertise_hosts                advertise DSN extensions to these hosts
ignore_fromline_hosts              allow "From " from these hosts
ignore_fromline_local              allow "From " from local SMTP
pipelining_advertise_hosts         advertise pipelining to these hosts
pipelining_connect_advertise_hosts advertise pipelining to these hosts
prdr_enable                        advertise PRDR to all hosts
smtputf8_advertise_hosts           advertise SMTPUTF8 to these hosts
tls_advertise_hosts                advertise TLS to these hosts


14.19 Processing messages
-------------------------

allow_domain_literals              recognize domain literal syntax
allow_mx_to_ip                     allow MX to point to IP address
allow_utf8_domains                 in addresses
check_rfc2047_length               check length of RFC 2047 "encoded words"
delivery_date_remove               from incoming messages
envelope_to_remove                 from incoming messages
extract_addresses_remove_arguments affects -t processing
headers_charset                    default for translations
qualify_domain                     default for senders
qualify_recipient                  default for recipients
return_path_remove                 from incoming messages
strip_excess_angle_brackets        in addresses
strip_trailing_dot                 at end of addresses
untrusted_set_sender               untrusted can set envelope sender


14.20 System filter
-------------------

system_filter                     locate system filter
system_filter_directory_transport transport for delivery to a directory
system_filter_file_transport      transport for delivery to a file
system_filter_group               group for filter running
system_filter_pipe_transport      transport for delivery to a pipe
system_filter_reply_transport     transport for autoreply delivery
system_filter_user                user for filter running


14.21 Routing and delivery
--------------------------

disable_ipv6             do no IPv6 processing
dns_again_means_nonexist for broken domains
dns_check_names_pattern  pre-DNS syntax check
dns_dnssec_ok            parameter for resolver
dns_ipv4_lookup          only v4 lookup for these domains
dns_retrans              parameter for resolver
dns_retry                parameter for resolver
dns_trust_aa             DNS zones trusted as authentic
dns_use_edns0            parameter for resolver
hold_domains             hold delivery for these domains
local_interfaces         for routing checks
queue_domains            no immediate delivery for these
queue_fast_ramp          parallel delivery with 2-phase queue run
queue_only               no immediate delivery at all
queue_only_file          no immediate delivery if file exists
queue_only_load          no immediate delivery if load is high
queue_only_load_latch    don't re-evaluate load for each message
queue_only_override      allow command line to override
queue_run_in_order       order of arrival
queue_run_max            of simultaneous queue runners
queue_smtp_domains       no immediate SMTP delivery for these
remote_max_parallel      parallel SMTP delivery per message
remote_sort_domains      order of remote deliveries
retry_data_expire        timeout for retry data
retry_interval_max       safety net for retry rules


14.22 Bounce and warning messages
---------------------------------

bounce_message_file          content of bounce
bounce_message_text          content of bounce
bounce_return_body           include body if returning message
bounce_return_linesize_limit limit on returned message line length
bounce_return_message        include original message in bounce
bounce_return_size_limit     limit on returned message
bounce_sender_authentication send authenticated sender with bounce
dsn_from                     set From: contents in bounces
errors_copy                  copy bounce messages
errors_reply_to              Reply-to: in bounces
delay_warning                time schedule
delay_warning_condition      condition for warning messages
ignore_bounce_errors_after   discard undeliverable bounces
smtp_return_error_details    give detail on rejections
warn_message_file            content of warning message


14.23 Alphabetical list of main options
---------------------------------------

Those options that undergo string expansion before use are marked with *.

+-----------------------------------------------------+
|accept_8bitmime|Use: main|Type: boolean|Default: true|
+-----------------------------------------------------+

This option causes Exim to send 8BITMIME in its response to an SMTP EHLO
command, and to accept the BODY= parameter on MAIL commands. However, though
Exim is 8-bit clean, it is not a protocol converter, and it takes no steps to
do anything special with messages received by this route.

Historically Exim kept this option off by default, but the maintainers feel
that in today's Internet, this causes more problems than it solves. It now
defaults to true. A more detailed analysis of the issues is provided by Dan
Bernstein:

https://cr.yp.to/smtp/8bitmime.html

To log received 8BITMIME status use

log_selector = +8bitmime

+---------------------------------------------------+
|acl_not_smtp|Use: main|Type: string*|Default: unset|
+---------------------------------------------------+

This option defines the ACL that is run when a non-SMTP message has been read
and is on the point of being accepted. See chapter 44 for further details.

+--------------------------------------------------------+
|acl_not_smtp_mime|Use: main|Type: string*|Default: unset|
+--------------------------------------------------------+

This option defines the ACL that is run for individual MIME parts of non-SMTP
messages. It operates in exactly the same way as acl_smtp_mime operates for
SMTP messages.

+---------------------------------------------------------+
|acl_not_smtp_start|Use: main|Type: string*|Default: unset|
+---------------------------------------------------------+

This option defines the ACL that is run before Exim starts reading a non-SMTP
message. See chapter 44 for further details.

+----------------------------------------------------+
|acl_smtp_auth|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run when an SMTP AUTH command is received.
See chapter 44 for further details.

+-------------------------------------------------------+
|acl_smtp_connect|Use: main|Type: string*|Default: unset|
+-------------------------------------------------------+

This option defines the ACL that is run when an SMTP connection is received.
See chapter 44 for further details.

+----------------------------------------------------+
|acl_smtp_data|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run after an SMTP DATA command has been
processed and the message itself has been received, but before the final
acknowledgment is sent. See chapter 44 for further details.

+----------------------------------------------------------+
|acl_smtp_data_prdr|Use: main|Type: string*|Default: accept|
+----------------------------------------------------------+

This option defines the ACL that, if the PRDR feature has been negotiated, is
run for each recipient after an SMTP DATA command has been processed and the
message itself has been received, but before the acknowledgment is sent. See
chapter 44 for further details.

+----------------------------------------------------+
|acl_smtp_dkim|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run for each DKIM signature (by default, or
as specified in the dkim_verify_signers option) of a received message. See
section 58.1.2 for further details.

+----------------------------------------------------+
|acl_smtp_etrn|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run when an SMTP ETRN command is received.
See chapter 44 for further details.

+----------------------------------------------------+
|acl_smtp_expn|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run when an SMTP EXPN command is received.
See chapter 44 for further details.

+----------------------------------------------------+
|acl_smtp_helo|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run when an SMTP EHLO or HELO command is
received. See chapter 44 for further details.

+----------------------------------------------------+
|acl_smtp_mail|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run when an SMTP MAIL command is received.
See chapter 44 for further details.

+--------------------------------------------------------+
|acl_smtp_mailauth|Use: main|Type: string*|Default: unset|
+--------------------------------------------------------+

This option defines the ACL that is run when there is an AUTH parameter on a
MAIL command. See chapter 44 for details of ACLs, and chapter 33 for details of
authentication.

+----------------------------------------------------+
|acl_smtp_mime|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option is available when Exim is built with the content-scanning
extension. It defines the ACL that is run for each MIME part in a message. See
section 45.4 for details.

+-------------------------------------------------------+
|acl_smtp_notquit|Use: main|Type: string*|Default: unset|
+-------------------------------------------------------+

This option defines the ACL that is run when an SMTP session ends without a
QUIT command being received. See chapter 44 for further details.

+-------------------------------------------------------+
|acl_smtp_predata|Use: main|Type: string*|Default: unset|
+-------------------------------------------------------+

This option defines the ACL that is run when an SMTP DATA command is received,
before the message itself is received. See chapter 44 for further details.

+----------------------------------------------------+
|acl_smtp_quit|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run when an SMTP QUIT command is received.
See chapter 44 for further details.

+----------------------------------------------------+
|acl_smtp_rcpt|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run when an SMTP RCPT command is received.
See chapter 44 for further details.

+--------------------------------------------------------+
|acl_smtp_starttls|Use: main|Type: string*|Default: unset|
+--------------------------------------------------------+

This option defines the ACL that is run when an SMTP STARTTLS command is
received. See chapter 44 for further details.

+----------------------------------------------------+
|acl_smtp_vrfy|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option defines the ACL that is run when an SMTP VRFY command is received.
See chapter 44 for further details.

+----------------------------------------------------------+
|add_environment|Use: main|Type: string list|Default: empty|
+----------------------------------------------------------+

This option adds individual environment variables that the currently linked
libraries and programs in child processes may use. Each list element should be
of the form "name=value".

See 29.4 for the environment of pipe transports.

+--------------------------------------------------------+
|admin_groups|Use: main|Type: string list*|Default: unset|
+--------------------------------------------------------+

This option is expanded just once, at the start of Exim's processing. If the
current group or any of the supplementary groups of an Exim caller is in this
colon-separated list, the caller has admin privileges. If all your system
programmers are in a specific group, for example, you can give them all Exim
admin privileges by putting that group in admin_groups. However, this does not
permit them to read Exim's spool files (whose group owner is the Exim gid). To
permit this, you have to add individuals to the Exim group.

+------------------------------------------------------------+
|allow_domain_literals|Use: main|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is set, the RFC 2822 domain literal format is permitted in email
addresses. The option is not set by default, because the domain literal format
is not normally required these days, and few people know about it. It has,
however, been exploited by mail abusers.

Unfortunately, it seems that some DNS black list maintainers are using this
format to report black listing to postmasters. If you want to accept messages
addressed to your hosts by IP address, you need to set allow_domain_literals
true, and also to add "@[]" to the list of local domains (defined in the named
domain list local_domains in the default configuration). This "magic string"
matches the domain literal form of all the local host's IP addresses.

+-----------------------------------------------------+
|allow_mx_to_ip|Use: main|Type: boolean|Default: false|
+-----------------------------------------------------+

It appears that more and more DNS zone administrators are breaking the rules
and putting domain names that look like IP addresses on the right hand side of
MX records. Exim follows the rules and rejects this, giving an error message
that explains the misconfiguration. However, some other MTAs support this
practice, so to avoid "Why can't Exim do this?" complaints, allow_mx_to_ip
exists, in order to enable this heinous activity. It is not recommended, except
when you have no other choice.

+---------------------------------------------------------+
|allow_utf8_domains|Use: main|Type: boolean|Default: false|
+---------------------------------------------------------+

Lots of discussion is going on about internationalized domain names. One camp
is strongly in favour of just using UTF-8 characters, and it seems that at
least two other MTAs permit this. This option allows Exim users to experiment
if they wish.

If it is set true, Exim's domain parsing function allows valid UTF-8
multicharacters to appear in domain name components, in addition to letters,
digits, and hyphens.

If Exim is built with internationalization support and the SMTPUTF8 ESMTP
option is in use (see chapter 60) this option can be left as default. Without
that, if you want to look up such domain names in the DNS, you must also adjust
the value of dns_check_names_pattern to match the extended form. A suitable
setting is:

dns_check_names_pattern = (?i)^(?>(?(1)\.|())[a-z0-9\xc0-\xff]\
  (?>[-a-z0-9\x80-\xff]*[a-z0-9\x80-\xbf])?)+$

Alternatively, you can just disable this feature by setting

dns_check_names_pattern =

That is, set the option to an empty string so that no check is done.

+----------------------------------------------------------+
|auth_advertise_hosts|Use: main|Type: host list*|Default: *|
+----------------------------------------------------------+

If any server authentication mechanisms are configured, Exim advertises them in
response to an EHLO command only if the calling host matches this list.
Otherwise, Exim does not advertise AUTH. Exim does not accept AUTH commands
from clients to which it has not advertised the availability of AUTH. The
advertising of individual authentication mechanisms can be controlled by the
use of the server_advertise_condition generic authenticator option on the
individual authenticators. See chapter 33 for further details.

Certain mail clients (for example, Netscape) require the user to provide a name
and password for authentication if AUTH is advertised, even though it may not
be needed (the host may accept messages from hosts on its local LAN without
authentication, for example). The auth_advertise_hosts option can be used to
make these clients more friendly by excluding them from the set of hosts to
which Exim advertises AUTH.

If you want to advertise the availability of AUTH only when the connection is
encrypted using TLS, you can make use of the fact that the value of this option
is expanded, with a setting like this:

auth_advertise_hosts = ${if eq{$tls_in_cipher}{}{}{*}}

If $tls_in_cipher is empty, the session is not encrypted, and the result of the
expansion is empty, thus matching no hosts. Otherwise, the result of the
expansion is *, which matches all hosts.

+------------------------------------------+
|auto_thaw|Use: main|Type: time|Default: 0s|
+------------------------------------------+

If this option is set to a time greater than zero, a queue runner will try a
new delivery attempt on any frozen message, other than a bounce message, if
this much time has passed since it was frozen. This may result in the message
being re-frozen if nothing has changed since the last attempt. It is a way of
saying "keep on trying, even though there are big problems".

Note: This is an old option, which predates timeout_frozen_after and 
ignore_bounce_errors_after. It is retained for compatibility, but it is not
thought to be very useful any more, and its use should probably be avoided.

+----------------------------------------------------+
|av_scanner|Use: main|Type: string|Default: see below|
+----------------------------------------------------+

This option is available if Exim is built with the content-scanning extension.
It specifies which anti-virus scanner to use. The default value is:

sophie:/var/run/sophie

If the value of av_scanner starts with a dollar character, it is expanded
before use. See section 45.1 for further details.

+------------------------------------------------+
|bi_command|Use: main|Type: string|Default: unset|
+------------------------------------------------+

This option supplies the name of a command that is run when Exim is called with
the -bi option (see chapter 5). The string value is just the command name, it
is not a complete command line. If an argument is required, it must come from
the -oA command line option.

+----------------------------------------------------------+
|bounce_message_file|Use: main|Type: string*|Default: unset|
+----------------------------------------------------------+

This option defines a template file containing paragraphs of text to be used
for constructing bounce messages. Details of the file's contents are given in
chapter 50. The option is expanded to give the file path, which must be
absolute and untainted. See also warn_message_file.

+---------------------------------------------------------+
|bounce_message_text|Use: main|Type: string|Default: unset|
+---------------------------------------------------------+

When this option is set, its contents are included in the default bounce
message immediately after "This message was created automatically by mail
delivery software." It is not used if bounce_message_file is set.

+--------------------------------------------------------+
|bounce_return_body|Use: main|Type: boolean|Default: true|
+--------------------------------------------------------+

This option controls whether the body of an incoming message is included in a
bounce message when bounce_return_message is true. The default setting causes
the entire message, both header and body, to be returned (subject to the value
of bounce_return_size_limit). If this option is false, only the message header
is included. In the case of a non-SMTP message containing an error that is
detected during reception, only those header lines preceding the point at which
the error was detected are returned.

+-----------------------------------------------------------------+
|bounce_return_linesize_limit|Use: main|Type: integer|Default: 998|
+-----------------------------------------------------------------+

This option sets a limit in bytes on the line length of messages that are
returned to senders due to delivery problems, when bounce_return_message is
true. The default value corresponds to RFC limits. If the message being
returned has lines longer than this value it is treated as if the 
bounce_return_size_limit (below) restriction was exceeded.

The option also applies to bounces returned when an error is detected during
reception of a message. In this case lines from the original are truncated.

The option does not apply to messages generated by an autoreply transport.

+-----------------------------------------------------------+
|bounce_return_message|Use: main|Type: boolean|Default: true|
+-----------------------------------------------------------+

If this option is set false, none of the original message is included in bounce
messages generated by Exim. See also bounce_return_size_limit and 
bounce_return_body.

+--------------------------------------------------------------+
|bounce_return_size_limit|Use: main|Type: integer|Default: 100K|
+--------------------------------------------------------------+

This option sets a limit in bytes on the size of messages that are returned to
senders as part of bounce messages when bounce_return_message is true. The
limit should be less than the value of the global message_size_limit and of any
message_size_limit settings on transports, to allow for the bounce text that
Exim generates. If this option is set to zero there is no limit.

When the body of any message that is to be included in a bounce message is
greater than the limit, it is truncated, and a comment pointing this out is
added at the top. The actual cutoff may be greater than the value given, owing
to the use of buffering for transferring the message in chunks (typically 8K in
size). The idea is to save bandwidth on those undeliverable 15-megabyte
messages.

+------------------------------------------------------------------+
|bounce_sender_authentication|Use: main|Type: string|Default: unset|
+------------------------------------------------------------------+

This option provides an authenticated sender address that is sent with any
bounce messages generated by Exim that are sent over an authenticated SMTP
connection. A typical setting might be:

bounce_sender_authentication = mailer-daemon@my.domain.example

which would cause bounce messages to be sent using the SMTP command:

MAIL FROM:<> AUTH=mailer-daemon@my.domain.example

The value of bounce_sender_authentication must always be a complete email
address.

+---------------------------------------------------------------+
|callout_domain_negative_expire|Use: main|Type: time|Default: 3h|
+---------------------------------------------------------------+

This option specifies the expiry time for negative callout cache data for a
domain. See section 44.22 for details of callout verification, and section
44.22.2 for details of the caching.

+---------------------------------------------------------------+
|callout_domain_positive_expire|Use: main|Type: time|Default: 7d|
+---------------------------------------------------------------+

This option specifies the expiry time for positive callout cache data for a
domain. See section 44.22 for details of callout verification, and section
44.22.2 for details of the caching.

+--------------------------------------------------------+
|callout_negative_expire|Use: main|Type: time|Default: 2h|
+--------------------------------------------------------+

This option specifies the expiry time for negative callout cache data for an
address. See section 44.22 for details of callout verification, and section
44.22.2 for details of the caching.

+---------------------------------------------------------+
|callout_positive_expire|Use: main|Type: time|Default: 24h|
+---------------------------------------------------------+

This option specifies the expiry time for positive callout cache data for an
address. See section 44.22 for details of callout verification, and section
44.22.2 for details of the caching.

+--------------------------------------------------------------------+
|callout_random_local_part|Use: main|Type: string*|Default: see below|
+--------------------------------------------------------------------+

This option defines the "random" local part that can be used as part of callout
verification. The default value is

$primary_hostname-$tod_epoch-testing

See section 44.22.1 for details of how this value is used.

+-----------------------------------------------------+
|check_log_inodes|Use: main|Type: integer|Default: 100|
|----------------+---------+-------------+------------|
|check_log_space |Use: main|Type: integer|Default: 10M|
+-----------------------------------------------------+

See check_spool_space below.

+----------------------------------------------------------+
|check_rfc2047_length|Use: main|Type: boolean|Default: true|
+----------------------------------------------------------+

RFC 2047 defines a way of encoding non-ASCII characters in headers using a
system of "encoded words". The RFC specifies a maximum length for an encoded
word; strings to be encoded that exceed this length are supposed to use
multiple encoded words. By default, Exim does not recognize encoded words that
exceed the maximum length. However, it seems that some software, in violation
of the RFC, generates overlong encoded words. If check_rfc2047_length is set
false, Exim recognizes encoded words of any length.

+-------------------------------------------------------+
|check_spool_inodes|Use: main|Type: integer|Default: 100|
|------------------+---------+-------------+------------|
|check_spool_space |Use: main|Type: integer|Default: 10M|
+-------------------------------------------------------+

The four check_... options allow for checking of disk resources before a
message is accepted.

When any of these options are nonzero, they apply to all incoming messages. If
you want to apply different checks to different kinds of message, you can do so
by testing the variables $log_inodes, $log_space, $spool_inodes, and
$spool_space in an ACL with appropriate additional conditions.

check_spool_space and check_spool_inodes check the spool partition if either
value is greater than zero, for example:

check_spool_space = 100M
check_spool_inodes = 100

The spool partition is the one that contains the directory defined by
SPOOL_DIRECTORY in Local/Makefile. It is used for holding messages in transit.

check_log_space and check_log_inodes check the partition in which log files are
written if either is greater than zero. These should be set only if 
log_file_path and spool_directory refer to different partitions.

If there is less space or fewer inodes than requested, Exim refuses to accept
incoming mail. In the case of SMTP input this is done by giving a 452 temporary
error response to the MAIL command. If ESMTP is in use and there was a SIZE
parameter on the MAIL command, its value is added to the check_spool_space
value, and the check is performed even if check_spool_space is zero, unless 
no_smtp_check_spool_space is set.

The values for check_spool_space and check_log_space are held as a number of
kilobytes (though specified in bytes). If a non-multiple of 1024 is specified,
it is rounded up.

For non-SMTP input and for batched SMTP input, the test is done at start-up; on
failure a message is written to stderr and Exim exits with a non-zero code, as
it obviously cannot send an error message of any kind.

There is a slight performance penalty for these checks. Versions of Exim
preceding 4.88 had these disabled by default; high-rate installations confident
they will never run out of resources may wish to deliberately disable them.

+--------------------------------------------------------------+
|chunking_advertise_hosts|Use: main|Type: host list*|Default: *|
+--------------------------------------------------------------+

The CHUNKING extension (RFC3030) will be advertised in the EHLO message to
these hosts. Hosts may use the BDAT command as an alternate to DATA.

+-------------------------------------------------------------------------+
|commandline_checks_require_admin|Use: main|Type: boolean|Default: "false"|
+-------------------------------------------------------------------------+

This option restricts various basic checking features to require an
administrative user. This affects most of the -b* options, such as -be.

+----------------------------------------------------+
|debug_store|Use: main|Type: boolean|Default: "false"|
+----------------------------------------------------+

This option, when true, enables extra checking in Exim's internal memory
management. For use when a memory corruption issue is being investigated, it
should normally be left as default.

+--------------------------------------------------------+
|daemon_smtp_ports|Use: main|Type: string|Default: "smtp"|
+--------------------------------------------------------+

This option specifies one or more default SMTP ports on which the Exim daemon
listens. See chapter 13 for details of how it is used. For backward
compatibility, daemon_smtp_port (singular) is a synonym.

+-----------------------------------------------------------+
|daemon_startup_retries|Use: main|Type: integer|  Default: 9|
|----------------------+---------+-------------+------------|
|daemon_startup_sleep  |Use: main| Type: time  |Default: 30s|
+-----------------------------------------------------------+

These options control the retrying done by the daemon at startup when it cannot
immediately bind a listening socket (typically because the socket is already in
use): daemon_startup_retries defines the number of retries after the first
failure, and daemon_startup_sleep defines the length of time to wait between
retries.

+----------------------------------------------------+
|delay_warning|Use: main|Type: time list|Default: 24h|
+----------------------------------------------------+

When a message is delayed, Exim sends a warning message to the sender at
intervals specified by this option. The data is a colon-separated list of times
after which to send warning messages. If the value of the option is an empty
string or a zero time, no warnings are sent. Up to 10 times may be given. If a
message has been in the queue for longer than the last time, the last interval
between the times is used to compute subsequent warning times. For example,
with

delay_warning = 4h:8h:24h

the first message is sent after 4 hours, the second after 8 hours, and the
third one after 24 hours. After that, messages are sent every 16 hours, because
that is the interval between the last two times on the list. If you set just
one time, it specifies the repeat interval. For example, with:

delay_warning = 6h

messages are repeated every six hours. To stop warnings after a given time, set
a very large time at the end of the list. For example:

delay_warning = 2h:12h:99d

Note that the option is only evaluated at the time a delivery attempt fails,
which depends on retry and queue-runner configuration. Typically retries will
be configured more frequently than warning messages.

+------------------------------------------------------------------+
|delay_warning_condition|Use: main|Type: string*|Default: see below|
+------------------------------------------------------------------+

The string is expanded at the time a warning message might be sent. If all the
deferred addresses have the same domain, it is set in $domain during the
expansion. Otherwise $domain is empty. If the result of the expansion is a
forced failure, an empty string, or a string matching any of "0", "no" or
"false" (the comparison being done caselessly) then the warning message is not
sent. The default is:

delay_warning_condition = ${if or {\
  { !eq{$h_list-id:$h_list-post:$h_list-subscribe:}{} }\
  { match{$h_precedence:}{(?i)bulk|list|junk} }\
  { match{$h_auto-submitted:}{(?i)auto-generated|auto-replied} }\
  } {no}{yes}}

This suppresses the sending of warnings for messages that contain List-ID:, 
List-Post:, or List-Subscribe: headers, or have "bulk", "list" or "junk" in a 
Precedence: header, or have "auto-generated" or "auto-replied" in an 
Auto-Submitted: header.

+-------------------------------------------------------------+
|deliver_drop_privilege|Use: main|Type: boolean|Default: false|
+-------------------------------------------------------------+

If this option is set true, Exim drops its root privilege at the start of a
delivery process, and runs as the Exim user throughout. This severely restricts
the kinds of local delivery that are possible, but is viable in certain types
of configuration. There is a discussion about the use of root privilege in
chapter 56.

+-----------------------------------------------------------------+
|deliver_queue_load_max|Use: main|Type: fixed-point|Default: unset|
+-----------------------------------------------------------------+

When this option is set, a queue run is abandoned if the system load average
becomes greater than the value of the option. The option has no effect on
ancient operating systems on which Exim cannot determine the load average. See
also queue_only_load and smtp_load_reserve.

+----------------------------------------------------------+
|delivery_date_remove|Use: main|Type: boolean|Default: true|
+----------------------------------------------------------+

Exim's transports have an option for adding a Delivery-date: header to a
message when it is delivered, in exactly the same way as Return-path: is
handled. Delivery-date: records the actual time of delivery. Such headers
should not be present in incoming messages, and this option causes them to be
removed at the time the message is received, to avoid any problems that might
occur when a delivered message is subsequently sent on to some other recipient.

+----------------------------------------------------+
|disable_fsync|Use: main|Type: boolean|Default: false|
+----------------------------------------------------+

This option is available only if Exim was built with the compile-time option
ENABLE_DISABLE_FSYNC. When this is not set, a reference to disable_fsync in a
runtime configuration generates an "unknown option" error. You should not build
Exim with ENABLE_DISABLE_FSYNC or set disable_fsync unless you really, really,
really understand what you are doing. No pre-compiled distributions of Exim
should ever make this option available.

When disable_fsync is set true, Exim no longer calls fsync() to force updated
files' data to be written to disc before continuing. Unexpected events such as
crashes and power outages may cause data to be lost or scrambled. Here be
Dragons. Beware.

+---------------------------------------------------+
|disable_ipv6|Use: main|Type: boolean|Default: false|
+---------------------------------------------------+

If this option is set true, even if the Exim binary has IPv6 support, no IPv6
activities take place. AAAA records are never looked up, and any IPv6 addresses
that are listed in local_interfaces, data for the manualroute router, etc. are
ignored. If IP literals are enabled, the ipliteral router declines to handle
IPv6 literal addresses.

+-----------------------------------------------------------------------+
|dkim_verify_hashes|Use: main|Type: string list|Default: sha256 : sha512|
+-----------------------------------------------------------------------+

This option gives a list of hash types which are acceptable in signatures, and
an order of processing. Signatures with algorithms not in the list will be
ignored.

Acceptable values include:

sha1
sha256
sha512

Note that the acceptance of sha1 violates RFC 8301.

+-----------------------------------------------------------------------+
|dkim_verify_keytypes|Use: main|Type: string list|Default: ed25519 : rsa|
+-----------------------------------------------------------------------+

This option gives a list of key types which are acceptable in signatures, and
an order of processing. Signatures with algorithms not in the list will be
ignored.

+-----------------------------------------------------------------------------+
|dkim_verify_min_keysizes| Use:   |  Type: string  |Default: rsa=1024 ed25519=|
|                        |  main  |      list      |                       250|
+-----------------------------------------------------------------------------+

This option gives a list of key sizes which are acceptable in signatures. The
list is keyed by the algorithm type for the key; the values are in bits.
Signatures with keys smaller than given by this option will fail verification.

The default enforces the RFC 8301 minimum key size for RSA signatures.

+----------------------------------------------------------+
|dkim_verify_minimal|Use: main|Type: boolean|Default: false|
+----------------------------------------------------------+

If set to true, verification of signatures will terminate after the first
success.

+-----------------------------------------------------------------------+
|dkim_verify_signers|Use: main|Type: domain list*|Default: $dkim_signers|
+-----------------------------------------------------------------------+

This option gives a list of DKIM domains for which the DKIM ACL is run. It is
expanded after the message is received; by default it runs the ACL once for
each signature in the message. See section 58.1.2.

+------------------------------------------------------------+
|dmarc_forensic_sender|Use: main|Type: string*|Default: unset|
+------------------------------------------------------------+

+--------------------------------------------------------+
|dmarc_history_file|Use: main|Type: string|Default: unset|
+--------------------------------------------------------+

+----------------------------------------------------+
|dmarc_tld_file|Use: main|Type: string|Default: unset|
+----------------------------------------------------+

These options control DMARC processing. See section 58.3 for details.

+--------------------------------------------------------------------+
|dns_again_means_nonexist|Use: main|Type: domain list*|Default: unset|
+--------------------------------------------------------------------+

DNS lookups give a "try again" response for the DNS errors "non-authoritative
host not found" and "SERVERFAIL". This can cause Exim to keep trying to deliver
a message, or to give repeated temporary errors to incoming mail. Sometimes the
effect is caused by a badly set up name server and may persist for a long time.
If a domain which exhibits this problem matches anything in 
dns_again_means_nonexist, it is treated as if it did not exist. This option
should be used with care. You can make it apply to reverse lookups by a setting
such as this:

dns_again_means_nonexist = *.in-addr.arpa

This option applies to all DNS lookups that Exim does,

except for TLSA lookups (where knowing about such failures is
security-relevant).

It also applies when the gethostbyname() or getipnodebyname() functions give
temporary errors, since these are most likely to be caused by DNS lookup
problems. The dnslookup router has some options of its own for controlling what
happens when lookups for MX or SRV records give temporary errors. These more
specific options are applied after this global option.

+-----------------------------------------------------------------+
|dns_check_names_pattern|Use: main|Type: string|Default: see below|
+-----------------------------------------------------------------+

When this option is set to a non-empty string, it causes Exim to check domain
names for characters that are not allowed in host names before handing them to
the DNS resolver, because some resolvers give temporary errors for names that
contain unusual characters. If a domain name contains any unwanted characters,
a "not found" result is forced, and the resolver is not called. The check is
done by matching the domain name against a regular expression, which is the
value of this option. The default pattern is

dns_check_names_pattern = \
  (?i)^(?>(?(1)\.|())[^\W_](?>[a-z0-9/-]*[^\W_])?)+$

which permits only letters, digits, slashes, and hyphens in components, but
they must start and end with a letter or digit. Slashes are not, in fact,
permitted in host names, but they are found in certain NS records (which can be
accessed in Exim by using a dnsdb lookup). If you set allow_utf8_domains, you
must modify this pattern, or set the option to an empty string.

+-------------------------------------------------------+
|dns_csa_search_limit|Use: main|Type: integer|Default: 5|
+-------------------------------------------------------+

This option controls the depth of parental searching for CSA SRV records in the
DNS, as described in more detail in section 44.26.

+---------------------------------------------------------+
|dns_csa_use_reverse|Use: main|Type: boolean|Default: true|
+---------------------------------------------------------+

This option controls whether or not an IP address, given as a CSA domain, is
reversed and looked up in the reverse DNS, as described in more detail in
section 44.26.

+--------------------------------------------------+
|dns_cname_loops|Use: main|Type: integer|Default: 1|
+--------------------------------------------------+

This option controls the following of CNAME chains, needed if the resolver does
not do it internally. As of 2018 most should, and the default can be left. If
you have an ancient one, a value of 10 is likely needed.

The default value of one CNAME-follow is needed thanks to the observed return
for an MX request, given no MX presence but a CNAME to an A, of the CNAME.

+-------------------------------------------------+
|dns_dnssec_ok|Use: main|Type: integer|Default: -1|
+-------------------------------------------------+

If this option is set to a non-negative number then Exim will initialise the
DNS resolver library to either use or not use DNSSEC, overriding the system
default. A value of 0 coerces DNSSEC off, a value of 1 coerces DNSSEC on.

If the resolver library does not support DNSSEC then this option has no effect.

On Linux with glibc 2.31 or newer this is insufficient, the resolver library
will default to stripping out a successful validation status. This will break a
previously working Exim installation. Provided that you do trust the resolver
(ie, is on localhost) you can tell glibc to pass through any successful
validation with a new option in /etc/resolv.conf:

options trust-ad

+-----------------------------------------------------------+
|dns_ipv4_lookup|Use: main|Type: domain list*|Default: unset|
+-----------------------------------------------------------+

When Exim is compiled with IPv6 support and disable_ipv6 is not set, it looks
for IPv6 address records (AAAA records) as well as IPv4 address records (A
records) when trying to find IP addresses for hosts, unless the host's domain
matches this list.

This is a fudge to help with name servers that give big delays or otherwise do
not work for the AAAA record type. In due course, when the world's name servers
have all been upgraded, there should be no need for this option. Note that all
lookups, including those done for verification, are affected; this will result
in verify failure for IPv6 connections or ones using names only valid for IPv6
addresses.

+--------------------------------------------+
|dns_retrans|Use: main|Type: time|Default: 0s|
+--------------------------------------------+

The options dns_retrans and dns_retry can be used to set the retransmission and
retry parameters for DNS lookups. Values of zero (the defaults) leave the
system default settings unchanged. The first value is the time between retries,
and the second is the number of retries. It isn't totally clear exactly how
these settings affect the total time a DNS lookup may take. I haven't found any
documentation about timeouts on DNS lookups; these parameter values are
available in the external resolver interface structure, but nowhere does it
seem to describe how they are used or what you might want to set in them. See
also the slow_lookup_log option.

+--------------------------------------------+
|dns_retry|Use: main|Type: integer|Default: 0|
+--------------------------------------------+

See dns_retrans above.

+--------------------------------------------------------+
|dns_trust_aa|Use: main|Type: domain list*|Default: unset|
+--------------------------------------------------------+

If this option is set then lookup results marked with the AA bit (Authoritative
Answer) are trusted the same way as if they were DNSSEC-verified. The authority
section's name of the answer must match with this expanded domain list.

Use this option only if you talk directly to a resolver that is authoritative
for some zones and does not set the AD (Authentic Data) bit in the answer. Some
DNS servers may have an configuration option to mark the answers from their own
zones as verified (they set the AD bit). Others do not have this option. It is
considered as poor practice using a resolver that is an authoritative server
for some zones.

Use this option only if you really have to (e.g. if you want to use DANE for
remote delivery to a server that is listed in the DNS zones that your resolver
is authoritative for).

If the DNS answer packet has the AA bit set and contains resource record in the
answer section, the name of the first NS record appearing in the authority
section is compared against the list. If the answer packet is authoritative but
the answer section is empty, the name of the first SOA record in the
authoritative section is used instead.

+-------------------------------------------------+
|dns_use_edns0|Use: main|Type: integer|Default: -1|
+-------------------------------------------------+

If this option is set to a non-negative number then Exim will initialise the
DNS resolver library to either use or not use EDNS0 extensions, overriding the
system default. A value of 0 coerces EDNS0 off, a value of 1 coerces EDNS0 on.

If the resolver library does not support EDNS0 then this option has no effect.

OpenBSD's asr resolver routines are known to ignore the EDNS0 option; this
means that DNSSEC will not work with Exim on that platform either, unless Exim
is linked against an alternative DNS client library.

+----------------------------------------------+
|drop_cr|Use: main|Type: boolean|Default: false|
+----------------------------------------------+

This is an obsolete option that is now a no-op. It used to affect the way Exim
handled CR and LF characters in incoming messages. What happens now is
described in section 48.2.

+-------------------------------------------------------------+
|dsn_advertise_hosts|Use: main|Type: host list*|Default: unset|
+-------------------------------------------------------------+

DSN extensions (RFC3461) will be advertised in the EHLO message to, and
accepted from, these hosts. Hosts may use the NOTIFY and ORCPT options on RCPT
TO commands, and RET and ENVID options on MAIL FROM commands. A NOTIFY=SUCCESS
option requests success-DSN messages. A NOTIFY= option with no argument
requests that no delay or failure DSNs are sent. Note: Supplying success-DSN
messages has been criticised on privacy grounds; it can leak details of
internal forwarding.

+---------------------------------------------------+
|dsn_from|Use: main|Type: string*|Default: see below|
+---------------------------------------------------+

This option can be used to vary the contents of From: header lines in bounces
and other automatically generated messages ("Delivery Status Notifications" -
hence the name of the option). The default setting is:

dsn_from = Mail Delivery System <Mailer-Daemon@$qualify_domain>

The value is expanded every time it is needed. If the expansion fails, a panic
is logged, and the default value is used.

+--------------------------------------------------------+
|envelope_to_remove|Use: main|Type: boolean|Default: true|
+--------------------------------------------------------+

Exim's transports have an option for adding an Envelope-to: header to a message
when it is delivered, in exactly the same way as Return-path: is handled. 
Envelope-to: records the original recipient address from the message's envelope
that caused the delivery to happen. Such headers should not be present in
incoming messages, and this option causes them to be removed at the time the
message is received, to avoid any problems that might occur when a delivered
message is subsequently sent on to some other recipient.

+-------------------------------------------------------+
|errors_copy|Use: main|Type: string list*|Default: unset|
+-------------------------------------------------------+

Setting this option causes Exim to send bcc copies of bounce messages that it
generates to other addresses. Note: This does not apply to bounce messages
coming from elsewhere. The value of the option is a colon-separated list of
items. Each item consists of a pattern, terminated by white space, followed by
a comma-separated list of email addresses. If a pattern contains spaces, it
must be enclosed in double quotes.

Each pattern is processed in the same way as a single item in an address list
(see section 10.5). When a pattern matches the recipient of the bounce message,
the message is copied to the addresses on the list. The items are scanned in
order, and once a matching one is found, no further items are examined. For
example:

errors_copy = spqr@mydomain   postmaster@mydomain.example :\
              rqps@mydomain   hostmaster@mydomain.example,\
                              postmaster@mydomain.example

The address list is expanded before use. The expansion variables $local_part
and $domain are set from the original recipient of the error message, and if
there was any wildcard matching in the pattern, the expansion variables $0, $1,
etc. are set in the normal way.

+-----------------------------------------------------+
|errors_reply_to|Use: main|Type: string|Default: unset|
+-----------------------------------------------------+

By default, Exim's bounce and delivery warning messages contain the header line

From: Mail Delivery System <Mailer-Daemon@qualify-domain>

where qualify-domain is the value of the qualify_domain option. A warning
message that is generated by the quota_warn_message option in an appendfile
transport may contain its own From: header line that overrides the default.

Experience shows that people reply to bounce messages. If the errors_reply_to
option is set, a Reply-To: header is added to bounce and warning messages. For
example:

errors_reply_to = postmaster@my.domain.example

The value of the option is not expanded. It must specify a valid RFC 2822
address. However, if a warning message that is generated by the 
quota_warn_message option in an appendfile transport contain its own Reply-To:
header line, the value of the errors_reply_to option is not used.

+---------------------------------------------------+
|event_action|Use: main|Type: string*|Default: unset|
+---------------------------------------------------+

This option declares a string to be expanded for Exim's events mechanism. For
details see chapter 61.

+------------------------------------------------------------------+
|exim_group|Use: main|Type: string|Default: compile-time configured|
+------------------------------------------------------------------+

This option changes the gid under which Exim runs when it gives up root
privilege. The default value is compiled into the binary. The value of this
option is used only when exim_user is also set. Unless it consists entirely of
digits, the string is looked up using getgrnam(), and failure causes a
configuration error. See chapter 56 for a discussion of security issues.

+---------------------------------------------------+
|exim_path|Use: main|Type: string|Default: see below|
+---------------------------------------------------+

This option specifies the path name of the Exim binary, which is used when Exim
needs to re-exec itself. The default is set up to point to the file exim in the
directory configured at compile time by the BIN_DIRECTORY setting. It is
necessary to change exim_path if, exceptionally, Exim is run from some other
place. Warning: Do not use a macro to define the value of this option, because
you will break those Exim utilities that scan the configuration file to find
where the binary is. (They then use the -bP option to extract option settings
such as the value of spool_directory.)

+-----------------------------------------------------------------+
|exim_user|Use: main|Type: string|Default: compile-time configured|
+-----------------------------------------------------------------+

This option changes the uid under which Exim runs when it gives up root
privilege. The default value is compiled into the binary. Ownership of the run
time configuration file and the use of the -C and -D command line options is
checked against the values in the binary, not what is set here.

Unless it consists entirely of digits, the string is looked up using getpwnam()
, and failure causes a configuration error. If exim_group is not also supplied,
the gid is taken from the result of getpwnam() if it is used. See chapter 56
for a discussion of security issues.

+------------------------------------------------------------+
|exim_version|Use: main|Type: string|Default: current version|
+------------------------------------------------------------+

This option overrides the $version_number/$exim_version that Exim reports in
various places. Use with care; this may fool stupid security scanners.

+-----------------------------------------------------------------+
|extra_local_interfaces|Use: main|Type: string list|Default: unset|
+-----------------------------------------------------------------+

This option defines network interfaces that are to be considered local when
routing, but which are not used for listening by the daemon. See section 13.8
for details.

+------------------------------------------------------------------------+
|extract_addresses_remove_arguments|Use: main|Type: boolean|Default: true|
+------------------------------------------------------------------------+

According to some Sendmail documentation (Sun, IRIX, HP-UX), if any addresses
are present on the command line when the -t option is used to build an envelope
from a message's To:, Cc: and Bcc: headers, the command line addresses are
removed from the recipients list. This is also how Smail behaves. However,
other Sendmail documentation (the O'Reilly book) states that command line
addresses are added to those obtained from the header lines. When 
extract_addresses_remove_arguments is true (the default), Exim subtracts
argument headers. If it is set false, Exim adds rather than removes argument
addresses.

+---------------------------------------------------+
|finduser_retries|Use: main|Type: integer|Default: 0|
+---------------------------------------------------+

On systems running NIS or other schemes in which user and group information is
distributed from a remote system, there can be times when getpwnam() and
related functions fail, even when given valid data, because things time out.
Unfortunately these failures cannot be distinguished from genuine "not found"
errors. If finduser_retries is set greater than zero, Exim will try that many
extra times to find a user or a group, waiting for one second between retries.

You should not set this option greater than zero if your user information is in
a traditional /etc/passwd file, because it will cause Exim needlessly to search
the file multiple times for non-existent users, and also cause delay.

+-----------------------------------------------------------------------+
|freeze_tell|Use: main|Type: string list, comma separated|Default: unset|
+-----------------------------------------------------------------------+

On encountering certain errors, or when configured to do so in a system filter,
ACL, or special router, Exim freezes a message. This means that no further
delivery attempts take place until an administrator thaws the message, or the 
auto_thaw, ignore_bounce_errors_after, or timeout_frozen_after feature cause it
to be processed. If freeze_tell is set, Exim generates a warning message
whenever it freezes something, unless the message it is freezing is a
locally-generated bounce message. (Without this exception there is the
possibility of looping.) The warning message is sent to the addresses supplied
as the comma-separated value of this option. If several of the message's
addresses cause freezing, only a single message is sent. If the freezing was
automatic, the reason(s) for freezing can be found in the message log. If you
configure freezing in a filter or ACL, you must arrange for any logging that
you require.

+----------------------------------------------------+
|gecos_name   |Use: main|Type: string*|Default: unset|
|-------------+---------+-------------+--------------|
|gecos_pattern|Use: main|Type: string |Default: unset|
+----------------------------------------------------+

Some operating systems, notably HP-UX, use the "gecos" field in the system
password file to hold other information in addition to users' real names. Exim
looks up this field for use when it is creating Sender: or From: headers. If
either gecos_pattern or gecos_name are unset, the contents of the field are
used unchanged, except that, if an ampersand is encountered, it is replaced by
the user's login name with the first character forced to upper case, since this
is a convention that is observed on many systems.

When these options are set, gecos_pattern is treated as a regular expression
that is to be applied to the field (again with & replaced by the login name),
and if it matches, gecos_name is expanded and used as the user's name.

Numeric variables such as $1, $2, etc. can be used in the expansion to pick up
sub-fields that were matched by the pattern. In HP-UX, where the user's name
terminates at the first comma, the following can be used:

gecos_pattern = ([^,]*)
gecos_name = $1

+---------------------------------------------------------+
|gnutls_compat_mode|Use: main|Type: boolean|Default: unset|
+---------------------------------------------------------+

This option controls whether GnuTLS is used in compatibility mode in an Exim
server. This reduces security slightly, but improves interworking with older
implementations of TLS.

+---------------------------------------------------------------+
|gnutls_allow_auto_pkcs11|Use: main|Type: boolean|Default: unset|
+---------------------------------------------------------------+

This option will let GnuTLS (2.12.0 or later) autoload PKCS11 modules with the
p11-kit configuration files in /etc/pkcs11/modules/.

See https://www.gnutls.org/manual/gnutls.html#Smart-cards-and-HSMs for
documentation.

+---------------------------------------------------------+
|headers_charset|Use: main|Type: string|Default: see below|
+---------------------------------------------------------+

This option sets a default character set for translating from encoded MIME
"words" in header lines, when referenced by an $h_xxx expansion item. The
default is the value of HEADERS_CHARSET in Local/Makefile. The ultimate default
is ISO-8859-1. For more details see the description of header insertions in
section 11.5.

+---------------------------------------------------------+
|header_maxsize|Use: main|Type: integer|Default: see below|
+---------------------------------------------------------+

This option controls the overall maximum size of a message's header section.
The default is the value of HEADER_MAXSIZE in Local/Makefile; the default for
that is 1M. Messages with larger header sections are rejected.

+------------------------------------------------------+
|header_line_maxsize|Use: main|Type: integer|Default: 0|
+------------------------------------------------------+

This option limits the length of any individual header line in a message, after
all the continuations have been joined together. Messages with individual
header lines that are longer than the limit are rejected. The default value of
zero means "no limit".

+----------------------------------------------------------------+
|helo_accept_junk_hosts|Use: main|Type: host list*|Default: unset|
+----------------------------------------------------------------+

Exim checks the syntax of HELO and EHLO commands for incoming SMTP mail, and
gives an error response for invalid data. Unfortunately, there are some SMTP
clients that send syntactic junk. They can be accommodated by setting this
option. Note that this is a syntax check only. See helo_verify_hosts if you
want to do semantic checking. See also helo_allow_chars for a way of extending
the permitted character set.

+------------------------------------------------------+
|helo_allow_chars|Use: main|Type: string|Default: unset|
+------------------------------------------------------+

This option can be set to a string of rogue characters that are permitted in
all EHLO and HELO names in addition to the standard letters, digits, hyphens,
and dots. If you really must allow underscores, you can set

helo_allow_chars = _

Note that the value is one string, not a list.

+-----------------------------------------------------------------+
|helo_lookup_domains|Use: main|Type: domain list*|Default: "@:@[]"|
+-----------------------------------------------------------------+

If the domain given by a client in a HELO or EHLO command matches this list, a
reverse lookup is done in order to establish the host's true name. The default
forces a lookup if the client host gives the server's name or any of its IP
addresses (in brackets), something that broken clients have been seen to do.

+---------------------------------------------------------------+
|helo_try_verify_hosts|Use: main|Type: host list*|Default: unset|
+---------------------------------------------------------------+

By default, Exim just checks the syntax of HELO and EHLO commands (see 
helo_accept_junk_hosts and helo_allow_chars). However, some sites like to do
more extensive checking of the data supplied by these commands. The ACL
condition "verify = helo" is provided to make this possible. Formerly, it was
necessary also to set this option (helo_try_verify_hosts) to force the check to
occur. From release 4.53 onwards, this is no longer necessary. If the check has
not been done before "verify = helo" is encountered, it is done at that time.
Consequently, this option is obsolete. Its specification is retained here for
backwards compatibility.

When an EHLO or HELO command is received, if the calling host matches 
helo_try_verify_hosts, Exim checks that the host name given in the HELO or EHLO
command either:

  * is an IP literal matching the calling address of the host, or

  * matches the host name that Exim obtains by doing a reverse lookup of the
    calling host address, or

  * when looked up in DNS yields the calling host address.

However, the EHLO or HELO command is not rejected if any of the checks fail.
Processing continues, but the result of the check is remembered, and can be
detected later in an ACL by the "verify = helo" condition.

If DNS was used for successful verification, the variable $helo_verify_dnssec
records the DNSSEC status of the lookups.

+-----------------------------------------------------------+
|helo_verify_hosts|Use: main|Type: host list*|Default: unset|
+-----------------------------------------------------------+

Like helo_try_verify_hosts, this option is obsolete, and retained only for
backwards compatibility. For hosts that match this option, Exim checks the host
name given in the HELO or EHLO in the same way as for helo_try_verify_hosts. If
the check fails, the HELO or EHLO command is rejected with a 550 error, and
entries are written to the main and reject logs. If a MAIL command is received
before EHLO or HELO, it is rejected with a 503 error.

+--------------------------------------------------------+
|hold_domains|Use: main|Type: domain list*|Default: unset|
+--------------------------------------------------------+

This option allows mail for particular domains to be held in the queue
manually. The option is overridden if a message delivery is forced with the -M,
-qf, -Rf or -Sf options, and also while testing or verifying addresses using 
-bt or -bv. Otherwise, if a domain matches an item in hold_domains, no routing
or delivery for that address is done, and it is deferred every time the message
is looked at.

This option is intended as a temporary operational measure for delaying the
delivery of mail while some problem is being sorted out, or some new
configuration tested. If you just want to delay the processing of some domains
until a queue run occurs, you should use queue_domains or queue_smtp_domains,
not hold_domains.

A setting of hold_domains does not override Exim's code for removing messages
from the queue if they have been there longer than the longest retry time in
any retry rule. If you want to hold messages for longer than the normal retry
times, insert a dummy retry rule with a long retry time.

+-----------------------------------------------------+
|host_lookup|Use: main|Type: host list*|Default: unset|
+-----------------------------------------------------+

Exim does not look up the name of a calling host from its IP address unless it
is required to compare against some host list, or the host matches 
helo_try_verify_hosts or helo_verify_hosts, or the host matches this option
(which normally contains IP addresses rather than host names). The default
configuration file contains

host_lookup = *

which causes a lookup to happen for all hosts. If the expense of these lookups
is felt to be too great, the setting can be changed or removed.

After a successful reverse lookup, Exim does a forward lookup on the name it
has obtained, to verify that it yields the IP address that it started with. If
this check fails, Exim behaves as if the name lookup failed.

After any kind of failure, the host name (in $sender_host_name) remains unset,
and $host_lookup_failed is set to the string "1". See also 
dns_again_means_nonexist, helo_lookup_domains, and "verify =
reverse_host_lookup" in ACLs.

+---------------------------------------------------------------------+
|host_lookup_order|Use: main|Type: string list|Default: "bydns:byaddr"|
+---------------------------------------------------------------------+

This option specifies the order of different lookup methods when Exim is trying
to find a host name from an IP address. The default is to do a DNS lookup
first, and then to try a local lookup (using gethostbyaddr() or equivalent) if
that fails. You can change the order of these lookups, or omit one entirely, if
you want.

Warning: The "byaddr" method does not always yield aliases when there are
multiple PTR records in the DNS and the IP address is not listed in /etc/hosts.
Different operating systems give different results in this case. That is why
the default tries a DNS lookup first.

+----------------------------------------------------------------+
|host_reject_connection|Use: main|Type: host list*|Default: unset|
+----------------------------------------------------------------+

If this option is set, incoming SMTP calls from the hosts listed are rejected
as soon as the connection is made. This option is obsolete, and retained only
for backward compatibility, because nowadays the ACL specified by 
acl_smtp_connect can also reject incoming connections immediately.

If the connection is on a TLS-on-connect port then the TCP connection is just
dropped. Otherwise, an SMTP error is sent first.

The ability to give an immediate rejection (either by this option or using an
ACL) is provided for use in unusual cases. Many hosts will just try again,
sometimes without much delay. Normally, it is better to use an ACL to reject
incoming messages at a later stage, such as after RCPT commands. See chapter 44
.

+----------------------------------------------------------------+
|hosts_connection_nolog|Use: main|Type: host list*|Default: unset|
+----------------------------------------------------------------+

This option defines a list of hosts for which connection logging does not
happen, even though the smtp_connection log selector is set. For example, you
might want not to log SMTP connections from local processes, or from 127.0.0.1,
or from your local LAN. This option is consulted in the main loop of the
daemon; you should therefore strive to restrict its value to a short inline
list of IP addresses and networks. To disable logging SMTP connections from
local processes, you must create a host list with an empty item. For example:

hosts_connection_nolog = :

The hosts affected by this option also do not log "no MAIL in SMTP connection"
lines, as may commonly be produced by a monitoring system.

+------------------------------------------------------------+
|hosts_require_alpn|Use: main|Type: host list*|Default: unset|
+------------------------------------------------------------+

If the TLS library supports ALPN then a successful negotiation of ALPN will be
required for any client matching the list, for TLS to be used. See also the 
tls_alpn option.

Note: prevention of fallback to in-clear connection is not managed by this
option, and should be done separately.

+--------------------------------------------------------+
|hosts_require_helo|Use: main|Type: host list*|Default: *|
+--------------------------------------------------------+

Exim will require an accepted HELO or EHLO command from a host matching this
list, before accepting a MAIL command.

+-----------------------------------------------------+
|hosts_proxy|Use: main|Type: host list*|Default: unset|
+-----------------------------------------------------+

This option enables use of Proxy Protocol proxies for incoming connections. For
details see section 59.1.

+----------------------------------------------------------------+
|hosts_treat_as_local|Use: main|Type: domain list*|Default: unset|
+----------------------------------------------------------------+

If this option is set, any host names that match the domain list are treated as
if they were the local host when Exim is scanning host lists obtained from MX
records or other sources. Note that the value of this option is a domain list,
not a host list, because it is always used to check host names, not IP
addresses.

This option also applies when Exim is matching the special items "@mx_any",
"@mx_primary", and "@mx_secondary" in a domain list (see section 10.3), and
when checking the hosts option in the smtp transport for the local host (see
the allow_localhost option in that transport). See also local_interfaces, 
extra_local_interfaces, and chapter 13, which contains a discussion about local
network interfaces and recognizing the local host.

+--------------------------------------------------------+
|ibase_servers|Use: main|Type: string list|Default: unset|
+--------------------------------------------------------+

This option provides a list of InterBase servers and associated connection
data, to be used in conjunction with ibase lookups (see section 9.13.1). The
option is available only if Exim has been built with InterBase support.

+------------------------------------------------------------+
|ignore_bounce_errors_after|Use: main|Type: time|Default: 10w|
+------------------------------------------------------------+

This option affects the processing of bounce messages that cannot be delivered,
that is, those that suffer a permanent delivery failure. (Bounce messages that
suffer temporary delivery failures are of course retried in the usual way.)

After a permanent delivery failure, bounce messages are frozen, because there
is no sender to whom they can be returned. When a frozen bounce message has
been in the queue for more than the given time, it is unfrozen at the next
queue run, and a further delivery is attempted. If delivery fails again, the
bounce message is discarded. This makes it possible to keep failed bounce
messages around for a shorter time than the normal maximum retry time for
frozen messages. For example,

ignore_bounce_errors_after = 12h

retries failed bounce message deliveries after 12 hours, discarding any further
failures. If the value of this option is set to a zero time period, bounce
failures are discarded immediately. Setting a very long time (as in the default
value) has the effect of disabling this option. For ways of automatically
dealing with other kinds of frozen message, see auto_thaw and 
timeout_frozen_after.

+---------------------------------------------------------------+
|ignore_fromline_hosts|Use: main|Type: host list*|Default: unset|
|---------------------+---------+----------------+--------------|
|ignore_fromline_local|Use: main| Type: boolean  |Default: false|
+---------------------------------------------------------------+

Some broken SMTP clients insist on sending a UUCP-like "From " line before the
headers of a message. By default this is treated as the start of the message's
body, which means that any following headers are not recognized as such. Exim
can be made to ignore it by setting ignore_fromline_hosts to match those hosts
that insist on sending it. If the sender is actually a local process rather
than a remote host, and is using -bs to inject the messages, 
ignore_fromline_local must be set to achieve this effect.

+-----------------------------------------------------------+
|keep_environment|Use: main|Type: string list|Default: unset|
+-----------------------------------------------------------+

This option contains a string list of environment variables to keep. You have
to trust these variables or you have to be sure that these variables do not
impose any security risk. Keep in mind that during the startup phase Exim is
running with an effective UID 0 in most installations. As the default value is
an empty list, the default environment for using libraries, running embedded
Perl code, or running external binaries is empty, and does not not even contain
PATH or HOME.

Actually the list is interpreted as a list of patterns (10.2), except that it
is not expanded first.

WARNING: Macro substitution is still done first, so having a macro FOO and
having FOO_HOME in your keep_environment option may have unexpected results.
You may work around this using a regular expression that does not match the
macro name: ^[F]OO_HOME$.

Current versions of Exim issue a warning during startup if you do not mention 
keep_environment in your runtime configuration file and if your current
environment is not empty. Future versions may not issue that warning anymore.

See the add_environment main config option for a way to set environment
variables to a fixed value. The environment for pipe transports is handled
separately, see section 29.4 for details.

+-----------------------------------------------+
|keep_malformed|Use: main|Type: time|Default: 4d|
+-----------------------------------------------+

This option specifies the length of time to keep messages whose spool files
have been corrupted in some way. This should, of course, never happen. At the
next attempt to deliver such a message, it gets removed. The incident is
logged.

+------------------------------------------------------+
|ldap_ca_cert_dir|Use: main|Type: string|Default: unset|
+------------------------------------------------------+

This option indicates which directory contains CA certificates for verifying a
TLS certificate presented by an LDAP server. While Exim does not provide a
default value, your SSL library may. Analogous to tls_verify_certificates but
as a client-side option for LDAP and constrained to be a directory.

+-------------------------------------------------------+
|ldap_ca_cert_file|Use: main|Type: string|Default: unset|
+-------------------------------------------------------+

This option indicates which file contains CA certificates for verifying a TLS
certificate presented by an LDAP server. While Exim does not provide a default
value, your SSL library may. Analogous to tls_verify_certificates but as a
client-side option for LDAP and constrained to be a file.

+----------------------------------------------------+
|ldap_cert_file|Use: main|Type: string|Default: unset|
+----------------------------------------------------+

This option indicates which file contains an TLS client certificate which Exim
should present to the LDAP server during TLS negotiation. Should be used
together with ldap_cert_key.

+---------------------------------------------------+
|ldap_cert_key|Use: main|Type: string|Default: unset|
+---------------------------------------------------+

This option indicates which file contains the secret/private key to use to
prove identity to the LDAP server during TLS negotiation. Should be used
together with ldap_cert_file, which contains the identity to be proven.

+-------------------------------------------------------+
|ldap_cipher_suite|Use: main|Type: string|Default: unset|
+-------------------------------------------------------+

This controls the TLS cipher-suite negotiation during TLS negotiation with the
LDAP server. See 43.4 for more details of the format of cipher-suite options
with OpenSSL (as used by LDAP client libraries).

+---------------------------------------------------------------+
|ldap_default_servers|Use: main|Type: string list|Default: unset|
+---------------------------------------------------------------+

This option provides a list of LDAP servers which are tried in turn when an
LDAP query does not contain a server. See section 9.11.1 for details of LDAP
queries. This option is available only when Exim has been built with LDAP
support.

+--------------------------------------------------------+
|ldap_require_cert|Use: main|Type: string|Default: unset.|
+--------------------------------------------------------+

This should be one of the values "hard", "demand", "allow", "try" or "never". A
value other than one of these is interpreted as "never". See the entry
"TLS_REQCERT" in your system man page for ldap.conf(5). Although Exim does not
set a default, the LDAP library probably defaults to hard/demand.

+-----------------------------------------------------+
|ldap_start_tls|Use: main|Type: boolean|Default: false|
+-----------------------------------------------------+

If set, Exim will attempt to negotiate TLS with the LDAP server when connecting
on a regular LDAP port. This is the LDAP equivalent of SMTP's "STARTTLS". This
is distinct from using "ldaps", which is the LDAP form of SSL-on-connect. In
the event of failure to negotiate TLS, the action taken is controlled by 
ldap_require_cert. This option is ignored for "ldapi" connections.

+---------------------------------------------------+
|ldap_version|Use: main|Type: integer|Default: unset|
+---------------------------------------------------+

This option can be used to force Exim to set a specific protocol version for
LDAP. If it option is unset, it is shown by the -bP command line option as -1.
When this is the case, the default is 3 if LDAP_VERSION3 is defined in the LDAP
headers; otherwise it is 2. This option is available only when Exim has been
built with LDAP support.

+------------------------------------------------------+
|local_from_check|Use: main|Type: boolean|Default: true|
+------------------------------------------------------+

When a message is submitted locally (that is, not over a TCP/IP connection) by
an untrusted user, Exim removes any existing Sender: header line, and checks
that the From: header line matches the login of the calling user and the domain
specified by qualify_domain.

Note: An unqualified address (no domain) in the From: header in a locally
submitted message is automatically qualified by Exim, unless the -bnq command
line option is used.

You can use local_from_prefix and local_from_suffix to permit affixes on the
local part. If the From: header line does not match, Exim adds a Sender: header
with an address constructed from the calling user's login and the default
qualify domain.

If local_from_check is set false, the From: header check is disabled, and no 
Sender: header is ever added. If, in addition, you want to retain Sender:
header lines supplied by untrusted users, you must also set local_sender_retain
to be true.

These options affect only the header lines in the message. The envelope sender
is still forced to be the login id at the qualify domain unless 
untrusted_set_sender permits the user to supply an envelope sender.

For messages received over TCP/IP, an ACL can specify "submission mode" to
request similar header line checking. See section 48.5.12, which has more
details about Sender: processing.

+-------------------------------------------------------+
|local_from_prefix|Use: main|Type: string|Default: unset|
|-----------------+---------+------------+--------------|
|local_from_suffix|Use: main|Type: string|Default: unset|
+-------------------------------------------------------+

When Exim checks the From: header line of locally submitted messages for
matching the login id (see local_from_check above), it can be configured to
ignore certain prefixes and suffixes in the local part of the address. This is
done by setting local_from_prefix and/or local_from_suffix to appropriate
lists, in the same form as the local_part_prefix and local_part_suffix router
options (see chapter 15). For example, if

local_from_prefix = *-

is set, a From: line containing

From: anything-user@your.domain.example

will not cause a Sender: header to be added if user@your.domain.example matches
the actual sender address that is constructed from the login name and qualify
domain.

+---------------------------------------------------------------+
|local_interfaces|Use: main|Type: string list|Default: see below|
+---------------------------------------------------------------+

This option controls which network interfaces are used by the daemon for
listening; they are also used to identify the local host when routing. Chapter
13 contains a full description of this option and the related options 
daemon_smtp_ports, extra_local_interfaces, hosts_treat_as_local, and 
tls_on_connect_ports. The default value for local_interfaces is

local_interfaces = 0.0.0.0

when Exim is built without IPv6 support; otherwise it is

local_interfaces = <; ::0 ; 0.0.0.0

+---------------------------------------------------+
|local_scan_timeout|Use: main|Type: time|Default: 5m|
+---------------------------------------------------+

This timeout applies to the local_scan() function (see chapter 46). Zero means
"no timeout". If the timeout is exceeded, the incoming message is rejected with
a temporary error if it is an SMTP message. For a non-SMTP message, the message
is dropped and Exim ends with a non-zero code. The incident is logged on the
main and reject logs.

+----------------------------------------------------------+
|local_sender_retain|Use: main|Type: boolean|Default: false|
+----------------------------------------------------------+

When a message is submitted locally (that is, not over a TCP/IP connection) by
an untrusted user, Exim removes any existing Sender: header line. If you do not
want this to happen, you must set local_sender_retain, and you must also set 
local_from_check to be false (Exim will complain if you do not). See also the
ACL modifier "control = suppress_local_fixups". Section 48.5.12 has more
details about Sender: processing.

+-------------------------------------------------------+
|localhost_number|Use: main|Type: string*|Default: unset|
+-------------------------------------------------------+

Exim's message ids are normally unique only within the local host. If
uniqueness among a set of hosts is required, each host must set a different
value for the localhost_number option. The string is expanded immediately after
reading the configuration file (so that a number can be computed from the host
name, for example) and the result of the expansion must be a number in the
range 0-16 (or 0-10 on operating systems with case-insensitive file systems).
This is available in subsequent string expansions via the variable
$localhost_number. When localhost_number is set, the final two characters of
the message id, instead of just being a fractional part of the time, are
computed from the time and the local host number as described in section 3.4.

+-----------------------------------------------------------------------+
|log_file_path|Use: main|Type: string list*|Default: set at compile time|
+-----------------------------------------------------------------------+

This option sets the path which is used to determine the names of Exim's log
files, or indicates that logging is to be to syslog, or both. It is expanded
when Exim is entered, so it can, for example, contain a reference to the host
name. If no specific path is set for the log files at compile or runtime, or if
the option is unset at runtime (i.e. "log_file_path = ") they are written in a
sub-directory called log in Exim's spool directory. A path must start with a
slash. To send to syslog, use the word "syslog". Chapter 53 contains further
details about Exim's logging, and section 53.1 describes how the contents of 
log_file_path are used. If this string is fixed at your installation (contains
no expansion variables) it is recommended that you do not set this option in
the configuration file, but instead supply the path using LOG_FILE_PATH in
Local/Makefile so that it is available to Exim for logging errors detected
early on - in particular, failure to read the configuration file.

+--------------------------------------------------+
|log_selector|Use: main|Type: string|Default: unset|
+--------------------------------------------------+

This option can be used to reduce or increase the number of things that Exim
writes to its log files. Its argument is made up of names preceded by plus or
minus characters. For example:

log_selector = +arguments -retry_defer

A list of possible names and what they control is given in the chapter on
logging, in section 53.15.

+---------------------------------------------------+
|log_timezone|Use: main|Type: boolean|Default: false|
+---------------------------------------------------+

By default, the timestamps on log lines are in local time without the timezone.
This means that if your timezone changes twice a year, the timestamps in log
lines are ambiguous for an hour when the clocks go back. One way of avoiding
this problem is to set the timezone to UTC. An alternative is to set 
log_timezone true. This turns on the addition of the timezone offset to
timestamps in log lines. Turning on this option can add quite a lot to the size
of log files because each line is extended by 6 characters. Note that the
$tod_log variable contains the log timestamp without the zone, but there is
another variable called $tod_zone that contains just the timezone offset.

+---------------------------------------------------+
|lookup_open_max|Use: main|Type: integer|Default: 25|
+---------------------------------------------------+

This option limits the number of simultaneously open files for single-key
lookups that use regular files (that is, lsearch, dbm, and cdb). Exim normally
keeps these files open during routing, because often the same file is required
several times. If the limit is reached, Exim closes the least recently used
file. Note that if you are using the ndbm library, it actually opens two files
for each logical DBM database, though it still counts as one for the purposes
of lookup_open_max. If you are getting "too many open files" errors with NDBM,
you need to reduce the value of lookup_open_max.

+------------------------------------------------------+
|max_username_length|Use: main|Type: integer|Default: 0|
+------------------------------------------------------+

Some operating systems are broken in that they truncate long arguments to
getpwnam() to eight characters, instead of returning "no such user". If this
option is set greater than zero, any attempt to call getpwnam() with an
argument that is longer behaves as if getpwnam() failed.

+---------------------------------------------------------+
|message_body_newlines|Use: main|Type: bool|Default: false|
+---------------------------------------------------------+

By default, newlines in the message body are replaced by spaces when setting
the $message_body and $message_body_end expansion variables. If this option is
set true, this no longer happens.

+---------------------------------------------------------+
|message_body_visible|Use: main|Type: integer|Default: 500|
+---------------------------------------------------------+

This option specifies how much of a message's body is to be included in the
$message_body and $message_body_end expansion variables.

+---------------------------------------------------------------+
|message_id_header_domain|Use: main|Type: string*|Default: unset|
+---------------------------------------------------------------+

If this option is set, the string is expanded and used as the right hand side
(domain) of the Message-ID: header that Exim creates if a locally-originated
incoming message does not have one. "Locally-originated" means "not received
over TCP/IP." Otherwise, the primary host name is used. Only letters, digits,
dot and hyphen are accepted; any other characters are replaced by hyphens. If
the expansion is forced to fail, or if the result is an empty string, the
option is ignored.

+-------------------------------------------------------------+
|message_id_header_text|Use: main|Type: string*|Default: unset|
+-------------------------------------------------------------+

If this variable is set, the string is expanded and used to augment the text of
the Message-id: header that Exim creates if a locally-originated incoming
message does not have one. The text of this header is required by RFC 2822 to
take the form of an address. By default, Exim uses its internal message id as
the local part, and the primary host name as the domain. If this option is set,
it is expanded, and provided the expansion is not forced to fail, and does not
yield an empty string, the result is inserted into the header immediately
before the @, separated from the internal message id by a dot. Any characters
that are illegal in an address are automatically converted into hyphens. This
means that variables such as $tod_log can be used, because the spaces and
colons will become hyphens.

+--------------------------------------------------+
|message_logs|Use: main|Type: boolean|Default: true|
+--------------------------------------------------+

If this option is turned off, per-message log files are not created in the
msglog spool sub-directory. This reduces the amount of disk I/O required by
Exim, by reducing the number of files involved in handling a message from a
minimum of four (header spool file, body spool file, delivery journal, and
per-message log) to three. The other major I/O activity is Exim's main log,
which is not affected by this option.

+-------------------------------------------------------+
|message_size_limit|Use: main|Type: string*|Default: 50M|
+-------------------------------------------------------+

This option limits the maximum size of message that Exim will process. The
value is expanded for each incoming connection so, for example, it can be made
to depend on the IP address of the remote host for messages arriving via TCP/
IP. After expansion, the value must be a sequence of decimal digits, optionally
followed by K or M.

If nonzero the value will be advertised as a parameter to the ESMTP SIZE
service extension keyword.

Note: This limit cannot be made to depend on a message's sender or any other
properties of an individual message, because it has to be advertised in the
server's response to EHLO. String expansion failure causes a temporary error. A
value of zero means no limit, but its use is not recommended. See also 
bounce_return_size_limit.

Incoming SMTP messages are failed with a 552 error if the limit is exceeded;
locally-generated messages either get a stderr message or a delivery failure
message to the sender, depending on the -oe setting. Rejection of an oversized
message is logged in both the main and the reject logs. See also the generic
transport option message_size_limit, which limits the size of message that an
individual transport can process.

If you use a virus-scanner and set this option to to a value larger than the
maximum size that your virus-scanner is configured to support, you may get
failures triggered by large mails. The right size to configure for the
virus-scanner depends upon what data is passed and the options in use but it's
probably safest to just set it to a little larger than this value. E.g., with a
default Exim message size of 50M and a default ClamAV StreamMaxLength of 10M,
some problems may result.

A value of 0 will disable size limit checking; Exim will still advertise the
SIZE extension in an EHLO response, but without a limit, so as to permit SMTP
clients to still indicate the message size along with the MAIL verb.

+-----------------------------------------------------------+
|move_frozen_messages|Use: main|Type: boolean|Default: false|
+-----------------------------------------------------------+

This option, which is available only if Exim has been built with the setting

SUPPORT_MOVE_FROZEN_MESSAGES=yes

in Local/Makefile, causes frozen messages and their message logs to be moved
from the input and msglog directories on the spool to Finput and Fmsglog,
respectively. There is currently no support in Exim or the standard utilities
for handling such moved messages, and they do not show up in lists generated by
-bp or by the Exim monitor.

+--------------------------------------------------+
|mua_wrapper|Use: main|Type: boolean|Default: false|
+--------------------------------------------------+

Setting this option true causes Exim to run in a very restrictive mode in which
it passes messages synchronously to a smart host. Chapter 52 contains a full
description of this facility.

+--------------------------------------------------------+
|mysql_servers|Use: main|Type: string list|Default: unset|
+--------------------------------------------------------+

This option provides a list of MySQL servers and associated connection data, to
be used in conjunction with mysql lookups (see section 9.13.1). The option is
available only if Exim has been built with MySQL support.

+-------------------------------------------------------+
|never_users|Use: main|Type: string list*|Default: unset|
+-------------------------------------------------------+

This option is expanded just once, at the start of Exim's processing. Local
message deliveries are normally run in processes that are setuid to the
recipient, and remote deliveries are normally run under Exim's own uid and gid.
It is usually desirable to prevent any deliveries from running as root, as a
safety precaution.

When Exim is built, an option called FIXED_NEVER_USERS can be set to a list of
users that must not be used for local deliveries. This list is fixed in the
binary and cannot be overridden by the configuration file. By default, it
contains just the single user name "root". The never_users runtime option can
be used to add more users to the fixed list.

If a message is to be delivered as one of the users on the fixed list or the 
never_users list, an error occurs, and delivery is deferred. A common example
is

never_users = root:daemon:bin

Including root is redundant if it is also on the fixed list, but it does no
harm. This option overrides the pipe_as_creator option of the pipe transport
driver.

+-----------------------------------------------------------------------------+
|notifier_socket| Use:   |  Type:    |              Default: $spool_directory/|
|               |  main  |  string   |                      exim_daemon_notify|
+-----------------------------------------------------------------------------+

This option gives the name for a unix-domain socket on which the daemon listens
for work and information-requests. Only installations running multiple daemons
sharing a spool directory should need to modify the default.

The option is expanded before use. If the platform supports Linux-style
abstract socket names, the result is used with a nul byte prefixed. Otherwise,
it should be a full path name and use a directory accessible to Exim.

If this option is set as empty, or the command line -oY option is used, or the
command line uses a -oX option and does not use -oP, then a notifier socket is
not created.

+-----------------------------------------------------------------------------+
|               |Use:  |  Type:   |Default: +no_sslv2 +no_sslv3 +single_dh_use|
|openssl_options| main |  string  |               +no_ticket +no_renegotiation|
|               |      |   list   |                                           |
+-----------------------------------------------------------------------------+

This option allows an administrator to adjust the SSL options applied by
OpenSSL to connections. It is given as a space-separated list of items, each
one to be +added or -subtracted from the current value.

This option is only available if Exim is built against OpenSSL. The values
available for this option vary according to the age of your OpenSSL install.
The "all" value controls a subset of flags which are available, typically the
bug workaround options. The SSL_CTX_set_options man page will list the values
known on your system and Exim should support all the "bug workaround" options
and many of the "modifying" options. The Exim names lose the leading "SSL_OP_"
and are lower-cased.

Note that adjusting the options can have severe impact upon the security of SSL
as used by Exim. It is possible to disable safety checks and shoot yourself in
the foot in various unpleasant ways. This option should not be adjusted
lightly. An unrecognised item will be detected at startup, by invoking Exim
with the -bV flag.

The option affects Exim operating both as a server and as a client.

Historical note: prior to release 4.80, Exim defaulted this value to
"+dont_insert_empty_fragments", which may still be needed for compatibility
with some clients, but which lowers security by increasing exposure to some now
infamous attacks.

Examples:

# Make both old MS and old Eudora happy:
openssl_options = -all +microsoft_big_sslv3_buffer \
                       +dont_insert_empty_fragments

# Disable older protocol versions:
openssl_options = +no_sslv2 +no_sslv3

Possible options may include:

  * "all"

  * "allow_unsafe_legacy_renegotiation"

  * "cipher_server_preference"

  * "dont_insert_empty_fragments"

  * "ephemeral_rsa"

  * "legacy_server_connect"

  * "microsoft_big_sslv3_buffer"

  * "microsoft_sess_id_bug"

  * "msie_sslv2_rsa_padding"

  * "netscape_challenge_bug"

  * "netscape_reuse_cipher_change_bug"

  * "no_compression"

  * "no_session_resumption_on_renegotiation"

  * "no_sslv2"

  * "no_sslv3"

  * "no_ticket"

  * "no_tlsv1"

  * "no_tlsv1_1"

  * "no_tlsv1_2"

  * "safari_ecdhe_ecdsa_bug"

  * "single_dh_use"

  * "single_ecdh_use"

  * "ssleay_080_client_dh_bug"

  * "sslref2_reuse_cert_type_bug"

  * "tls_block_padding_bug"

  * "tls_d5_bug"

  * "tls_rollback_bug"

As an aside, the "safari_ecdhe_ecdsa_bug" item is a misnomer and affects all
clients connecting using the MacOS SecureTransport TLS facility prior to MacOS
10.8.4, including email clients. If you see old MacOS clients failing to
negotiate TLS then this option value might help, provided that your OpenSSL
release is new enough to contain this work-around. This may be a situation
where you have to upgrade OpenSSL to get buggy clients working.

+---------------------------------------------------------+
|oracle_servers|Use: main|Type: string list|Default: unset|
+---------------------------------------------------------+

This option provides a list of Oracle servers and associated connection data,
to be used in conjunction with oracle lookups (see section 9.13.1). The option
is available only if Exim has been built with Oracle support.

+-----------------------------------------------------+
|panic_coredump|Use: main|Type: boolean|Default: false|
+-----------------------------------------------------+

This option is rarely needed but can help for some debugging investigations. If
set, when an internal error is detected by Exim which is sufficient to
terminate the process (all such are logged in the paniclog) then a coredump is
requested.

Note that most systems require additional administrative configuration to
permit write a core file for a setuid program, which is Exim's common installed
configuration.

+----------------------------------------------------------------+
|percent_hack_domains|Use: main|Type: domain list*|Default: unset|
+----------------------------------------------------------------+

The "percent hack" is the convention whereby a local part containing a percent
sign is re-interpreted as a new email address, with the percent replaced by @.
This is sometimes called "source routing", though that term is also applied to
RFC 2822 addresses that begin with an @ character. If this option is set, Exim
implements the percent facility for those domains listed, but no others. This
happens before an incoming SMTP address is tested against an ACL.

Warning: The "percent hack" has often been abused by people who are trying to
get round relaying restrictions. For this reason, it is best avoided if at all
possible. Unfortunately, a number of less security-conscious MTAs implement it
unconditionally. If you are running Exim on a gateway host, and routing mail
through to internal MTAs without processing the local parts, it is a good idea
to reject recipient addresses with percent characters in their local parts.
Exim's default configuration does this.

+----------------------------------------------------+
|perl_at_start|Use: main|Type: boolean|Default: false|
|-------------+---------+-------------+--------------|
|perl_startup |Use: main|Type: string |Default: unset|
+----------------------------------------------------+

These options are available only when Exim is built with an embedded Perl
interpreter. See chapter 12 for details of their use.

+-----------------------------------------------------+
|perl_taintmode|Use: main|Type: boolean|Default: false|
+-----------------------------------------------------+

This option enables the taint mode of the embedded Perl interpreter.

+--------------------------------------------------------+
|pgsql_servers|Use: main|Type: string list|Default: unset|
+--------------------------------------------------------+

This option provides a list of PostgreSQL servers and associated connection
data, to be used in conjunction with pgsql lookups (see section 9.13.1). The
option is available only if Exim has been built with PostgreSQL support.

+------------------------------------------------------------------+
|pid_file_path|Use: main|Type: string*|Default: set at compile time|
+------------------------------------------------------------------+

This option sets the name of the file to which the Exim daemon writes its
process id. The string is expanded, so it can contain, for example, references
to the host name:

pid_file_path = /var/log/$primary_hostname/exim.pid

If no path is set, the pid is written to the file exim-daemon.pid in Exim's
spool directory. The value set by the option can be overridden by the -oP
command line option. A pid file is not written if a "non-standard" daemon is
run by means of the -oX option, unless a path is explicitly supplied by -oP.

+----------------------------------------------------------------+
|pipelining_advertise_hosts|Use: main|Type: host list*|Default: *|
+----------------------------------------------------------------+

This option can be used to suppress the advertisement of the SMTP PIPELINING
extension to specific hosts. See also the no_pipelining control in section
44.13. When PIPELINING is not advertised and smtp_enforce_sync is true, an Exim
server enforces strict synchronization for each SMTP command and response. When
PIPELINING is advertised, Exim assumes that clients will use it; "out of order"
commands that are "expected" do not count as protocol errors (see 
smtp_max_synprot_errors).

+------------------------------------------------------------------------+
|pipelining_connect_advertise_hosts|Use: main|Type: host list*|Default: *|
+------------------------------------------------------------------------+

If Exim is built without the DISABLE_PIPE_CONNECT build option this option
controls which hosts the facility is advertised to and from which pipeline
early-connection (before MAIL) SMTP commands are acceptable. When used, the
pipelining saves on roundtrip times.

See also the hosts_pipe_connect smtp transport option.

The SMTP service extension keyword advertised is "PIPECONNECT"; it permits the
client to pipeline TCP connection and hello command (inclear phase), or
TLS-establishment and hello command (encrypted phase), on later connections to
the same host.

+--------------------------------------------------+
|prdr_enable|Use: main|Type: boolean|Default: false|
+--------------------------------------------------+

This option can be used to enable the Per-Recipient Data Response extension to
SMTP, defined by Eric Hall. If the option is set, PRDR is advertised by Exim
when operating as a server. If the client requests PRDR, and more than one
recipient, for a message an additional ACL is called for each recipient after
the message content is received. See section 44.2.7.

+------------------------------------------------------------+
|preserve_message_logs|Use: main|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is set, message log files are not deleted when messages are
completed. Instead, they are moved to a sub-directory of the spool directory
called msglog.OLD, where they remain available for statistical or debugging
purposes. This is a dangerous option to set on systems with any appreciable
volume of mail. Use with care!

+----------------------------------------------------------+
|primary_hostname|Use: main|Type: string|Default: see below|
+----------------------------------------------------------+

This specifies the name of the current host. It is used in the default EHLO or
HELO command for outgoing SMTP messages (changeable via the helo_data option in
the smtp transport), and as the default for qualify_domain. The value is also
used by default in some SMTP response messages from an Exim server. This can be
changed dynamically by setting smtp_active_hostname.

If primary_hostname is not set, Exim calls uname() to find the host name. If
this fails, Exim panics and dies. If the name returned by uname() contains only
one component, Exim passes it to gethostbyname() (or getipnodebyname() when
available) in order to obtain the fully qualified version. The variable
$primary_hostname contains the host name, whether set explicitly by this
option, or defaulted.

+--------------------------------------------------------+
|print_topbitchars|Use: main|Type: boolean|Default: false|
+--------------------------------------------------------+

By default, Exim considers only those characters whose codes lie in the range
32-126 to be printing characters. In a number of circumstances (for example,
when writing log entries) non-printing characters are converted into escape
sequences, primarily to avoid messing up the layout. If print_topbitchars is
set, code values of 128 and above are also considered to be printing
characters.

This option also affects the header syntax checks performed by the autoreply
transport, and whether Exim uses RFC 2047 encoding of the user's full name when
constructing From: and Sender: addresses (as described in section 48.7).
Setting this option can cause Exim to generate eight bit message headers that
do not conform to the standards.

+------------------------------------------------------+
|process_log_path|Use: main|Type: string|Default: unset|
+------------------------------------------------------+

This option sets the name of the file to which an Exim process writes its
"process log" when sent a USR1 signal. This is used by the exiwhat utility
script. If this option is unset, the file called exim-process.info in Exim's
spool directory is used. The ability to specify the name explicitly can be
useful in environments where two different Exims are running, using different
spool directories.

+---------------------------------------------------------+
|prod_requires_admin|Use: main|Type: boolean|Default: true|
+---------------------------------------------------------+

The -M, -R, and -q command-line options require the caller to be an admin user
unless prod_requires_admin is set false. See also queue_list_requires_admin and
commandline_checks_require_admin.

+-------------------------------------------------------+
|proxy_protocol_timeout|Use: main|Type: time|Default: 3s|
+-------------------------------------------------------+

This option sets the timeout for proxy protocol negotiation. For details see
section 59.1.

+--------------------------------------------------------+
|qualify_domain|Use: main|Type: string|Default: see below|
+--------------------------------------------------------+

This option specifies the domain name that is added to any envelope sender
addresses that do not have a domain qualification. It also applies to recipient
addresses if qualify_recipient is not set. Unqualified addresses are accepted
by default only for locally-generated messages. Qualification is also applied
to addresses in header lines such as From: and To: for locally-generated
messages, unless the -bnq command line option is used.

Messages from external sources must always contain fully qualified addresses,
unless the sending host matches sender_unqualified_hosts or 
recipient_unqualified_hosts (as appropriate), in which case incoming addresses
are qualified with qualify_domain or qualify_recipient as necessary.
Internally, Exim always works with fully qualified envelope addresses. If 
qualify_domain is not set, it defaults to the primary_hostname value.

+-----------------------------------------------------------+
|qualify_recipient|Use: main|Type: string|Default: see below|
+-----------------------------------------------------------+

This option allows you to specify a different domain for qualifying recipient
addresses to the one that is used for senders. See qualify_domain above.

+---------------------------------------------------------+
|queue_domains|Use: main|Type: domain list*|Default: unset|
+---------------------------------------------------------+

This option lists domains for which immediate delivery is not required. A
delivery process is started whenever a message is received, but only those
domains that do not match are processed. All other deliveries wait until the
next queue run. See also hold_domains and queue_smtp_domains.

+-----------------------------------------------------+
|queue_fast_ramp|Use: main|Type: boolean|Default: true|
+-----------------------------------------------------+

If set to true, two-phase queue runs, initiated using -qq on the command line,
may start parallel delivery processes during their first phase. This will be
done when a threshold number of messages have been routed for a single host.

+---------------------------------------------------------------+
|queue_list_requires_admin|Use: main|Type: boolean|Default: true|
+---------------------------------------------------------------+

The -bp command-line option, which lists the messages that are on the queue,
requires the caller to be an admin user unless queue_list_requires_admin is set
false. See also prod_requires_admin and commandline_checks_require_admin.

+-------------------------------------------------+
|queue_only|Use: main|Type: boolean|Default: false|
+-------------------------------------------------+

If queue_only is set, a delivery process is not automatically started whenever
a message is received. Instead, the message waits in the queue for the next
queue run. Even if queue_only is false, incoming messages may not get delivered
immediately when certain conditions (such as heavy load) occur.

The -odq command line has the same effect as queue_only. The -odb and -odi
command line options override queue_only unless queue_only_override is set
false. See also queue_only_file, queue_only_load, and smtp_accept_queue.

+----------------------------------------------------------+
|queue_only_file|Use: main|Type: string list|Default: unset|
+----------------------------------------------------------+

This option can be set to a colon-separated list of absolute path names, each
one optionally preceded by "smtp". When Exim is receiving a message, it tests
for the existence of each listed path using a call to stat(). For each path
that exists, the corresponding queueing option is set. For paths with no
prefix, queue_only is set; for paths prefixed by "smtp", queue_smtp_domains is
set to match all domains. So, for example,

queue_only_file = smtp/some/file

causes Exim to behave as if queue_smtp_domains were set to "*" whenever /some/
file exists.

+----------------------------------------------------------+
|queue_only_load|Use: main|Type: fixed-point|Default: unset|
+----------------------------------------------------------+

If the system load average is higher than this value, incoming messages from
all sources are queued, and no automatic deliveries are started. If this
happens during local or remote SMTP input, all subsequent messages received on
the same SMTP connection are queued by default, whatever happens to the load in
the meantime, but this can be changed by setting queue_only_load_latch false.

Deliveries will subsequently be performed by queue runner processes. This
option has no effect on ancient operating systems on which Exim cannot
determine the load average. See also deliver_queue_load_max and 
smtp_load_reserve.

+-----------------------------------------------------------+
|queue_only_load_latch|Use: main|Type: boolean|Default: true|
+-----------------------------------------------------------+

When this option is true (the default), once one message has been queued
because the load average is higher than the value set by queue_only_load, all
subsequent messages received on the same SMTP connection are also queued. This
is a deliberate choice; even though the load average may fall below the
threshold, it doesn't seem right to deliver later messages on the same
connection when not delivering earlier ones. However, there are special
circumstances such as very long-lived connections from scanning appliances
where this is not the best strategy. In such cases, queue_only_load_latch
should be set false. This causes the value of the load average to be
re-evaluated for each message.

+---------------------------------------------------------+
|queue_only_override|Use: main|Type: boolean|Default: true|
+---------------------------------------------------------+

When this option is true, the -odx command line options override the setting of
queue_only or queue_only_file in the configuration file. If queue_only_override
is set false, the -odx options cannot be used to override; they are accepted,
but ignored.

+---------------------------------------------------------+
|queue_run_in_order|Use: main|Type: boolean|Default: false|
+---------------------------------------------------------+

If this option is set, queue runs happen in order of message arrival instead of
in an arbitrary order. For this to happen, a complete list of the entire queue
must be set up before the deliveries start. When the queue is all held in a
single directory (the default), a single list is created for both the ordered
and the non-ordered cases. However, if split_spool_directory is set, a single
list is not created when queue_run_in_order is false. In this case, the
sub-directories are processed one at a time (in a random order), and this
avoids setting up one huge list for the whole queue. Thus, setting 
queue_run_in_order with split_spool_directory may degrade performance when the
queue is large, because of the extra work in setting up the single, large list.
In most situations, queue_run_in_order should not be set.

+-------------------------------------------------+
|queue_run_max|Use: main|Type: integer*|Default: 5|
+-------------------------------------------------+

This controls the maximum number of queue runner processes that an Exim daemon
can run simultaneously. This does not mean that it starts them all at once, but
rather that if the maximum number are still running when the time comes to
start another one, it refrains from starting another one. This can happen with
very large queues and/or very sluggish deliveries. This option does not,
however, interlock with other processes, so additional queue runners can be
started by other means, or by killing and restarting the daemon.

Setting this option to zero does not suppress queue runs; rather, it disables
the limit, allowing any number of simultaneous queue runner processes to be
run. If you do not want queue runs to occur, omit the -qxx setting on the
daemon's command line.

To set limits for different named queues use an expansion depending on the
$queue_name variable.

+--------------------------------------------------------------+
|queue_smtp_domains|Use: main|Type: domain list*|Default: unset|
+--------------------------------------------------------------+

When this option is set, a delivery process is started whenever a message is
received, routing is performed, and local deliveries take place. However, if
any SMTP deliveries are required for domains that match queue_smtp_domains,
they are not immediately delivered, but instead the message waits in the queue
for the next queue run. Since routing of the message has taken place, Exim
knows to which remote hosts it must be delivered, and so when the queue run
happens, multiple messages for the same host are delivered over a single SMTP
connection. The -odqs command line option causes all SMTP deliveries to be
queued in this way, and is equivalent to setting queue_smtp_domains to "*". See
also hold_domains and queue_domains.

+------------------------------------------------+
|receive_timeout|Use: main|Type: time|Default: 0s|
+------------------------------------------------+

This option sets the timeout for accepting a non-SMTP message, that is, the
maximum time that Exim waits when reading a message on the standard input. If
the value is zero, it will wait forever. This setting is overridden by the -or
command line option. The timeout for incoming SMTP messages is controlled by 
smtp_receive_timeout.

+---------------------------------------------------------------+
|received_header_text|Use: main|Type: string*|Default: see below|
+---------------------------------------------------------------+

This string defines the contents of the Received: message header that is added
to each message, except for the timestamp, which is automatically added on at
the end (preceded by a semicolon). The string is expanded each time it is used.
If the expansion yields an empty string, no Received: header line is added to
the message. Otherwise, the string should start with the text "Received:" and
conform to the RFC 2822 specification for Received: header lines. The default
setting is:

received_header_text = Received: \
  ${if def:sender_rcvhost {from $sender_rcvhost\n\t}\
    {${if def:sender_ident \
      {from ${quote_local_part:$sender_ident} }}\
        ${if def:sender_helo_name {(helo=$sender_helo_name)\n\t}}}}\
  by $primary_hostname \
  ${if def:received_protocol {with $received_protocol }}\
  ${if def:tls_in_ver        { ($tls_in_ver)}}\
  ${if def:tls_in_cipher_std { tls $tls_in_cipher_std\n\t}}\
  (Exim $version_number)\n\t\
  ${if def:sender_address \
  {(envelope-from <$sender_address>)\n\t}}\
  id $message_exim_id\
  ${if def:received_for {\n\tfor $received_for}}

The references to the TLS version and cipher are omitted when Exim is built
without TLS support. The use of conditional expansions ensures that this works
for both locally generated messages and messages received from remote hosts,
giving header lines such as the following:

Received: from scrooge.carol.example ([192.168.12.25] ident=root)
by marley.carol.example with esmtp (Exim 4.00)
(envelope-from <bob@carol.example>)
id 16IOWa-00019l-00
for chas@dickens.example; Tue, 25 Dec 2001 14:43:44 +0000
Received: by scrooge.carol.example with local (Exim 4.00)
id 16IOWW-000083-00; Tue, 25 Dec 2001 14:43:41 +0000

Until the body of the message has been received, the timestamp is the time when
the message started to be received. Once the body has arrived, and all policy
checks have taken place, the timestamp is updated to the time at which the
message was accepted.

+--------------------------------------------------------+
|received_headers_max|Use: main|Type: integer|Default: 30|
+--------------------------------------------------------+

When a message is to be delivered, the number of Received: headers is counted,
and if it is greater than this parameter, a mail loop is assumed to have
occurred, the delivery is abandoned, and an error message is generated. This
applies to both local and remote deliveries.

+---------------------------------------------------------------------+
|recipient_unqualified_hosts|Use: main|Type: host list*|Default: unset|
+---------------------------------------------------------------------+

This option lists those hosts from which Exim is prepared to accept unqualified
recipient addresses in message envelopes. The addresses are made fully
qualified by the addition of the qualify_recipient value. This option also
affects message header lines. Exim does not reject unqualified recipient
addresses in headers, but it qualifies them only if the message came from a
host that matches recipient_unqualified_hosts, or if the message was submitted
locally (not using TCP/IP), and the -bnq option was not set.

+-----------------------------------------------------+
|recipients_max|Use: main|Type: integer|Default: 50000|
+-----------------------------------------------------+

If this option is set greater than zero, it specifies the maximum number of
original recipients for any message. Additional recipients that are generated
by aliasing or forwarding do not count. SMTP messages get a 452 response for
all recipients over the limit; earlier recipients are delivered as normal.
Non-SMTP messages with too many recipients are failed, and no deliveries are
done.

Note: The RFCs specify that an SMTP server should accept at least 100 RCPT
commands in a single message.

+------------------------------------------------------------+
|recipients_max_reject|Use: main|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is set true, Exim rejects SMTP messages containing too many
recipients by giving 552 errors to the surplus RCPT commands, and a 554 error
to the eventual DATA command. Otherwise (the default) it gives a 452 error to
the surplus RCPT commands and accepts the message on behalf of the initial set
of recipients. The remote server should then re-send the message for the
remaining recipients at a later time.

+------------------------------------------------------+
|remote_max_parallel|Use: main|Type: integer|Default: 4|
+------------------------------------------------------+

This option controls parallel delivery of one message to a number of remote
hosts. If the value is less than 2, parallel delivery is disabled, and Exim
does all the remote deliveries for a message one by one. Otherwise, if a single
message has to be delivered to more than one remote host, or if several copies
have to be sent to the same remote host, up to remote_max_parallel deliveries
are done simultaneously. If more than remote_max_parallel deliveries are
required, the maximum number of processes are started, and as each one
finishes, another is begun. The order of starting processes is the same as if
sequential delivery were being done, and can be controlled by the 
remote_sort_domains option. If parallel delivery takes place while running with
debugging turned on, the debugging output from each delivery process is tagged
with its process id.

This option controls only the maximum number of parallel deliveries for one
message in one Exim delivery process. Because Exim has no central queue
manager, there is no way of controlling the total number of simultaneous
deliveries if the configuration allows a delivery attempt as soon as a message
is received.

See also the max_parallel generic transport option, and the serialize_hosts
smtp transport option.

If you want to control the total number of deliveries on the system, you need
to set the queue_only option. This ensures that all incoming messages are added
to the queue without starting a delivery process. Then set up an Exim daemon to
start queue runner processes at appropriate intervals (probably fairly often,
for example, every minute), and limit the total number of queue runners by
setting the queue_run_max parameter. Because each queue runner delivers only
one message at a time, the maximum number of deliveries that can then take
place at once is queue_run_max multiplied by remote_max_parallel.

If it is purely remote deliveries you want to control, use queue_smtp_domains
instead of queue_only. This has the added benefit of doing the SMTP routing
before queueing, so that several messages for the same host will eventually get
delivered down the same connection.

+---------------------------------------------------------------+
|remote_sort_domains|Use: main|Type: domain list*|Default: unset|
+---------------------------------------------------------------+

When there are a number of remote deliveries for a message, they are sorted by
domain into the order given by this list. For example,

remote_sort_domains = *.cam.ac.uk:*.uk

would attempt to deliver to all addresses in the cam.ac.uk domain first, then
to those in the uk domain, then to any others.

+--------------------------------------------------+
|retry_data_expire|Use: main|Type: time|Default: 7d|
+--------------------------------------------------+

This option sets a "use before" time on retry information in Exim's hints
database. Any older retry data is ignored. This means that, for example, once a
host has not been tried for 7 days, Exim behaves as if it has no knowledge of
past failures.

+----------------------------------------------------+
|retry_interval_max|Use: main|Type: time|Default: 24h|
+----------------------------------------------------+

Chapter 32 describes Exim's mechanisms for controlling the intervals between
delivery attempts for messages that cannot be delivered straight away. This
option sets an overall limit to the length of time between retries. It cannot
be set greater than 24 hours; any attempt to do so forces the default value.

+--------------------------------------------------------+
|return_path_remove|Use: main|Type: boolean|Default: true|
+--------------------------------------------------------+

RFC 2821, section 4.4, states that an SMTP server must insert a Return-path:
header line into a message when it makes a "final delivery". The Return-path:
header preserves the sender address as received in the MAIL command. This
description implies that this header should not be present in an incoming
message. If return_path_remove is true, any existing Return-path: headers are
removed from messages at the time they are received. Exim's transports have
options for adding Return-path: headers at the time of delivery. They are
normally used only for final local deliveries.

+-------------------------------------------------------+
|return_size_limit|Use: main|Type: integer|Default: 100K|
+-------------------------------------------------------+

This option is an obsolete synonym for bounce_return_size_limit.

+-----------------------------------------------------+
|rfc1413_hosts|Use: main|Type: host list*|Default: @[]|
+-----------------------------------------------------+

RFC 1413 identification calls are made to any client host which matches an item
in the list. The default value specifies just this host, being any local
interface for the system.

+------------------------------------------------------+
|rfc1413_query_timeout|Use: main|Type: time|Default: 0s|
+------------------------------------------------------+

This sets the timeout on RFC 1413 identification calls. If it is set to zero,
no RFC 1413 calls are ever made.

+------------------------------------------------------------------+
|sender_unqualified_hosts|Use: main|Type: host list*|Default: unset|
+------------------------------------------------------------------+

This option lists those hosts from which Exim is prepared to accept unqualified
sender addresses. The addresses are made fully qualified by the addition of 
qualify_domain. This option also affects message header lines. Exim does not
reject unqualified addresses in headers that contain sender addresses, but it
qualifies them only if the message came from a host that matches 
sender_unqualified_hosts, or if the message was submitted locally (not using
TCP/IP), and the -bnq option was not set.

+--------------------------------------------------+
|slow_lookup_log|Use: main|Type: integer|Default: 0|
+--------------------------------------------------+

This option controls logging of slow lookups. If the value is nonzero it is
taken as a number of milliseconds and lookups taking longer than this are
logged. Currently this applies only to DNS lookups.

+-----------------------------------------------------------+
|smtp_accept_keepalive|Use: main|Type: boolean|Default: true|
+-----------------------------------------------------------+

This option controls the setting of the SO_KEEPALIVE option on incoming TCP/IP
socket connections. When set, it causes the kernel to probe idle connections
periodically, by sending packets with "old" sequence numbers. The other end of
the connection should send an acknowledgment if the connection is still okay or
a reset if the connection has been aborted. The reason for doing this is that
it has the beneficial effect of freeing up certain types of connection that can
get stuck when the remote host is disconnected without tidying up the TCP/IP
call properly. The keepalive mechanism takes several hours to detect
unreachable hosts.

+---------------------------------------------------+
|smtp_accept_max|Use: main|Type: integer|Default: 20|
+---------------------------------------------------+

This option specifies the maximum number of simultaneous incoming SMTP calls
that Exim will accept. It applies only to the listening daemon; there is no
control (in Exim) when incoming SMTP is being handled by inetd. If the value is
set to zero, no limit is applied. However, it is required to be non-zero if
either smtp_accept_max_per_host or smtp_accept_queue is set. See also 
smtp_accept_reserve and smtp_load_reserve.

A new SMTP connection is immediately rejected if the smtp_accept_max limit has
been reached. If not, Exim first checks smtp_accept_max_per_host. If that limit
has not been reached for the client host, smtp_accept_reserve and 
smtp_load_reserve are then checked before accepting the connection.

+-----------------------------------------------------------+
|smtp_accept_max_nonmail|Use: main|Type: integer|Default: 10|
+-----------------------------------------------------------+

Exim counts the number of "non-mail" commands in an SMTP session, and drops the
connection if there are too many. This option defines "too many". The check
catches some denial-of-service attacks, repeated failing AUTHs, or a mad client
looping sending EHLO, for example. The check is applied only if the client host
matches smtp_accept_max_nonmail_hosts.

When a new message is expected, one occurrence of RSET is not counted. This
allows a client to send one RSET between messages (this is not necessary, but
some clients do it). Exim also allows one uncounted occurrence of HELO or EHLO,
and one occurrence of STARTTLS between messages. After starting up a TLS
session, another EHLO is expected, and so it too is not counted. The first
occurrence of AUTH in a connection, or immediately following STARTTLS is not
counted. Otherwise, all commands other than MAIL, RCPT, DATA, and QUIT are
counted.

+-------------------------------------------------------------------+
|smtp_accept_max_nonmail_hosts|Use: main|Type: host list*|Default: *|
+-------------------------------------------------------------------+

You can control which hosts are subject to the smtp_accept_max_nonmail check by
setting this option. The default value makes it apply to all hosts. By changing
the value, you can exclude any badly-behaved hosts that you have to live with.

+---------------------------------------------------------------------+
|smtp_accept_max_per_connection|Use: main|Type: integer*|Default: 1000|
+---------------------------------------------------------------------+

The value of this option limits the number of MAIL commands that Exim is
prepared to accept over a single SMTP connection, whether or not each command
results in the transfer of a message. After the limit is reached, a 421
response is given to subsequent MAIL commands. This limit is a safety
precaution against a client that goes mad (incidents of this type have been
seen). The option is expanded after the HELO or EHLO is received and may depend
on values available at that time. An empty or zero value after expansion
removes the limit.

+---------------------------------------------------------------+
|smtp_accept_max_per_host|Use: main|Type: string*|Default: unset|
+---------------------------------------------------------------+

This option restricts the number of simultaneous IP connections from a single
host (strictly, from a single IP address) to the Exim daemon. The option is
expanded, to enable different limits to be applied to different hosts by
reference to $sender_host_address. Once the limit is reached, additional
connection attempts from the same host are rejected with error code 421. This
is entirely independent of smtp_accept_reserve. The option's default value of
zero imposes no limit. If this option is set greater than zero, it is required
that smtp_accept_max be non-zero.

Warning: When setting this option you should not use any expansion
constructions that take an appreciable amount of time. The expansion and test
happen in the main daemon loop, in order to reject additional connections
without forking additional processes (otherwise a denial-of-service attack
could cause a vast number or processes to be created). While the daemon is
doing this processing, it cannot accept any other incoming connections.

+----------------------------------------------------+
|smtp_accept_queue|Use: main|Type: integer|Default: 0|
+----------------------------------------------------+

If the number of simultaneous incoming SMTP connections being handled via the
listening daemon exceeds this value, messages received by SMTP are just placed
in the queue; no delivery processes are started automatically. The count is
fixed at the start of an SMTP connection. It cannot be updated in the
subprocess that receives messages, and so the queueing or not queueing applies
to all messages received in the same connection.

A value of zero implies no limit, and clearly any non-zero value is useful only
if it is less than the smtp_accept_max value (unless that is zero). See also 
queue_only, queue_only_load, queue_smtp_domains, and the various -odx command
line options.

+--------------------------------------------------------------------+
|smtp_accept_queue_per_connection|Use: main|Type: integer|Default: 10|
+--------------------------------------------------------------------+

This option limits the number of delivery processes that Exim starts
automatically when receiving messages via SMTP, whether via the daemon or by
the use of -bs or -bS. If the value of the option is greater than zero, and the
number of messages received in a single SMTP session exceeds this number,
subsequent messages are placed in the queue, but no delivery processes are
started. This helps to limit the number of Exim processes when a server
restarts after downtime and there is a lot of mail waiting for it on other
systems. On large systems, the default should probably be increased, and on
dial-in client systems it should probably be set to zero (that is, disabled).

+------------------------------------------------------+
|smtp_accept_reserve|Use: main|Type: integer|Default: 0|
+------------------------------------------------------+

When smtp_accept_max is set greater than zero, this option specifies a number
of SMTP connections that are reserved for connections from the hosts that are
specified in smtp_reserve_hosts. The value set in smtp_accept_max includes this
reserve pool. The specified hosts are not restricted to this number of
connections; the option specifies a minimum number of connection slots for
them, not a maximum. It is a guarantee that this group of hosts can always get
at least smtp_accept_reserve connections. However, the limit specified by 
smtp_accept_max_per_host is still applied to each individual host.

For example, if smtp_accept_max is set to 50 and smtp_accept_reserve is set to
5, once there are 45 active connections (from any hosts), new connections are
accepted only from hosts listed in smtp_reserve_hosts, provided the other
criteria for acceptance are met.

+-----------------------------------------------------------+
|smtp_active_hostname|Use: main|Type: string*|Default: unset|
+-----------------------------------------------------------+

This option is provided for multi-homed servers that want to masquerade as
several different hosts. At the start of an incoming SMTP connection, its value
is expanded and used instead of the value of $primary_hostname in SMTP
responses. For example, it is used as domain name in the response to an
incoming HELO or EHLO command.

The active hostname is placed in the $smtp_active_hostname variable, which is
saved with any messages that are received. It is therefore available for use in
routers and transports when the message is later delivered.

If this option is unset, or if its expansion is forced to fail, or if the
expansion results in an empty string, the value of $primary_hostname is used.
Other expansion failures cause a message to be written to the main and panic
logs, and the SMTP command receives a temporary error. Typically, the value of 
smtp_active_hostname depends on the incoming interface address. For example:

smtp_active_hostname = ${if eq{$received_ip_address}{10.0.0.1}\
  {cox.mydomain}{box.mydomain}}

Although $smtp_active_hostname is primarily concerned with incoming messages,
it is also used as the default for HELO commands in callout verification if
there is no remote transport from which to obtain a helo_data value.

+-------------------------------------------------------+
|smtp_backlog_monitor|Use: main|Type: integer|Default: 0|
+-------------------------------------------------------+

If this option is set to greater than zero, and the backlog of available TCP
connections on a socket listening for SMTP is larger than it, a line is logged
giving the value and the socket address and port. The value is retrived jsut
before an accept call. This facility is only available on Linux.

+------------------------------------------------------+
|smtp_banner|Use: main|Type: string*|Default: see below|
+------------------------------------------------------+

If a connect ACL does not supply a message, this string (which is expanded
every time it is used) is output as the initial positive response to an SMTP
connection. The default setting is:

smtp_banner = $smtp_active_hostname ESMTP Exim \
  $version_number $tod_full

Failure to expand the string causes a panic error; a forced fail just closes
the connection.

If you want to create a multiline response to the initial SMTP connection, use
"\n" in the string at appropriate points, but not at the end. Note that the 220
code is not included in this string. Exim adds it automatically (several times
in the case of a multiline response).

+------------------------------------------------------------+
|smtp_check_spool_space|Use: main|Type: boolean|Default: true|
+------------------------------------------------------------+

When this option is set, if an incoming SMTP session encounters the SIZE option
on a MAIL command, it checks that there is enough space in the spool
directory's partition to accept a message of that size, while still leaving
free the amount specified by check_spool_space (even if that value is zero). If
there isn't enough space, a temporary error code is returned.

+--------------------------------------------------------+
|smtp_connect_backlog|Use: main|Type: integer|Default: 20|
+--------------------------------------------------------+

This option specifies a maximum number of waiting SMTP connections. Exim passes
this value to the TCP/IP system when it sets up its listener. Once this number
of connections are waiting for the daemon's attention, subsequent connection
attempts are refused at the TCP/IP level. At least, that is what the manuals
say; in some circumstances such connection attempts have been observed to time
out instead. For large systems it is probably a good idea to increase the value
(to 50, say). It also gives some protection against denial-of-service attacks
by SYN flooding.

+-------------------------------------------------------+
|smtp_enforce_sync|Use: main|Type: boolean|Default: true|
+-------------------------------------------------------+

The SMTP protocol specification requires the client to wait for a response from
the server at certain points in the dialogue. Without PIPELINING these
synchronization points are after every command; with PIPELINING they are fewer,
but they still exist.

Some spamming sites send out a complete set of SMTP commands without waiting
for any response. Exim protects against this by rejecting a message if the
client has sent further input when it should not have. The error response "554
SMTP synchronization error" is sent, and the connection is dropped. Testing for
this error cannot be perfect because of transmission delays (unexpected input
may be on its way but not yet received when Exim checks). However, it does
detect many instances.

The check can be globally disabled by setting smtp_enforce_sync false. If you
want to disable the check selectively (for example, only for certain hosts),
you can do so by an appropriate use of a control modifier in an ACL (see
section 44.13). See also pipelining_advertise_hosts.

+--------------------------------------------------------+
|smtp_etrn_command|Use: main|Type: string*|Default: unset|
+--------------------------------------------------------+

If this option is set, the given command is run whenever an SMTP ETRN command
is received from a host that is permitted to issue such commands (see chapter
44). The string is split up into separate arguments which are independently
expanded. The expansion variable $domain is set to the argument of the ETRN
command, and no syntax checking is done on it. For example:

smtp_etrn_command = /etc/etrn_command $domain \
                    $sender_host_address

If the option is not set, the argument for the ETRN command must be a #
followed by an address string. In this case an exim -R <string> command is
used; if the ETRN ACL has set up a named-queue then -MCG <queue> is appended.

A new process is created to run the command, but Exim does not wait for it to
complete. Consequently, its status cannot be checked. If the command cannot be
run, a line is written to the panic log, but the ETRN caller still receives a
250 success response. Exim is normally running under its own uid when receiving
SMTP, so it is not possible for it to change the uid before running the
command.

+---------------------------------------------------------+
|smtp_etrn_serialize|Use: main|Type: boolean|Default: true|
+---------------------------------------------------------+

When this option is set, it prevents the simultaneous execution of more than
one identical command as a result of ETRN in an SMTP connection. See section
49.2.5 for details.

+------------------------------------------------------------+
|smtp_load_reserve|Use: main|Type: fixed-point|Default: unset|
+------------------------------------------------------------+

If the system load average ever gets higher than this, incoming SMTP calls are
accepted only from those hosts that match an entry in smtp_reserve_hosts. If 
smtp_reserve_hosts is not set, no incoming SMTP calls are accepted when the
load is over the limit. The option has no effect on ancient operating systems
on which Exim cannot determine the load average. See also 
deliver_queue_load_max and queue_only_load.

+----------------------------------------------------------+
|smtp_max_synprot_errors|Use: main|Type: integer|Default: 3|
+----------------------------------------------------------+

Exim rejects SMTP commands that contain syntax or protocol errors. In
particular, a syntactically invalid email address, as in this command:

RCPT TO:<abc xyz@a.b.c>

causes immediate rejection of the command, before any other tests are done.
(The ACL cannot be run if there is no valid address to set up for it.) An
example of a protocol error is receiving RCPT before MAIL. If there are too
many syntax or protocol errors in one SMTP session, the connection is dropped.
The limit is set by this option.

When the PIPELINING extension to SMTP is in use, some protocol errors are
"expected", for instance, a RCPT command after a rejected MAIL command. Exim
assumes that PIPELINING will be used if it advertises it (see 
pipelining_advertise_hosts), and in this situation, "expected" errors do not
count towards the limit.

+------------------------------------------------------------+
|smtp_max_unknown_commands|Use: main|Type: integer|Default: 3|
+------------------------------------------------------------+

If there are too many unrecognized commands in an incoming SMTP session, an
Exim server drops the connection. This is a defence against some kinds of abuse
that subvert web clients into making connections to SMTP ports; in these
circumstances, a number of non-SMTP command lines are sent first.

+--------------------------------------------------------------+
|smtp_ratelimit_hosts|Use: main|Type: host list*|Default: unset|
|--------------------+---------+----------------+--------------|
|smtp_ratelimit_mail |Use: main|  Type: string  |Default: unset|
|--------------------+---------+----------------+--------------|
|smtp_ratelimit_rcpt |Use: main|  Type: string  |Default: unset|
+--------------------------------------------------------------+

Some sites find it helpful to be able to limit the rate at which certain hosts
can send them messages, and the rate at which an individual message can specify
recipients.

Exim has two rate-limiting facilities. This section describes the older
facility, which can limit rates within a single connection. The newer ratelimit
ACL condition can limit rates across all connections. See section 44.20 for
details of the newer facility.

When a host matches smtp_ratelimit_hosts, the values of smtp_ratelimit_mail and
smtp_ratelimit_rcpt are used to control the rate of acceptance of MAIL and RCPT
commands in a single SMTP session, respectively. Each option, if set, must
contain a set of four comma-separated values:

  * A threshold, before which there is no rate limiting.

  * An initial time delay. Unlike other times in Exim, numbers with decimal
    fractional parts are allowed here.

  * A factor by which to increase the delay each time.

  * A maximum value for the delay. This should normally be less than 5 minutes,
    because after that time, the client is liable to timeout the SMTP command.

For example, these settings have been used successfully at the site which first
suggested this feature, for controlling mail from their customers:

smtp_ratelimit_mail = 2,0.5s,1.05,4m
smtp_ratelimit_rcpt = 4,0.25s,1.015,4m

The first setting specifies delays that are applied to MAIL commands after two
have been received over a single connection. The initial delay is 0.5 seconds,
increasing by a factor of 1.05 each time. The second setting applies delays to
RCPT commands when more than four occur in a single message.

+------------------------------------------------------+
|smtp_receive_timeout|Use: main|Type: time*|Default: 5m|
+------------------------------------------------------+

This sets a timeout value for SMTP reception. It applies to all forms of SMTP
input, including batch SMTP. If a line of input (either an SMTP command or a
data line) is not received within this time, the SMTP connection is dropped and
the message is abandoned. A line is written to the log containing one of the
following messages:

SMTP command timeout on connection from...
SMTP data timeout on connection from...

The former means that Exim was expecting to read an SMTP command; the latter
means that it was in the DATA phase, reading the contents of a message.

If the first character of the option is a "$" the option is expanded before use
and may depend on $sender_host_name, $sender_host_address and $sender_host_port
.

The value set by this option can be overridden by the -os command-line option.
A setting of zero time disables the timeout, but this should never be used for
SMTP over TCP/IP. (It can be useful in some cases of local input using -bs or 
-bS.) For non-SMTP input, the reception timeout is controlled by 
receive_timeout and -or.

+------------------------------------------------------------+
|smtp_reserve_hosts|Use: main|Type: host list*|Default: unset|
+------------------------------------------------------------+

This option defines hosts for which SMTP connections are reserved; see 
smtp_accept_reserve and smtp_load_reserve above.

+----------------------------------------------------------------+
|smtp_return_error_details|Use: main|Type: boolean|Default: false|
+----------------------------------------------------------------+

In the default state, Exim uses bland messages such as "Administrative
prohibition" when it rejects SMTP commands for policy reasons. Many sysadmins
like this because it gives away little information to spammers. However, some
other sysadmins who are applying strict checking policies want to give out much
fuller information about failures. Setting smtp_return_error_details true
causes Exim to be more forthcoming. For example, instead of "Administrative
prohibition", it might give:

550-Rejected after DATA: '>' missing at end of address:
550 failing address in "From" header is: <user@dom.ain

+--------------------------------------------------------------+
|smtputf8_advertise_hosts|Use: main|Type: host list*|Default: *|
+--------------------------------------------------------------+

When Exim is built with support for internationalised mail names, the
availability thereof is advertised in response to EHLO only to those client
hosts that match this option. See chapter 60 for details of Exim's support for
internationalisation.

+-----------------------------------------------------------+
|spamd_address|Use: main|Type: string|Default: 127.0.0.1 783|
+-----------------------------------------------------------+

This option is available when Exim is compiled with the content-scanning
extension. It specifies how Exim connects to SpamAssassin's spamd daemon. See
section 45.2 for more details.

+--------------------------------------------------------------------+
|spf_guess|Use: main|Type: string|Default: v=spf1 a/24 mx/24 ptr ?all|
+--------------------------------------------------------------------+

This option is available when Exim is compiled with SPF support. See section
58.2 for more details.

+-----------------------------------------------------------------------------+
|spf_smtp_comment_template|Use:  | Type:   |     Default: Please%_see%_http://|
|                         | main | string* |              www.open-spf.org/Why|
+-----------------------------------------------------------------------------+

This option is available when Exim is compiled with SPF support. It allows the
customisation of the SMTP comment that the SPF library generates. You are
strongly encouraged to link to your own explanative site. The template must not
contain spaces. If you need spaces in the output, use the proper placeholder.
If libspf2 can not parse the template, it uses a built-in default broken link.
The following placeholders (along with Exim variables (but see below)) are
allowed in the template:

  * %_: A space.

  * %{L}: Envelope sender's local part.

  * %{S}: Envelope sender.

  * %{O}: Envelope sender's domain.

  * %{D}: Current(?) domain.

  * %{I}: SMTP client Ip.

  * %{C}: SMTP client pretty IP.

  * %{T}: Epoch time (UTC).

  * %{P}: SMTP client domain name.

  * %{V}: IP version.

  * %{H}: EHLO/HELO domain.

  * %{R}: Receiving domain.

The capitalized placeholders do proper URL encoding, if you use them
lowercased, no encoding takes place. This list was compiled from the libspf2
sources.

A note on using Exim variables: As currently the SPF library is initialized
before the SMTP EHLO phase, the variables useful for expansion are quite
limited.

+------------------------------------------------------------+
|split_spool_directory|Use: main|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is set, it causes Exim to split its input directory into 62
subdirectories, each with a single alphanumeric character as its name. The
sixth character of the message id is used to allocate messages to
subdirectories; this is the least significant base-62 digit of the time of
arrival of the message.

Splitting up the spool in this way may provide better performance on systems
where there are long mail queues, by reducing the number of files in any one
directory. The msglog directory is also split up in a similar way to the input
directory; however, if preserve_message_logs is set, all old msglog files are
still placed in the single directory msglog.OLD.

It is not necessary to take any special action for existing messages when
changing split_spool_directory. Exim notices messages that are in the "wrong"
place, and continues to process them. If the option is turned off after a
period of being on, the subdirectories will eventually empty and be
automatically deleted.

When split_spool_directory is set, the behaviour of queue runner processes
changes. Instead of creating a list of all messages in the queue, and then
trying to deliver each one, in turn, it constructs a list of those in one
sub-directory and tries to deliver them, before moving on to the next
sub-directory. The sub-directories are processed in a random order. This
spreads out the scanning of the input directories, and uses less memory. It is
particularly beneficial when there are lots of messages in the queue. However,
if queue_run_in_order is set, none of this new processing happens. The entire
queue has to be scanned and sorted before any deliveries can start.

+--------------------------------------------------------------------+
|spool_directory|Use: main|Type: string*|Default: set at compile time|
+--------------------------------------------------------------------+

This defines the directory in which Exim keeps its spool, that is, the messages
it is waiting to deliver. The default value is taken from the compile-time
configuration setting, if there is one. If not, this option must be set. The
string is expanded, so it can contain, for example, a reference to
$primary_hostname.

If the spool directory name is fixed on your installation, it is recommended
that you set it at build time rather than from this option, particularly if the
log files are being written to the spool directory (see log_file_path).
Otherwise log files cannot be used for errors that are detected early on, such
as failures in the configuration file.

By using this option to override the compiled-in path, it is possible to run
tests of Exim without using the standard spool.

+-------------------------------------------------------+
|spool_wireformat|Use: main|Type: boolean|Default: false|
+-------------------------------------------------------+

If this option is set, Exim may for some messages use an alternative format for
data-files in the spool which matches the wire format. Doing this permits more
efficient message reception and transmission. Currently it is only done for
messages received using the ESMTP CHUNKING option.

The following variables will not have useful values:

$max_received_linelength
$body_linecount
$body_zerocount

Users of the local_scan() API (see 46), and any external programs which are
passed a reference to a message data file (except via the "regex", "malware" or
"spam") ACL conditions) will need to be aware of the different formats
potentially available.

Using any of the ACL conditions noted will negate the reception benefit (as a
Unix-mbox-format file is constructed for them). The transmission benefit is
maintained.

+----------------------------------------------------+
|sqlite_lock_timeout|Use: main|Type: time|Default: 5s|
+----------------------------------------------------+

This option controls the timeout that the sqlite lookup uses when trying to
access an SQLite database. See section 9.13.5 for more details.

+------------------------------------------------------+
|strict_acl_vars|Use: main|Type: boolean|Default: false|
+------------------------------------------------------+

This option controls what happens if a syntactically valid but undefined ACL
variable is referenced. If it is false (the default), an empty string is
substituted; if it is true, an error is generated. See section 44.10 for
details of ACL variables.

+------------------------------------------------------------------+
|strip_excess_angle_brackets|Use: main|Type: boolean|Default: false|
+------------------------------------------------------------------+

If this option is set, redundant pairs of angle brackets round "route-addr"
items in addresses are stripped. For example, <<xxx@a.b.c.d>> is treated as 
<xxx@a.b.c.d>. If this is in the envelope and the message is passed on to
another MTA, the excess angle brackets are not passed on. If this option is not
set, multiple pairs of angle brackets cause a syntax error.

+---------------------------------------------------------+
|strip_trailing_dot|Use: main|Type: boolean|Default: false|
+---------------------------------------------------------+

If this option is set, a trailing dot at the end of a domain in an address is
ignored. If this is in the envelope and the message is passed on to another
MTA, the dot is not passed on. If this option is not set, a dot at the end of a
domain causes a syntax error. However, addresses in header lines are checked
only when an ACL requests header syntax checking.

+--------------------------------------------------------+
|syslog_duplication|Use: main|Type: boolean|Default: true|
+--------------------------------------------------------+

When Exim is logging to syslog, it writes the log lines for its three separate
logs at different syslog priorities so that they can in principle be separated
on the logging hosts. Some installations do not require this separation, and in
those cases, the duplication of certain log lines is a nuisance. If 
syslog_duplication is set false, only one copy of any particular log line is
written to syslog. For lines that normally go to both the main log and the
reject log, the reject log version (possibly containing message header lines)
is written, at LOG_NOTICE priority. Lines that normally go to both the main and
the panic log are written at the LOG_ALERT priority.

+-----------------------------------------------------+
|syslog_facility|Use: main|Type: string|Default: unset|
+-----------------------------------------------------+

This option sets the syslog "facility" name, used when Exim is logging to
syslog. The value must be one of the strings "mail", "user", "news", "uucp",
"daemon", or "localx" where x is a digit between 0 and 7. If this option is
unset, "mail" is used. See chapter 53 for details of Exim's logging.

+------------------------------------------------+
|syslog_pid|Use: main|Type: boolean|Default: true|
+------------------------------------------------+

If syslog_pid is set false, the PID on Exim's log lines are omitted when these
lines are sent to syslog. (Syslog normally prefixes the log lines with the PID
of the logging process automatically.) You need to enable the "+pid" log
selector item, if you want Exim to write it's PID into the logs.) See chapter
53 for details of Exim's logging.

+---------------------------------------------------------+
|syslog_processname|Use: main|Type: string|Default: "exim"|
+---------------------------------------------------------+

This option sets the syslog "ident" name, used when Exim is logging to syslog.
The value must be no longer than 32 characters. See chapter 53 for details of
Exim's logging.

+------------------------------------------------------+
|syslog_timestamp|Use: main|Type: boolean|Default: true|
+------------------------------------------------------+

If syslog_timestamp is set false, the timestamps on Exim's log lines are
omitted when these lines are sent to syslog. See chapter 53 for details of
Exim's logging.

+----------------------------------------------------+
|system_filter|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option specifies an Exim filter file that is applied to all messages at
the start of each delivery attempt, before any routing is done. System filters
must be Exim filters; they cannot be Sieve filters. If the system filter
generates any deliveries to files or pipes, or any new mail messages, the
appropriate system_filter_..._transport option(s) must be set, to define which
transports are to be used. Details of this facility are given in chapter 47. A
forced expansion failure results in no filter operation.

+------------------------------------------------------------------------+
|system_filter_directory_transport|Use: main|Type: string*|Default: unset|
+------------------------------------------------------------------------+

This sets the name of the transport driver that is to be used when the save
command in a system message filter specifies a path ending in "/", implying
delivery of each message into a separate file in some directory. During the
delivery, the variable $address_file contains the path name.

+-------------------------------------------------------------------+
|system_filter_file_transport|Use: main|Type: string*|Default: unset|
+-------------------------------------------------------------------+

This sets the name of the transport driver that is to be used when the save
command in a system message filter specifies a path not ending in "/". During
the delivery, the variable $address_file contains the path name.

+---------------------------------------------------------+
|system_filter_group|Use: main|Type: string|Default: unset|
+---------------------------------------------------------+

This option is used only when system_filter_user is also set. It sets the gid
under which the system filter is run, overriding any gid that is associated
with the user. The value may be numerical or symbolic.

+-------------------------------------------------------------------+
|system_filter_pipe_transport|Use: main|Type: string*|Default: unset|
+-------------------------------------------------------------------+

This specifies the transport driver that is to be used when a pipe command is
used in a system filter. During the delivery, the variable $address_pipe
contains the pipe command.

+--------------------------------------------------------------------+
|system_filter_reply_transport|Use: main|Type: string*|Default: unset|
+--------------------------------------------------------------------+

This specifies the transport driver that is to be used when a mail command is
used in a system filter.

+--------------------------------------------------------+
|system_filter_user|Use: main|Type: string|Default: unset|
+--------------------------------------------------------+

If this option is set to root, the system filter is run in the main Exim
delivery process, as root. Otherwise, the system filter runs in a separate
process, as the given user, defaulting to the Exim run-time user. Unless the
string consists entirely of digits, it is looked up in the password data.
Failure to find the named user causes a configuration error. The gid is either
taken from the password data, or specified by system_filter_group. When the uid
is specified numerically, system_filter_group is required to be set.

If the system filter generates any pipe, file, or reply deliveries, the uid
under which the filter is run is used when transporting them, unless a
transport option overrides.

+-------------------------------------------------+
|tcp_nodelay|Use: main|Type: boolean|Default: true|
+-------------------------------------------------+

If this option is set false, it stops the Exim daemon setting the TCP_NODELAY
option on its listening sockets. Setting TCP_NODELAY turns off the "Nagle
algorithm", which is a way of improving network performance in interactive
(character-by-character) situations. Turning it off should improve Exim's
performance a bit, so that is what happens by default. However, it appears that
some broken clients cannot cope, and time out. Hence this option. It affects
only those sockets that are set up for listening by the daemon. Sockets created
by the smtp transport for delivering mail always set TCP_NODELAY.

+-----------------------------------------------------+
|timeout_frozen_after|Use: main|Type: time|Default: 0s|
+-----------------------------------------------------+

If timeout_frozen_after is set to a time greater than zero, a frozen message of
any kind that has been in the queue for longer than the given time is
automatically cancelled at the next queue run. If the frozen message is a
bounce message, it is just discarded; otherwise, a bounce is sent to the
sender, in a similar manner to cancellation by the -Mg command line option. If
you want to timeout frozen bounce messages earlier than other kinds of frozen
message, see ignore_bounce_errors_after.

Note: the default value of zero means no timeouts; with this setting, frozen
messages remain in the queue forever (except for any frozen bounce messages
that are released by ignore_bounce_errors_after).

+----------------------------------------------+
|timezone|Use: main|Type: string|Default: unset|
+----------------------------------------------+

The value of timezone is used to set the environment variable TZ while running
Exim (if it is different on entry). This ensures that all timestamps created by
Exim are in the required timezone. If you want all your timestamps to be in UTC
(aka GMT) you should set

timezone = UTC

The default value is taken from TIMEZONE_DEFAULT in Local/Makefile, or, if that
is not set, from the value of the TZ environment variable when Exim is built.
If timezone is set to the empty string, either at build or run time, any
existing TZ variable is removed from the environment when Exim runs. This is
appropriate behaviour for obtaining wall-clock time on some, but unfortunately
not all, operating systems.

+---------------------------------------------------------+
|tls_advertise_hosts|Use: main|Type: host list*|Default: *|
+---------------------------------------------------------+

When Exim is built with support for TLS encrypted connections, the availability
of the STARTTLS command to set up an encrypted session is advertised in
response to EHLO only to those client hosts that match this option. See chapter
43 for details of Exim's support for TLS. Note that the default value requires
that a certificate be supplied using the tls_certificate option. If TLS support
for incoming connections is not required the tls_advertise_hosts option should
be set empty.

+-----------------------------------------------------------+
|tls_alpn|Use: main|Type: string list*|Default: smtp : esmtp|
+-----------------------------------------------------------+

If this option is set, the TLS library supports ALPN, and the client offers
either more than one ALPN name or a name which does not match the list, the TLS
connection is declined.

+-----------------------------------------------------------+
|tls_certificate|Use: main|Type: string list*|Default: unset|
+-----------------------------------------------------------+

The value of this option is expanded, and must then be a list of absolute paths
to files which contain the server's certificates (in PEM format). Commonly only
one file is needed. The server's private key is also assumed to be in this file
if tls_privatekey is unset. See chapter 43 for further details.

Note: The certificates defined by this option are used only when Exim is
receiving incoming messages as a server. If you want to supply certificates for
use when sending messages as a client, you must set the tls_certificate option
in the relevant smtp transport.

Note: If you use filenames based on IP addresses, change the list separator in
the usual way (6.21) to avoid confusion under IPv6.

Note: Under versions of OpenSSL preceding 1.1.1, when a list of more than one
file is used, the $tls_in_ourcert variable is unreliable. The macro
"_TLS_BAD_MULTICERT_IN_OURCERT" will be defined for those versions.

If the option contains $tls_out_sni and Exim is built against OpenSSL, then if
the OpenSSL build supports TLS extensions and the TLS client sends the Server
Name Indication extension, then this option and others documented in 43.8 will
be re-expanded.

If this option is unset or empty a self-signed certificate will be used. Under
Linux this is generated at daemon startup; on other platforms it will be
generated fresh for every connection.

+----------------------------------------------+
|tls_crl|Use: main|Type: string*|Default: unset|
+----------------------------------------------+

This option specifies a certificate revocation list. The expanded value must be
the name of a file that contains CRLs in PEM format.

Under OpenSSL the option can specify a directory with CRL files.

Note: Under OpenSSL the option must, if given, supply a CRL for each signing
element of the certificate chain (i.e. all but the leaf). For the file variant
this can be multiple PEM blocks in the one file.

See 43.8 for discussion of when this option might be re-expanded.

+-----------------------------------------------------+
|tls_dh_max_bits|Use: main|Type: integer|Default: 2236|
+-----------------------------------------------------+

The number of bits used for Diffie-Hellman key-exchange may be suggested by the
chosen TLS library. That value might prove to be too high for interoperability.
This option provides a maximum clamp on the value suggested, trading off
security for interoperability.

The value must be at least 1024.

The value 2236 was chosen because, at time of adding the option, it was the
hard-coded maximum value supported by the NSS cryptographic library, as used by
Thunderbird, while GnuTLS was suggesting 2432 bits as normal.

If you prefer more security and are willing to break some clients, raise this
number.

Note that the value passed to GnuTLS for *generating* a new prime may be a
little less than this figure, because GnuTLS is inexact and may produce a
larger prime than requested.

+--------------------------------------------------+
|tls_dhparam|Use: main|Type: string*|Default: unset|
+--------------------------------------------------+

The value of this option is expanded and indicates the source of DH parameters
to be used by Exim.

Note: The Exim Maintainers strongly recommend using a filename with
site-generated local DH parameters, which has been supported across all
versions of Exim. The other specific constants available are a fallback so that
even when "unconfigured", Exim can offer Perfect Forward Secrecy in older
ciphersuites in TLS.

If tls_dhparam is a filename starting with a "/", then it names a file from
which DH parameters should be loaded. If the file exists, it should hold a
PEM-encoded PKCS#3 representation of the DH prime. If the file does not exist,
for OpenSSL it is an error. For GnuTLS, Exim will attempt to create the file
and fill it with a generated DH prime. For OpenSSL, if the DH bit-count from
loading the file is greater than tls_dh_max_bits then it will be ignored, and
treated as though the tls_dhparam were set to "none".

If this option expands to the string "none", then no DH parameters will be
loaded by Exim.

If this option expands to the string "historic" and Exim is using GnuTLS, then
Exim will attempt to load a file from inside the spool directory. If the file
does not exist, Exim will attempt to create it. See section 43.3 for further
details.

If Exim is using OpenSSL and this option is empty or unset, then Exim will load
a default DH prime; the default is Exim-specific but lacks verifiable
provenance.

In older versions of Exim the default was the 2048 bit prime described in
section 2.2 of RFC 5114, "2048-bit MODP Group with 224-bit Prime Order
Subgroup", which in IKE is assigned number 23.

Otherwise, the option must expand to the name used by Exim for any of a number
of DH primes specified in RFC 2409, RFC 3526, RFC 5114, RFC 7919, or from other
sources. As names, Exim uses a standard specified name, else "ike" followed by
the number used by IKE, or "default" which corresponds to
"exim.dev.20160529.3".

The available standard primes are: "ffdhe2048", "ffdhe3072", "ffdhe4096",
"ffdhe6144", "ffdhe8192", "ike1", "ike2", "ike5", "ike14", "ike15", "ike16",
"ike17", "ike18", "ike22", "ike23" and "ike24".

The available additional primes are: "exim.dev.20160529.1",
"exim.dev.20160529.2" and "exim.dev.20160529.3".

Some of these will be too small to be accepted by clients. Some may be too
large to be accepted by clients. The open cryptographic community has
suspicions about the integrity of some of the later IKE values, which led into
RFC7919 providing new fixed constants (the "ffdhe" identifiers).

At this point, all of the "ike" values should be considered obsolete; they are
still in Exim to avoid breaking unusual configurations, but are candidates for
removal the next time we have backwards-incompatible changes. Two of them in
particular ("ike1" and "ike22") are called out by RFC 8247 as MUST NOT use for
IPSEC, and two more ("ike23" and "ike24") as SHOULD NOT. Because of this, Exim
regards them as deprecated; if either of the first pair are used, warnings will
be logged in the paniclog, and if any are used then warnings will be logged in
the mainlog. All four will be removed in a future Exim release.

The TLS protocol does not negotiate an acceptable size for this; clients tend
to hard-drop connections if what is offered by the server is unacceptable,
whether too large or too small, and there's no provision for the client to tell
the server what these constraints are. Thus, as a server operator, you need to
make an educated guess as to what is most likely to work for your userbase.

Some known size constraints suggest that a bit-size in the range 2048 to 2236
is most likely to maximise interoperability. The upper bound comes from
applications using the Mozilla Network Security Services (NSS) library, which
used to set its "DH_MAX_P_BITS" upper-bound to 2236. This affects many mail
user agents (MUAs). The lower bound comes from Debian installs of Exim4 prior
to the 4.80 release, as Debian used to patch Exim to raise the minimum
acceptable bound from 1024 to 2048.

+-------------------------------------------------+
|tls_eccurve|Use: main|Type: string|Default: list*|
+-------------------------------------------------+

This option selects EC curves for use by Exim when used with OpenSSL. It has no
effect when Exim is used with GnuTLS (the equivalent can be done using a
priority string for the tls_require_ciphers option).

After expansion it must contain

one or (only for OpenSSL versiona 1.1.1 onwards) more

EC curve names, such as "prime256v1", "secp384r1", or "P-521". Consult your
OpenSSL manual for valid curve names.

For OpenSSL versions before (and not including) 1.0.2, the string "auto"
selects "prime256v1". For more recent OpenSSL versions "auto" tells the library
to choose.

If the option expands to an empty string, the effect is undefined.

+----------------------------------------------------+
|tls_ocsp_file|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This option must if set expand to the absolute path to a file which contains a
current status proof for the server's certificate, as obtained from the
Certificate Authority.

Usable for GnuTLS 3.4.4 or 3.3.17 or OpenSSL 1.1.0 (or later). The macro
"_HAVE_TLS_OCSP" will be defined for those versions.

For OpenSSL 1.1.0 or later, and for GnuTLS 3.5.6 or later the expanded value of
this option can be a list of files, to match a list given for the 
tls_certificate option. The ordering of the two lists must match. The macro
"_HAVE_TLS_OCSP_LIST" will be defined for those versions.

The file(s) should be in DER format, except for GnuTLS 3.6.3 or later or for
OpenSSL, when an optional filetype prefix can be used. The prefix must be one
of "DER" or "PEM", followed by a single space. If one is used it sets the
format for subsequent files in the list; the initial format is DER. If multiple
proofs are wanted, for multiple chain elements (this only works under TLS1.3)
they must be coded as a combined OCSP response.

Although GnuTLS will accept PEM files with multiple separate PEM blobs (ie.
separate OCSP responses), it sends them in the TLS Certificate record
interleaved with the certificates of the chain; although a GnuTLS client is
happy with that, an OpenSSL client is not.

+---------------------------------------------------------------+
|tls_on_connect_ports|Use: main|Type: string list|Default: unset|
+---------------------------------------------------------------+

This option specifies a list of incoming SSMTP (aka SMTPS) ports that should
operate the SSMTP (SMTPS) protocol, where a TLS session is immediately set up
without waiting for the client to issue a STARTTLS command. For further
details, see section 13.4.

+----------------------------------------------------------+
|tls_privatekey|Use: main|Type: string list*|Default: unset|
+----------------------------------------------------------+

The value of this option is expanded, and must then be a list of absolute paths
to files which contains the server's private keys. If this option is unset, or
if the expansion is forced to fail, or the result is an empty string, the
private key is assumed to be in the same file as the server's certificates. See
chapter 43 for further details.

See 43.8 for discussion of when this option might be re-expanded.

+---------------------------------------------------------+
|tls_remember_esmtp|Use: main|Type: boolean|Default: false|
+---------------------------------------------------------+

If this option is set true, Exim violates the RFCs by remembering that it is in
"esmtp" state after successfully negotiating a TLS session. This provides
support for broken clients that fail to send a new EHLO after starting a TLS
session.

+----------------------------------------------------------+
|tls_require_ciphers|Use: main|Type: string*|Default: unset|
+----------------------------------------------------------+

This option controls which ciphers can be used for incoming TLS connections.
The smtp transport has an option of the same name for controlling outgoing
connections. This option is expanded for each connection, so can be varied for
different clients if required. The value of this option must be a list of
permitted cipher suites. The OpenSSL and GnuTLS libraries handle cipher control
in somewhat different ways. If GnuTLS is being used, the client controls the
preference order of the available ciphers. Details are given in sections 43.4
and 43.5.

+--------------------------------------------------------------+
|tls_resumption_hosts|Use: main|Type: host list*|Default: unset|
+--------------------------------------------------------------+

This option controls which connections to offer the TLS resumption feature. See
43.11 for details.

+--------------------------------------------------------------+
|tls_try_verify_hosts|Use: main|Type: host list*|Default: unset|
+--------------------------------------------------------------+

See tls_verify_hosts below.

+---------------------------------------------------------------+
|tls_verify_certificates|Use: main|Type: string*|Default: system|
+---------------------------------------------------------------+

The value of this option is expanded, and must then be either the word "system"
or the absolute path to a file or directory containing permitted certificates
for clients that match tls_verify_hosts or tls_try_verify_hosts.

The "system" value for the option will use a system default location compiled
into the SSL library. This is not available for GnuTLS versions preceding
3.0.20, and will be taken as empty; an explicit location must be specified.

The use of a directory for the option value is not available for GnuTLS
versions preceding 3.3.6 and a single file must be used.

With OpenSSL the certificates specified explicitly either by file or directory
are added to those given by the system default location.

These certificates should be for the certificate authorities trusted, rather
than the public cert of individual clients. With both OpenSSL and GnuTLS, if
the value is a file then the certificates are sent by Exim as a server to
connecting clients, defining the list of accepted certificate authorities. Thus
the values defined should be considered public data. To avoid this, use the
explicit directory version. (If your peer is Exim up to 4.85, using GnuTLS, you
may need to send the CAs (thus using the file variant). Otherwise the peer
doesn't send its certificate.)

See 43.8 for discussion of when this option might be re-expanded.

A forced expansion failure or setting to an empty string is equivalent to being
unset.

+----------------------------------------------------------+
|tls_verify_hosts|Use: main|Type: host list*|Default: unset|
+----------------------------------------------------------+

This option, along with tls_try_verify_hosts, controls the checking of
certificates from clients. The expected certificates are defined by 
tls_verify_certificates, which must be set. A configuration error occurs if
either tls_verify_hosts or tls_try_verify_hosts is set and 
tls_verify_certificates is not set.

Any client that matches tls_verify_hosts is constrained by 
tls_verify_certificates. When the client initiates a TLS session, it must
present one of the listed certificates. If it does not, the connection is
aborted. Warning: Including a host in tls_verify_hosts does not require the
host to use TLS. It can still send SMTP commands through unencrypted
connections. Forcing a client to use TLS has to be done separately using an ACL
to reject inappropriate commands when the connection is not encrypted.

A weaker form of checking is provided by tls_try_verify_hosts. If a client
matches this option (but not tls_verify_hosts), Exim requests a certificate and
checks it against tls_verify_certificates, but does not abort the connection if
there is no certificate or if it does not match. This state can be detected in
an ACL, which makes it possible to implement policies such as "accept for relay
only if a verified certificate has been received, but accept for local delivery
if encrypted, even without a verified certificate".

Client hosts that match neither of these lists are not asked to present
certificates.

+----------------------------------------------------------+
|trusted_groups|Use: main|Type: string list*|Default: unset|
+----------------------------------------------------------+

This option is expanded just once, at the start of Exim's processing. If this
option is set, any process that is running in one of the listed groups, or
which has one of them as a supplementary group, is trusted. The groups can be
specified numerically or by name. See section 5.2 for details of what trusted
callers are permitted to do. If neither trusted_groups nor trusted_users is
set, only root and the Exim user are trusted.

+---------------------------------------------------------+
|trusted_users|Use: main|Type: string list*|Default: unset|
+---------------------------------------------------------+

This option is expanded just once, at the start of Exim's processing. If this
option is set, any process that is running as one of the listed users is
trusted. The users can be specified numerically or by name. See section 5.2 for
details of what trusted callers are permitted to do. If neither trusted_groups
nor trusted_users is set, only root and the Exim user are trusted.

+----------------------------------------------------+
|unknown_login|Use: main|Type: string*|Default: unset|
+----------------------------------------------------+

This is a specialized feature for use in unusual configurations. By default, if
the uid of the caller of Exim cannot be looked up using getpwuid(), Exim gives
up. The unknown_login option can be used to set a login name to be used in this
circumstance. It is expanded, so values like user$caller_uid can be set. When 
unknown_login is used, the value of unknown_username is used for the user's
real name (gecos field), unless this has been set by the -F option.

+------------------------------------------------------+
|unknown_username|Use: main|Type: string|Default: unset|
+------------------------------------------------------+

See unknown_login.

+-----------------------------------------------------------------+
|untrusted_set_sender|Use: main|Type: address list*|Default: unset|
+-----------------------------------------------------------------+

When an untrusted user submits a message to Exim using the standard input, Exim
normally creates an envelope sender address from the user's login and the
default qualification domain. Data from the -f option (for setting envelope
senders on non-SMTP messages) or the SMTP MAIL command (if -bs or -bS is used)
is ignored.

However, untrusted users are permitted to set an empty envelope sender address,
to declare that a message should never generate any bounces. For example:

exim -f '<>' user@domain.example

The untrusted_set_sender option allows you to permit untrusted users to set
other envelope sender addresses in a controlled way. When it is set, untrusted
users are allowed to set envelope sender addresses that match any of the
patterns in the list. Like all address lists, the string is expanded. The
identity of the user is in $sender_ident, so you can, for example, restrict
users to setting senders that start with their login ids followed by a hyphen
by a setting like this:

untrusted_set_sender = ^$sender_ident-

If you want to allow untrusted users to set envelope sender addresses without
restriction, you can use

untrusted_set_sender = *

The untrusted_set_sender option applies to all forms of local input, but only
to the setting of the envelope sender. It does not permit untrusted users to
use the other options which trusted user can use to override message
parameters. Furthermore, it does not stop Exim from removing an existing 
Sender: header in the message, or from adding a Sender: header if necessary.
See local_sender_retain and local_from_check for ways of overriding these
actions. The handling of the Sender: header is also described in section
48.5.12.

The log line for a message's arrival shows the envelope sender following "<=".
For local messages, the user's login always follows, after "U=". In -bp
displays, and in the Exim monitor, if an untrusted user sets an envelope sender
address, the user's login is shown in parentheses after the sender address.

+-----------------------------------------------------------+
|uucp_from_pattern|Use: main|Type: string|Default: see below|
+-----------------------------------------------------------+

Some applications that pass messages to an MTA via a command line interface use
an initial line starting with "From " to pass the envelope sender. In
particular, this is used by UUCP software. Exim recognizes such a line by means
of a regular expression that is set in uucp_from_pattern. When the pattern
matches, the sender address is constructed by expanding the contents of 
uucp_from_sender, provided that the caller of Exim is a trusted user. The
default pattern recognizes lines in the following two forms:

From ph10 Fri Jan  5 12:35 GMT 1996
From ph10 Fri, 7 Jan 97 14:00:00 GMT

The pattern can be seen by running

exim -bP uucp_from_pattern

It checks only up to the hours and minutes, and allows for a 2-digit or 4-digit
year in the second case. The first word after "From " is matched in the regular
expression by a parenthesized subpattern. The default value for 
uucp_from_sender is "$1", which therefore just uses this first word ("ph10" in
the example above) as the message's sender. See also ignore_fromline_hosts.

+------------------------------------------------------+
|uucp_from_sender|Use: main|Type: string*|Default: "$1"|
+------------------------------------------------------+

See uucp_from_pattern above.

+--------------------------------------------------------+
|warn_message_file|Use: main|Type: string*|Default: unset|
+--------------------------------------------------------+

This option defines a template file containing paragraphs of text to be used
for constructing the warning message which is sent by Exim when a message has
been in the queue for a specified amount of time, as specified by delay_warning
. Details of the file's contents are given in chapter 50. The option is
expanded to give the file path, which must be absolute and untainted. See also 
bounce_message_file.

+-----------------------------------------------------+
|write_rejectlog|Use: main|Type: boolean|Default: true|
+-----------------------------------------------------+

If this option is set false, Exim no longer writes anything to the reject log.
See chapter 53 for details of what Exim writes to its logs.



===============================================================================
15. GENERIC OPTIONS FOR ROUTERS

This chapter describes the generic options that apply to all routers. Those
that are preconditions are marked with ** in the "use" field.

For a general description of how a router operates, see sections 3.10 and 3.12.
The latter specifies the order in which the preconditions are tested. The order
of expansion of the options that provide data for a transport is: errors_to, 
headers_add, headers_remove, transport.

The name of a router is limited to be 64 ASCII characters long; prior to Exim
4.95 names would be silently truncated at this length, but now it is enforced.

+------------------------------------------------------+
|address_data|Use: routers|Type: string*|Default: unset|
+------------------------------------------------------+

The string is expanded just before the router is run, that is, after all the
precondition tests have succeeded. If the expansion is forced to fail, the
router declines, the value of address_data remains unchanged, and the more
option controls what happens next. Other expansion failures cause delivery of
the address to be deferred.

When the expansion succeeds, the value is retained with the address, and can be
accessed using the variable $address_data in the current router, subsequent
routers, and the eventual transport.

Warning: If the current or any subsequent router is a redirect router that runs
a user's filter file, the contents of $address_data are accessible in the
filter. This is not normally a problem, because such data is usually either not
confidential or it "belongs" to the current user, but if you do put
confidential data into $address_data you need to remember this point.

Even if the router declines or passes, the value of $address_data remains with
the address, though it can be changed by another address_data setting on a
subsequent router. If a router generates child addresses, the value of
$address_data propagates to them. This also applies to the special kind of
"child" that is generated by a router with the unseen option.

The idea of address_data is that you can use it to look up a lot of data for
the address once, and then pick out parts of the data later. For example, you
could use a single LDAP lookup to return a string of the form

uid=1234 gid=5678 mailbox=/mail/xyz forward=/home/xyz/.forward

In the transport you could pick out the mailbox by a setting such as

file = ${extract{mailbox}{$address_data}}

This makes the configuration file less messy, and also reduces the number of
lookups (though Exim does cache lookups).

See also the set option below.

The address_data facility is also useful as a means of passing information from
one router to another, and from a router to a transport. In addition, if
$address_data is set by a router when verifying a recipient address from an
ACL, it remains available for use in the rest of the ACL statement. After
verifying a sender, the value is transferred to $sender_address_data.

+-------------------------------------------------------+
|address_test|Use: routers**|Type: boolean|Default: true|
+-------------------------------------------------------+

If this option is set false, the router is skipped when routing is being tested
by means of the -bt command line option. This can be a convenience when your
first router sends messages to an external scanner, because it saves you having
to set the "already scanned" indicator when testing real address routing.

+--------------------------------------------------------------+
|cannot_route_message|Use: routers|Type: string*|Default: unset|
+--------------------------------------------------------------+

This option specifies a text message that is used when an address cannot be
routed because Exim has run out of routers. The default message is "Unrouteable
address". This option is useful only on routers that have more set false, or on
the very last router in a configuration, because the value that is used is
taken from the last router that is considered. This includes a router that is
skipped because its preconditions are not met, as well as a router that
declines. For example, using the default configuration, you could put:

cannot_route_message = Remote domain not found in DNS

on the first router, which is a dnslookup router with more set false, and

cannot_route_message = Unknown local user

on the final router that checks for local users. If string expansion fails for
this option, the default message is used. Unless the expansion failure was
explicitly forced, a message about the failure is written to the main and panic
logs, in addition to the normal message about the routing failure.

+------------------------------------------------------------+
|caseful_local_part|Use: routers|Type: boolean|Default: false|
+------------------------------------------------------------+

By default, routers handle the local parts of addresses in a case-insensitive
manner, though the actual case is preserved for transmission with the message.
If you want the case of letters to be significant in a router, you must set
this option true. For individual router options that contain address or local
part lists (for example, local_parts), case-sensitive matching can be turned on
by "+caseful" as a list item. See section 10.5.1 for more details.

The value of the $local_part variable is forced to lower case while a router is
running unless caseful_local_part is set. When a router assigns an address to a
transport, the value of $local_part when the transport runs is the same as it
was in the router. Similarly, when a router generates child addresses by
aliasing or forwarding, the values of $original_local_part and
$parent_local_part are those that were used by the redirecting router.

This option applies to the processing of an address by a router. When a
recipient address is being processed in an ACL, there is a separate control
modifier that can be used to specify case-sensitive processing within the ACL
(see section 44.13).

+------------------------------------------------------------+
|check_local_user|Use: routers**|Type: boolean|Default: false|
+------------------------------------------------------------+

When this option is true, Exim checks that the local part of the recipient
address (with affixes removed if relevant) is the name of an account on the
local system. The check is done by calling the getpwnam() function rather than
trying to read /etc/passwd directly. This means that other methods of holding
password data (such as NIS) are supported. If the local part is a local user, 
$local_part_data is set to an untainted version of the local part and $home is
set from the password data. The latter can be tested in other preconditions
that are evaluated after this one (the order of evaluation is given in section
3.12). However, the value of $home can be overridden by router_home_directory.
If the local part is not a local user, the router is skipped.

If you want to check that the local part is either the name of a local user or
matches something else, you cannot combine check_local_user with a setting of 
local_parts, because that specifies the logical and of the two conditions.
However, you can use a passwd lookup in a local_parts setting to achieve this.
For example:

local_parts = passwd;$local_part : lsearch;/etc/other/users

Note, however, that the side effects of check_local_user (such as setting up a
home directory) do not occur when a passwd lookup is used in a local_parts (or
any other) precondition.

+-----------------------------------------------------+
|condition|Use: routers**|Type: string*|Default: unset|
+-----------------------------------------------------+

This option specifies a general precondition test that has to succeed for the
router to be called. The condition option is the last precondition to be
evaluated (see section 3.12). The string is expanded, and if the result is a
forced failure, or an empty string, or one of the strings "0" or "no" or
"false" (checked without regard to the case of the letters), the router is
skipped, and the address is offered to the next one.

If the result is any other value, the router is run (as this is the last
precondition to be evaluated, all the other preconditions must be true).

This option is unusual in that multiple condition options may be present. All 
condition options must succeed.

The condition option provides a means of applying custom conditions to the
running of routers. Note that in the case of a simple conditional expansion,
the default expansion values are exactly what is wanted. For example:

condition = ${if >{$message_age}{600}}

Because of the default behaviour of the string expansion, this is equivalent to

condition = ${if >{$message_age}{600}{true}{}}

A multiple condition example, which succeeds:

condition = ${if >{$message_age}{600}}
condition = ${if !eq{${lc:$local_part}}{postmaster}}
condition = foobar

If the expansion fails (other than forced failure) delivery is deferred. Some
of the other precondition options are common special cases that could in fact
be specified using condition.

Historical note: We have condition on ACLs and on Routers. Routers are far
older, and use one set of semantics. ACLs are newer and when they were created,
the ACL condition process was given far stricter parse semantics. The bool{}
expansion condition uses the same rules as ACLs. The bool_lax{} expansion
condition uses the same rules as Routers. More pointedly, the bool_lax{} was
written to match the existing Router rules processing behavior.

This is best illustrated in an example:

# If used in an ACL condition will fail with a syntax error, but
# in a router condition any extra characters are treated as a string

$ exim -be '${if eq {${lc:GOOGLE.com}} {google.com}} {yes} {no}}'
true {yes} {no}}

$ exim -be '${if eq {${lc:WHOIS.com}} {google.com}} {yes} {no}}'
 {yes} {no}}

In each example above, the if statement actually ends after "{google.com}}".
Since no true or false braces were defined, the default if behavior is to
return a boolean true or a null answer (which evaluates to false). The rest of
the line is then treated as a string. So the first example resulted in the
boolean answer "true" with the string " {yes} {no}}" appended to it. The second
example resulted in the null output (indicating false) with the string " {yes}
{no}}" appended to it.

In fact you can put excess forward braces in too. In the router condition,
Exim's parser only looks for "{" symbols when they mean something, like after a
"$" or when required as part of a conditional. But otherwise "{" and "}" are
treated as ordinary string characters.

Thus, in a Router, the above expansion strings will both always evaluate true,
as the result of expansion is a non-empty string which doesn't match an
explicit false value. This can be tricky to debug. By contrast, in an ACL
either of those strings will always result in an expansion error because the
result doesn't look sufficiently boolean.

+-----------------------------------------------------+
|debug_print|Use: routers|Type: string*|Default: unset|
+-----------------------------------------------------+

If this option is set and debugging is enabled (see the -d command line option)
or in address-testing mode (see the -bt command line option), the string is
expanded and included in the debugging output. If expansion of the string
fails, the error message is written to the debugging output, and Exim carries
on processing. This option is provided to help with checking out the values of
variables and so on when debugging router configurations. For example, if a 
condition option appears not to be working, debug_print can be used to output
the variables it references. The output happens after checks for domains, 
local_parts, and check_local_user but before any other preconditions are
tested. A newline is added to the text if it does not end with one. The
variable $router_name contains the name of the router.

+---------------------------------------------------------+
|disable_logging|Use: routers|Type: boolean|Default: false|
+---------------------------------------------------------+

If this option is set true, nothing is logged for any routing errors or for any
deliveries caused by this router. You should not set this option unless you
really, really know what you are doing. See also the generic transport option
of the same name.

+-----------------------------------------------------------------+
|dnssec_request_domains|Use: routers|Type: domain list*|Default: *|
+-----------------------------------------------------------------+

DNS lookups for domains matching dnssec_request_domains will be done with the
DNSSEC request bit set. This applies to all of the SRV, MX, AAAA, A lookup
sequence.

+---------------------------------------------------------------------+
|dnssec_require_domains|Use: routers|Type: domain list*|Default: unset|
+---------------------------------------------------------------------+

DNS lookups for domains matching dnssec_require_domains will be done with the
DNSSEC request bit set. Any returns not having the Authenticated Data bit (AD
bit) set will be ignored and logged as a host-lookup failure. This applies to
all of the SRV, MX, AAAA, A lookup sequence.

+--------------------------------------------------------+
|domains|Use: routers**|Type: domain list*|Default: unset|
+--------------------------------------------------------+

If this option is set, the router is skipped unless the current domain matches
the list. The data returned by the list check is placed in $domain_data for use
in string expansions of the driver's private options and in the transport. See
section 3.12 for a list of the order in which preconditions are evaluated.

A complex example, using a file like:

alice@dom1
bill@dom1
maggie@dom1

and checking both domain and local_part

domains =         ${domain:${lookup {$local_part@$domain} lseach,ret=key {/path/to/accountsfile}}}
local_parts = ${local_part:${lookup {$local_part@$domain} lseach,ret=key {/path/to/accountsfile}}}

+-----------------------------------------------+
|driver|Use: routers|Type: string|Default: unset|
+-----------------------------------------------+

This option must always be set. It specifies which of the available routers is
to be used.

+-----------------------------------------------------+
|dsn_lasthop|Use: routers|Type: boolean|Default: false|
+-----------------------------------------------------+

If this option is set true, and extended DSN (RFC3461) processing is in effect,
Exim will not pass on DSN requests to downstream DSN-aware hosts but will
instead send a success DSN as if the next hop does not support DSN. Not
effective on redirect routers.

+---------------------------------------------------+
|errors_to|Use: routers|Type: string*|Default: unset|
+---------------------------------------------------+

If a router successfully handles an address, it may assign the address to a
transport for delivery or it may generate child addresses. In both cases, if
there is a delivery problem during later processing, the resulting bounce
message is sent to the address that results from expanding this string,
provided that the address verifies successfully. The errors_to option is
expanded before headers_add, headers_remove, and transport.

The errors_to setting associated with an address can be overridden if it
subsequently passes through other routers that have their own errors_to
settings, or if the message is delivered by a transport with a return_path
setting.

If errors_to is unset, or the expansion is forced to fail, or the result of the
expansion fails to verify, the errors address associated with the incoming
address is used. At top level, this is the envelope sender. A non-forced
expansion failure causes delivery to be deferred.

If an address for which errors_to has been set ends up being delivered over
SMTP, the envelope sender for that delivery is the errors_to value, so that any
bounces that are generated by other MTAs on the delivery route are also sent
there. You can set errors_to to the empty string by either of these settings:

errors_to =
errors_to = ""

An expansion item that yields an empty string has the same effect. If you do
this, a locally detected delivery error for addresses processed by this router
no longer gives rise to a bounce message; the error is discarded. If the
address is delivered to a remote host, the return path is set to "<>", unless
overridden by the return_path option on the transport.

If for some reason you want to discard local errors, but use a non-empty MAIL
command for remote delivery, you can preserve the original return path in
$address_data in the router, and reinstate it in the transport by setting 
return_path.

The most common use of errors_to is to direct mailing list bounces to the
manager of the list, as described in section 51.2, or to implement VERP
(Variable Envelope Return Paths) (see section 51.6).

+-----------------------------------------------+
|expn|Use: routers**|Type: boolean|Default: true|
+-----------------------------------------------+

If this option is turned off, the router is skipped when testing an address as
a result of processing an SMTP EXPN command. You might, for example, want to
turn it off on a router for users' .forward files, while leaving it on for the
system alias file. See section 3.12 for a list of the order in which
preconditions are evaluated.

The use of the SMTP EXPN command is controlled by an ACL (see chapter 44). When
Exim is running an EXPN command, it is similar to testing an address with -bt.
Compare VRFY, whose counterpart is -bv.

+-----------------------------------------------------+
|fail_verify|Use: routers|Type: boolean|Default: false|
+-----------------------------------------------------+

Setting this option has the effect of setting both fail_verify_sender and 
fail_verify_recipient to the same value.

+---------------------------------------------------------------+
|fail_verify_recipient|Use: routers|Type: boolean|Default: false|
+---------------------------------------------------------------+

If this option is true and an address is accepted by this router when verifying
a recipient, verification fails.

+------------------------------------------------------------+
|fail_verify_sender|Use: routers|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is true and an address is accepted by this router when verifying
a sender, verification fails.

+------------------------------------------------------------+
|fallback_hosts|Use: routers|Type: string list|Default: unset|
+------------------------------------------------------------+

String expansion is not applied to this option. The argument must be a
colon-separated list of host names or IP addresses. The list separator can be
changed (see section 6.21), and a port can be specified with each name or
address. In fact, the format of each item is exactly the same as defined for
the list of hosts in a manualroute router (see section 20.5).

If a router queues an address for a remote transport, this host list is
associated with the address, and used instead of the transport's fallback host
list. If hosts_randomize is set on the transport, the order of the list is
randomized for each use. See the fallback_hosts option of the smtp transport
for further details.

+---------------------------------------------------+
|group|Use: routers|Type: string*|Default: see below|
+---------------------------------------------------+

When a router queues an address for a transport, and the transport does not
specify a group, the group given here is used when running the delivery
process. The group may be specified numerically or by name. If expansion fails,
the error is logged and delivery is deferred. The default is unset, unless 
check_local_user is set, when the default is taken from the password
information. See also initgroups and user and the discussion in chapter 23.

+---------------------------------------------------+
|headers_add|Use: routers|Type: list*|Default: unset|
+---------------------------------------------------+

This option specifies a list of text headers, newline-separated (by default,
changeable in the usual way 6.21), that is associated with any addresses that
are accepted by the router. Each item is separately expanded, at routing time.
However, this option has no effect when an address is just being verified. The
way in which the text is used to add header lines at transport time is
described in section 48.6. New header lines are not actually added until the
message is in the process of being transported. This means that references to
header lines in string expansions in the transport's configuration do not "see"
the added header lines.

The headers_add option is expanded after errors_to, but before headers_remove
and transport. If an item is empty, or if an item expansion is forced to fail,
the item has no effect. Other expansion failures are treated as configuration
errors.

Unlike most options, headers_add can be specified multiple times for a router;
all listed headers are added.

Warning 1: The headers_add option cannot be used for a redirect router that has
the one_time option set.

Warning 2: If the unseen option is set on the router, all header additions are
deleted when the address is passed on to subsequent routers. For a redirect
router, if a generated address is the same as the incoming address, this can
lead to duplicate addresses with different header modifications. Exim does not
do duplicate deliveries (except, in certain circumstances, to pipes -- see
section 22.7), but it is undefined which of the duplicates is discarded, so
this ambiguous situation should be avoided. The repeat_use option of the 
redirect router may be of help.

+------------------------------------------------------+
|headers_remove|Use: routers|Type: list*|Default: unset|
+------------------------------------------------------+

This option specifies a list of text headers, colon-separated (by default,
changeable in the usual way 6.21), that is associated with any addresses that
are accepted by the router. However, the option has no effect when an address
is just being verified. Each list item is separately expanded, at transport
time. If an item ends in *, it will match any header with the given prefix. The
way in which the text is used to remove header lines at transport time is
described in section 48.6. Header lines are not actually removed until the
message is in the process of being transported. This means that references to
header lines in string expansions in the transport's configuration still "see"
the original header lines.

The headers_remove option is handled after errors_to and headers_add, but
before transport. If an item expansion is forced to fail, the item has no
effect. Other expansion failures are treated as configuration errors.

Unlike most options, headers_remove can be specified multiple times for a
router; all listed headers are removed.

Warning 1: The headers_remove option cannot be used for a redirect router that
has the one_time option set.

Warning 2: If the unseen option is set on the router, all header removal
requests are deleted when the address is passed on to subsequent routers, and
this can lead to problems with duplicates -- see the similar warning for 
headers_add above.

Warning 3: Because of the separate expansion of the list items, items that
contain a list separator must have it doubled. To avoid this, change the list
separator (6.21).

+----------------------------------------------------------------+
|ignore_target_hosts|Use: routers|Type: host list*|Default: unset|
+----------------------------------------------------------------+

Although this option is a host list, it should normally contain IP address
entries rather than names. If any host that is looked up by the router has an
IP address that matches an item in this list, Exim behaves as if that IP
address did not exist. This option allows you to cope with rogue DNS entries
like

remote.domain.example.  A  127.0.0.1

by setting

ignore_target_hosts = 127.0.0.1

on the relevant router. If all the hosts found by a dnslookup router are
discarded in this way, the router declines. In a conventional configuration, an
attempt to mail to such a domain would normally provoke the "unrouteable
domain" error, and an attempt to verify an address in the domain would fail.
Similarly, if ignore_target_hosts is set on an ipliteral router, the router
declines if presented with one of the listed addresses.

You can use this option to disable the use of IPv4 or IPv6 for mail delivery by
means of the first or the second of the following settings, respectively:

ignore_target_hosts = 0.0.0.0/0
ignore_target_hosts = <; 0::0/0

The pattern in the first line matches all IPv4 addresses, whereas the pattern
in the second line matches all IPv6 addresses.

This option may also be useful for ignoring link-local and site-local IPv6
addresses. Because, like all host lists, the value of ignore_target_hosts is
expanded before use as a list, it is possible to make it dependent on the
domain that is being routed.

During its expansion, $host_address is set to the IP address that is being
checked.

+----------------------------------------------------+
|initgroups|Use: routers|Type: boolean|Default: false|
+----------------------------------------------------+

If the router queues an address for a transport, and this option is true, and
the uid supplied by the router is not overridden by the transport, the
initgroups() function is called when running the transport to ensure that any
additional groups associated with the uid are set up. See also group and user
and the discussion in chapter 23.

+-----------------------------------------------------------------+
|local_part_prefix|Use: routers**|Type: string list|Default: unset|
+-----------------------------------------------------------------+

If this option is set, the router is skipped unless the local part starts with
one of the given strings, or local_part_prefix_optional is true. See section
3.12 for a list of the order in which preconditions are evaluated.

The list is scanned from left to right, and the first prefix that matches is
used. A limited form of wildcard is available; if the prefix begins with an
asterisk, it matches the longest possible sequence of arbitrary characters at
the start of the local part. An asterisk should therefore always be followed by
some character that does not occur in normal local parts. Wildcarding can be
used to set up multiple user mailboxes, as described in section 51.8.

During the testing of the local_parts option, and while the router is running,
the prefix is removed from the local part, and is available in the expansion
variable $local_part_prefix. When a message is being delivered, if the router
accepts the address, this remains true during subsequent delivery by a
transport. In particular, the local part that is transmitted in the RCPT
command for LMTP, SMTP, and BSMTP deliveries has the prefix removed by default.
This behaviour can be overridden by setting rcpt_include_affixes true on the
relevant transport.

If wildcarding (above) was used then the part of the prefix matching the
wildcard is available in $local_part_prefix_v.

When an address is being verified, local_part_prefix affects only the behaviour
of the router. If the callout feature of verification is in use, this means
that the full address, including the prefix, will be used during the callout.

The prefix facility is commonly used to handle local parts of the form 
owner-something. Another common use is to support local parts of the form 
real-username to bypass a user's .forward file - helpful when trying to tell a
user their forwarding is broken - by placing a router like this one immediately
before the router that handles .forward files:

real_localuser:
  driver = accept
  local_part_prefix = real-
  check_local_user
  transport = local_delivery

For security, it would probably be a good idea to restrict the use of this
router to locally-generated messages, using a condition such as this:

  condition = ${if match {$sender_host_address}\
                         {\N^(|127\.0\.0\.1)$\N}}

If both local_part_prefix and local_part_suffix are set for a router, both
conditions must be met if not optional. Care must be taken if wildcards are
used in both a prefix and a suffix on the same router. Different separator
characters must be used to avoid ambiguity.

+--------------------------------------------------------------------+
|local_part_prefix_optional|Use: routers|Type: boolean|Default: false|
+--------------------------------------------------------------------+

See local_part_prefix above.

+-----------------------------------------------------------------+
|local_part_suffix|Use: routers**|Type: string list|Default: unset|
+-----------------------------------------------------------------+

This option operates in the same way as local_part_prefix, except that the
local part must end (rather than start) with the given string, the 
local_part_suffix_optional option determines whether the suffix is mandatory,
and the wildcard * character, if present, must be the last character of the
suffix. This option facility is commonly used to handle local parts of the form
something-request and multiple user mailboxes of the form username-foo.

+--------------------------------------------------------------------+
|local_part_suffix_optional|Use: routers|Type: boolean|Default: false|
+--------------------------------------------------------------------+

See local_part_suffix above.

+----------------------------------------------------------------+
|local_parts|Use: routers**|Type: local part list*|Default: unset|
+----------------------------------------------------------------+

The router is run only if the local part of the address matches the list. See
section 3.12 for a list of the order in which preconditions are evaluated, and
section 10.6 for a discussion of local part lists. Because the string is
expanded, it is possible to make it depend on the domain, for example:

local_parts = dbm;/usr/local/specials/$domain_data

the data returned by the list check for the local part is placed in the
variable $local_part_data for use in expansions of the router's private options
or in the transport. You might use this option, for example, if you have a
large number of local virtual domains, and you want to send all postmaster mail
to the same place without having to set up an alias in each virtual domain:

postmaster:
  driver = redirect
  local_parts = postmaster
  data = postmaster@real.domain.example

+----------------------------------------------------------+
|log_as_local|Use: routers|Type: boolean|Default: see below|
+----------------------------------------------------------+

Exim has two logging styles for delivery, the idea being to make local
deliveries stand out more visibly from remote ones. In the "local" style, the
recipient address is given just as the local part, without a domain. The use of
this style is controlled by this option. It defaults to true for the accept
router, and false for all the others. This option applies only when a router
assigns an address to a transport. It has no effect on routers that redirect
addresses.

+----------------------------------------------+
|more|Use: routers|Type: boolean*|Default: true|
+----------------------------------------------+

The result of string expansion for this option must be a valid boolean value,
that is, one of the strings "yes", "no", "true", or "false". Any other result
causes an error, and delivery is deferred. If the expansion is forced to fail,
the default value for the option (true) is used. Other failures cause delivery
to be deferred.

If this option is set false, and the router declines to handle the address, no
further routers are tried, routing fails, and the address is bounced. However,
if the router explicitly passes an address to the following router by means of
the setting

self = pass

or otherwise, the setting of more is ignored. Also, the setting of more does
not affect the behaviour if one of the precondition tests fails. In that case,
the address is always passed to the next router.

Note that address_data is not considered to be a precondition. If its expansion
is forced to fail, the router declines, and the value of more controls what
happens next.

+---------------------------------------------------------+
|pass_on_timeout|Use: routers|Type: boolean|Default: false|
+---------------------------------------------------------+

If a router times out during a host lookup, it normally causes deferral of the
address. If pass_on_timeout is set, the address is passed on to the next
router, overriding no_more. This may be helpful for systems that are
intermittently connected to the Internet, or those that want to pass to a smart
host any messages that cannot immediately be delivered.

There are occasional other temporary errors that can occur while doing DNS
lookups. They are treated in the same way as a timeout, and this option applies
to all of them.

+----------------------------------------------------+
|pass_router|Use: routers|Type: string|Default: unset|
+----------------------------------------------------+

Routers that recognize the generic self option (dnslookup, ipliteral, and 
manualroute) are able to return "pass", forcing routing to continue, and
overriding a false setting of more. When one of these routers returns "pass",
the address is normally handed on to the next router in sequence. This can be
changed by setting pass_router to the name of another router. However (unlike 
redirect_router) the named router must be below the current router, to avoid
loops. Note that this option applies only to the special case of "pass". It
does not apply when a router returns "decline" because it cannot handle an
address.

+--------------------------------------------------------+
|redirect_router|Use: routers|Type: string|Default: unset|
+--------------------------------------------------------+

Sometimes an administrator knows that it is pointless to reprocess addresses
generated from alias or forward files with the same router again. For example,
if an alias file translates real names into login ids there is no point
searching the alias file a second time, especially if it is a large file.

The redirect_router option can be set to the name of any router instance. It
causes the routing of any generated addresses to start at the named router
instead of at the first router. This option has no effect if the router in
which it is set does not generate new addresses.

+--------------------------------------------------------------+
|require_files|Use: routers**|Type: string list*|Default: unset|
+--------------------------------------------------------------+

This option provides a general mechanism for predicating the running of a
router on the existence or non-existence of certain files or directories.
Before running a router, as one of its precondition tests, Exim works its way
through the require_files list, expanding each item separately.

Because the list is split before expansion, any colons in expansion items must
be doubled, or the facility for using a different list separator must be used (
6.21). If any expansion is forced to fail, the item is ignored. Other expansion
failures cause routing of the address to be deferred.

If any expanded string is empty, it is ignored. Otherwise, except as described
below, each string must be a fully qualified file path, optionally preceded by
"!". The paths are passed to the stat() function to test for the existence of
the files or directories. The router is skipped if any paths not preceded by "!
" do not exist, or if any paths preceded by "!" do exist.

If stat() cannot determine whether a file exists or not, delivery of the
message is deferred. This can happen when NFS-mounted filesystems are
unavailable.

This option is checked after the domains, local_parts, and senders options, so
you cannot use it to check for the existence of a file in which to look up a
domain, local part, or sender. (See section 3.12 for a full list of the order
in which preconditions are evaluated.) However, as these options are all
expanded, you can use the exists expansion condition to make such tests. The 
require_files option is intended for checking files that the router may be
going to use internally, or which are needed by a transport (e.g., .procmailrc
).

During delivery, the stat() function is run as root, but there is a facility
for some checking of the accessibility of a file by another user. This is not a
proper permissions check, but just a "rough" check that operates as follows:

If an item in a require_files list does not contain any forward slash
characters, it is taken to be the user (and optional group, separated by a
comma) to be checked for subsequent files in the list. If no group is specified
but the user is specified symbolically, the gid associated with the uid is
used. For example:

require_files = mail:/some/file
require_files = $local_part_data:$home/.procmailrc

If a user or group name in a require_files list does not exist, the 
require_files condition fails.

Exim performs the check by scanning along the components of the file path, and
checking the access for the given uid and gid. It checks for "x" access on
directories, and "r" access on the final file. Note that this means that file
access control lists, if the operating system has them, are ignored.

Warning 1: When the router is being run to verify addresses for an incoming
SMTP message, Exim is not running as root, but under its own uid. This may
affect the result of a require_files check. In particular, stat() may yield the
error EACCES ("Permission denied"). This means that the Exim user is not
permitted to read one of the directories on the file's path.

Warning 2: Even when Exim is running as root while delivering a message, stat()
can yield EACCES for a file in an NFS directory that is mounted without root
access. In this case, if a check for access by a particular user is requested,
Exim creates a subprocess that runs as that user, and tries the check again in
that process.

The default action for handling an unresolved EACCES is to consider it to be
caused by a configuration error, and routing is deferred because the existence
or non-existence of the file cannot be determined. However, in some
circumstances it may be desirable to treat this condition as if the file did
not exist. If the filename (or the exclamation mark that precedes the filename
for non-existence) is preceded by a plus sign, the EACCES error is treated as
if the file did not exist. For example:

require_files = +/some/file

If the router is not an essential part of verification (for example, it handles
users' .forward files), another solution is to set the verify option false so
that the router is skipped when verifying.

+------------------------------------------------------------------+
|retry_use_local_part|Use: routers|Type: boolean|Default: see below|
+------------------------------------------------------------------+

When a delivery suffers a temporary routing failure, a retry record is created
in Exim's hints database. For addresses whose routing depends only on the
domain, the key for the retry record should not involve the local part, but for
other addresses, both the domain and the local part should be included.
Usually, remote routing is of the former kind, and local routing is of the
latter kind.

This option controls whether the local part is used to form the key for retry
hints for addresses that suffer temporary errors while being handled by this
router. The default value is true for any router that has any of 
check_local_user, local_parts, condition, local_part_prefix, local_part_suffix,
senders or require_files set, and false otherwise. Note that this option does
not apply to hints keys for transport delays; they are controlled by a generic
transport option of the same name.

Failing to set this option when it is needed (because a remote router handles
only some of the local-parts for a domain) can result in incorrect error
messages being generated.

The setting of retry_use_local_part applies only to the router on which it
appears. If the router generates child addresses, they are routed
independently; this setting does not become attached to them.

+---------------------------------------------------------------+
|router_home_directory|Use: routers|Type: string*|Default: unset|
+---------------------------------------------------------------+

This option sets a home directory for use while the router is running. (Compare
transport_home_directory, which sets a home directory for later transporting.)
In particular, if used on a redirect router, this option sets a value for $home
while a filter is running. The value is expanded; forced expansion failure
causes the option to be ignored - other failures cause the router to defer.

Expansion of router_home_directory happens immediately after the 
check_local_user test (if configured), before any further expansions take
place. (See section 3.12 for a list of the order in which preconditions are
evaluated.) While the router is running, router_home_directory overrides the
value of $home that came from check_local_user.

When a router accepts an address and assigns it to a local transport (including
the cases when a redirect router generates a pipe, file, or autoreply
delivery), the home directory setting for the transport is taken from the first
of these values that is set:

  * The home_directory option on the transport;

  * The transport_home_directory option on the router;

  * The password data if check_local_user is set on the router;

  * The router_home_directory option on the router.

In other words, router_home_directory overrides the password data for the
router, but not for the transport.

+----------------------------------------------+
|self|Use: routers|Type: string|Default: freeze|
+----------------------------------------------+

This option applies to those routers that use a recipient address to find a
list of remote hosts. Currently, these are the dnslookup, ipliteral, and 
manualroute routers. Certain configurations of the queryprogram router can also
specify a list of remote hosts. Usually such routers are configured to send the
message to a remote host via an smtp transport. The self option specifies what
happens when the first host on the list turns out to be the local host. The way
in which Exim checks for the local host is described in section 13.8.

Normally this situation indicates either an error in Exim's configuration (for
example, the router should be configured not to process this domain), or an
error in the DNS (for example, the MX should not point to this host). For this
reason, the default action is to log the incident, defer the address, and
freeze the message. The following alternatives are provided for use in special
cases:

defer

    Delivery of the message is tried again later, but the message is not
    frozen.

reroute: <domain>

    The domain is changed to the given domain, and the address is passed back
    to be reprocessed by the routers. No rewriting of headers takes place. This
    behaviour is essentially a redirection.

reroute: rewrite: <domain>

    The domain is changed to the given domain, and the address is passed back
    to be reprocessed by the routers. Any headers that contain the original
    domain are rewritten.

pass

    The router passes the address to the next router, or to the router named in
    the pass_router option if it is set. This overrides no_more. During
    subsequent routing and delivery, the variable $self_hostname contains the
    name of the local host that the router encountered. This can be used to
    distinguish between different cases for hosts with multiple names. The
    combination

    self = pass
    no_more

    ensures that only those addresses that routed to the local host are passed
    on. Without no_more, addresses that were declined for other reasons would
    also be passed to the next router.

fail

    Delivery fails and an error report is generated.

send

    The anomaly is ignored and the address is queued for the transport. This
    setting should be used with extreme caution. For an smtp transport, it
    makes sense only in cases where the program that is listening on the SMTP
    port is not this version of Exim. That is, it must be some other MTA, or
    Exim with a different configuration file that handles the domain in another
    way.

+---------------------------------------------------------+
|senders|Use: routers**|Type: address list*|Default: unset|
+---------------------------------------------------------+

If this option is set, the router is skipped unless the message's sender
address matches something on the list. See section 3.12 for a list of the order
in which preconditions are evaluated.

There are issues concerning verification when the running of routers is
dependent on the sender. When Exim is verifying the address in an errors_to
setting, it sets the sender to the null string. When using the -bt option to
check a configuration file, it is necessary also to use the -f option to set an
appropriate sender. For incoming mail, the sender is unset when verifying the
sender, but is available when verifying any recipients. If the SMTP VRFY
command is enabled, it must be used after MAIL if the sender address matters.

+-------------------------------------------------+
|set|Use: routers|Type: string list|Default: unset|
+-------------------------------------------------+

This option may be used multiple times on a router; because of this the list
aspect is mostly irrelevant. The list separator is a semicolon but can be
changed in the usual way.

Each list-element given must be of the form "name = value" and the names used
must start with the string "r_". Values containing a list-separator should have
them doubled. When a router runs, the strings are evaluated in order, to create
variables which are added to the set associated with the address. This is done
immediately after all the preconditions, before the evaluation of the 
address_data option. The variable is set with the expansion of the value. The
variables can be used by the router options (not including any preconditions)
and by the transport. Later definitions of a given named variable will override
former ones. Variable use is via the usual $r_... syntax.

This is similar to the address_data option, except that many independent
variables can be used, with choice of naming.

+--------------------------------------------------------------+
|translate_ip_address|Use: routers|Type: string*|Default: unset|
+--------------------------------------------------------------+

There exist some rare networking situations (for example, packet radio) where
it is helpful to be able to translate IP addresses generated by normal routing
mechanisms into other IP addresses, thus performing a kind of manual IP
routing. This should be done only if the normal IP routing of the TCP/IP stack
is inadequate or broken. Because this is an extremely uncommon requirement, the
code to support this option is not included in the Exim binary unless
SUPPORT_TRANSLATE_IP_ADDRESS=yes is set in Local/Makefile.

The translate_ip_address string is expanded for every IP address generated by
the router, with the generated address set in $host_address. If the expansion
is forced to fail, no action is taken. For any other expansion error, delivery
of the message is deferred. If the result of the expansion is an IP address,
that replaces the original address; otherwise the result is assumed to be a
host name - this is looked up using gethostbyname() (or getipnodebyname() when
available) to produce one or more replacement IP addresses. For example, to
subvert all IP addresses in some specific networks, this could be added to a
router:

translate_ip_address = \
  ${lookup{${mask:$host_address/26}}lsearch{/some/file}\
    {$value}fail}}

The file would contain lines like

10.2.3.128/26    some.host
10.8.4.34/26     10.44.8.15

You should not make use of this facility unless you really understand what you
are doing.

+---------------------------------------------------+
|transport|Use: routers|Type: string*|Default: unset|
+---------------------------------------------------+

This option specifies the transport to be used when a router accepts an address
and sets it up for delivery. A transport is never needed if a router is used
only for verification. The value of the option is expanded at routing time,
after the expansion of errors_to, headers_add, and headers_remove, and result
must be the name of one of the configured transports. If it is not, delivery is
deferred.

The transport option is not used by the redirect router, but it does have some
private options that set up transports for pipe and file deliveries (see
chapter 22).

+---------------------------------------------------------------------+
|transport_current_directory|Use: routers|Type: string*|Default: unset|
+---------------------------------------------------------------------+

This option associates a current directory with any address that is routed to a
local transport. This can happen either because a transport is explicitly
configured for the router, or because it generates a delivery to a file or a
pipe. During the delivery process (that is, at transport time), this option
string is expanded and is set as the current directory, unless overridden by a
setting on the transport. If the expansion fails for any reason, including
forced failure, an error is logged, and delivery is deferred. See chapter 23
for details of the local delivery environment.

+----------------------------------------------------------------------+
|transport_home_directory|Use: routers|Type: string*|Default: see below|
+----------------------------------------------------------------------+

This option associates a home directory with any address that is routed to a
local transport. This can happen either because a transport is explicitly
configured for the router, or because it generates a delivery to a file or a
pipe. During the delivery process (that is, at transport time), the option
string is expanded and is set as the home directory, unless overridden by a
setting of home_directory on the transport. If the expansion fails for any
reason, including forced failure, an error is logged, and delivery is deferred.

If the transport does not specify a home directory, and 
transport_home_directory is not set for the router, the home directory for the
transport is taken from the password data if check_local_user is set for the
router. Otherwise it is taken from router_home_directory if that option is set;
if not, no home directory is set for the transport.

See chapter 23 for further details of the local delivery environment.

+-------------------------------------------------+
|unseen|Use: routers|Type: boolean*|Default: false|
+-------------------------------------------------+

The result of string expansion for this option must be a valid boolean value,
that is, one of the strings "yes", "no", "true", or "false". Any other result
causes an error, and delivery is deferred. If the expansion is forced to fail,
the default value for the option (false) is used. Other failures cause delivery
to be deferred.

When this option is set true, routing does not cease if the router accepts the
address. Instead, a copy of the incoming address is passed to the next router,
overriding a false setting of more. There is little point in setting more false
if unseen is always true, but it may be useful in cases when the value of 
unseen contains expansion items (and therefore, presumably, is sometimes true
and sometimes false).

Setting the unseen option has a similar effect to the unseen command qualifier
in filter files. It can be used to cause copies of messages to be delivered to
some other destination, while also carrying out a normal delivery. In effect,
the current address is made into a "parent" that has two children - one that is
delivered as specified by this router, and a clone that goes on to be routed
further. For this reason, unseen may not be combined with the one_time option
in a redirect router.

Warning: Header lines added to the address (or specified for removal) by this
router or by previous routers affect the "unseen" copy of the message only. The
clone that continues to be processed by further routers starts with no added
headers and none specified for removal. For a redirect router, if a generated
address is the same as the incoming address, this can lead to duplicate
addresses with different header modifications. Exim does not do duplicate
deliveries (except, in certain circumstances, to pipes -- see section 22.7),
but it is undefined which of the duplicates is discarded, so this ambiguous
situation should be avoided. The repeat_use option of the redirect router may
be of help.

Unlike the handling of header modifications, any data that was set by the 
address_data option in the current or previous routers is passed on to
subsequent routers.

+--------------------------------------------------+
|user|Use: routers|Type: string*|Default: see below|
+--------------------------------------------------+

When a router queues an address for a transport, and the transport does not
specify a user, the user given here is used when running the delivery process.
The user may be specified numerically or by name. If expansion fails, the error
is logged and delivery is deferred. This user is also used by the redirect
router when running a filter file. The default is unset, except when 
check_local_user is set. In this case, the default is taken from the password
information. If the user is specified as a name, and group is not set, the
group associated with the user is used. See also initgroups and group and the
discussion in chapter 23.

+-------------------------------------------------+
|verify|Use: routers**|Type: boolean|Default: true|
+-------------------------------------------------+

Setting this option has the effect of setting verify_sender and 
verify_recipient to the same value.

+-------------------------------------------------------+
|verify_only|Use: routers**|Type: boolean|Default: false|
+-------------------------------------------------------+

If this option is set, the router is used only when verifying an address,
delivering in cutthrough mode or testing with the -bv option, not when actually
doing a delivery, testing with the -bt option, or running the SMTP EXPN
command. It can be further restricted to verifying only senders or recipients
by means of verify_sender and verify_recipient.

Warning: When the router is being run to verify addresses for an incoming SMTP
message, Exim is not running as root, but under its own uid. If the router
accesses any files, you need to make sure that they are accessible to the Exim
user or group.

+-----------------------------------------------------------+
|verify_recipient|Use: routers**|Type: boolean|Default: true|
+-----------------------------------------------------------+

If this option is false, the router is skipped when verifying recipient
addresses, delivering in cutthrough mode or testing recipient verification
using -bv. See section 3.12 for a list of the order in which preconditions are
evaluated. See also the $verify_mode variable.

+--------------------------------------------------------+
|verify_sender|Use: routers**|Type: boolean|Default: true|
+--------------------------------------------------------+

If this option is false, the router is skipped when verifying sender addresses
or testing sender verification using -bvs. See section 3.12 for a list of the
order in which preconditions are evaluated. See also the $verify_mode variable.



===============================================================================
16. THE ACCEPT ROUTER

The accept router has no private options of its own. Unless it is being used
purely for verification (see verify_only) a transport is required to be defined
by the generic transport option. If the preconditions that are specified by
generic options are met, the router accepts the address and queues it for the
given transport. The most common use of this router is for setting up
deliveries to local mailboxes. For example:

localusers:
  driver = accept
  domains = mydomain.example
  check_local_user
  transport = local_delivery

The domains condition in this example checks the domain of the address, and 
check_local_user checks that the local part is the login of a local user. When
both preconditions are met, the accept router runs, and queues the address for
the local_delivery transport.



===============================================================================
17. THE DNSLOOKUP ROUTER

The dnslookup router looks up the hosts that handle mail for the recipient's
domain in the DNS. A transport must always be set for this router, unless 
verify_only is set.

If SRV support is configured (see check_srv below), Exim first searches for SRV
records. If none are found, or if SRV support is not configured, MX records are
looked up. If no MX records exist, address records are sought. However, 
mx_domains can be set to disable the direct use of address records.

MX records of equal priority are sorted by Exim into a random order. Exim then
looks for address records for the host names obtained from MX or SRV records.
When a host has more than one IP address, they are sorted into a random order,
except that IPv6 addresses are sorted before IPv4 addresses. If all the IP
addresses found are discarded by a setting of the ignore_target_hosts generic
option, the router declines.

Unless they have the highest priority (lowest MX value), MX records that point
to the local host, or to any host name that matches hosts_treat_as_local, are
discarded, together with any other MX records of equal or lower priority.

If the host pointed to by the highest priority MX record, or looked up as an
address record, is the local host, or matches hosts_treat_as_local, what
happens is controlled by the generic self option.


17.1 Problems with DNS lookups
------------------------------

There have been problems with DNS servers when SRV records are looked up. Some
misbehaving servers return a DNS error or timeout when a non-existent SRV
record is sought. Similar problems have in the past been reported for MX
records. The global dns_again_means_nonexist option can help with this problem,
but it is heavy-handed because it is a global option.

For this reason, there are two options, srv_fail_domains and mx_fail_domains,
that control what happens when a DNS lookup in a dnslookup router results in a
DNS failure or a "try again" response. If an attempt to look up an SRV or MX
record causes one of these results, and the domain matches the relevant list,
Exim behaves as if the DNS had responded "no such record". In the case of an
SRV lookup, this means that the router proceeds to look for MX records; in the
case of an MX lookup, it proceeds to look for A or AAAA records, unless the
domain matches mx_domains, in which case routing fails.


17.2 Declining addresses by dnslookup
-------------------------------------

There are a few cases where a dnslookup router will decline to accept an
address; if such a router is expected to handle "all remaining non-local
domains", then it is important to set no_more.

The router will defer rather than decline if the domain is found in the 
fail_defer_domains router option.

Reasons for a dnslookup router to decline currently include:

  * The domain does not exist in DNS

  * The domain exists but the MX record's host part is just "."; this is a
    common convention (borrowed from SRV) used to indicate that there is no
    such service for this domain and to not fall back to trying A/AAAA records.

  * Ditto, but for SRV records, when check_srv is set on this router.

  * MX record points to a non-existent host.

  * MX record points to an IP address and the main section option 
    allow_mx_to_ip is not set.

  * MX records exist and point to valid hosts, but all hosts resolve only to
    addresses blocked by the ignore_target_hosts generic option on this router.

  * The domain is not syntactically valid (see also allow_utf8_domains and 
    dns_check_names_pattern for handling one variant of this)

  * check_secondary_mx is set on this router but the local host can not be
    found in the MX records (see below)


17.3 Private options for dnslookup
----------------------------------

The private options for the dnslookup router are as follows:

+--------------------------------------------------------------+
|check_secondary_mx|Use: dnslookup|Type: boolean|Default: false|
+--------------------------------------------------------------+

If this option is set, the router declines unless the local host is found in
(and removed from) the list of hosts obtained by MX lookup. This can be used to
process domains for which the local host is a secondary mail exchanger
differently to other domains. The way in which Exim decides whether a host is
the local host is described in section 13.8.

+-----------------------------------------------------+
|check_srv|Use: dnslookup|Type: string*|Default: unset|
+-----------------------------------------------------+

The dnslookup router supports the use of SRV records (see RFC 2782) in addition
to MX and address records. The support is disabled by default. To enable SRV
support, set the check_srv option to the name of the service required. For
example,

check_srv = smtp

looks for SRV records that refer to the normal smtp service. The option is
expanded, so the service name can vary from message to message or address to
address. This might be helpful if SRV records are being used for a submission
service. If the expansion is forced to fail, the check_srv option is ignored,
and the router proceeds to look for MX records in the normal way.

When the expansion succeeds, the router searches first for SRV records for the
given service (it assumes TCP protocol). A single SRV record with a host name
that consists of just a single dot indicates "no such service for this domain";
if this is encountered, the router declines. If other kinds of SRV record are
found, they are used to construct a host list for delivery according to the
rules of RFC 2782. MX records are not sought in this case.

When no SRV records are found, MX records (and address records) are sought in
the traditional way. In other words, SRV records take precedence over MX
records, just as MX records take precedence over address records. Note that
this behaviour is not sanctioned by RFC 2782, though a previous draft RFC
defined it. It is apparently believed that MX records are sufficient for email
and that SRV records should not be used for this purpose. However, SRV records
have an additional "weight" feature which some people might find useful when
trying to split an SMTP load between hosts of different power.

See section 17.1 above for a discussion of Exim's behaviour when there is a DNS
lookup error.

+-------------------------------------------------------------------+
|fail_defer_domains|Use: dnslookup|Type: domain list*|Default: unset|
+-------------------------------------------------------------------+

DNS lookups for domains matching fail_defer_domains which find no matching
record will cause the router to defer rather than the default behaviour of
decline. This maybe be useful for queueing messages for a newly created domain
while the DNS configuration is not ready. However, it will result in any
message with mistyped domains also being queued.

+-------------------------------------------+
|ipv4_only|Use: string*|Type: unset|Default:|
+-------------------------------------------+

The string is expanded, and if the result is anything but a forced failure, or
an empty string, or one of the strings ?0? or ?no? or ?false? (checked without
regard to the case of the letters), only A records are used.

+---------------------------------------------+
|ipv4_prefer|Use: string*|Type: unset|Default:|
+---------------------------------------------+

The string is expanded, and if the result is anything but a forced failure, or
an empty string, or one of the strings ?0? or ?no? or ?false? (checked without
regard to the case of the letters), A records are sorted before AAAA records
(inverting the default).

+-----------------------------------------------------------+
|mx_domains|Use: dnslookup|Type: domain list*|Default: unset|
+-----------------------------------------------------------+

A domain that matches mx_domains is required to have either an MX or an SRV
record in order to be recognized. (The name of this option could be improved.)
For example, if all the mail hosts in fict.example are known to have MX
records, except for those in discworld.fict.example, you could use this
setting:

mx_domains = ! *.discworld.fict.example : *.fict.example

This specifies that messages addressed to a domain that matches the list but
has no MX record should be bounced immediately instead of being routed using
the address record.

+----------------------------------------------------------------+
|mx_fail_domains|Use: dnslookup|Type: domain list*|Default: unset|
+----------------------------------------------------------------+

If the DNS lookup for MX records for one of the domains in this list causes a
DNS lookup error, Exim behaves as if no MX records were found. See section 17.1
for more discussion.

+---------------------------------------------------------+
|qualify_single|Use: dnslookup|Type: boolean|Default: true|
+---------------------------------------------------------+

When this option is true, the resolver option RES_DEFNAMES is set for DNS
lookups. Typically, but not standardly, this causes the resolver to qualify
single-component names with the default domain. For example, on a machine
called dictionary.ref.example, the domain thesaurus would be changed to 
thesaurus.ref.example inside the resolver. For details of what your resolver
actually does, consult your man pages for resolver and resolv.conf.

+----------------------------------------------------------+
|rewrite_headers|Use: dnslookup|Type: boolean|Default: true|
+----------------------------------------------------------+

If the domain name in the address that is being processed is not fully
qualified, it may be expanded to its full form by a DNS lookup. For example, if
an address is specified as dormouse@teaparty, the domain might be expanded to 
teaparty.wonderland.fict.example. Domain expansion can also occur as a result
of setting the widen_domains option. If rewrite_headers is true, all
occurrences of the abbreviated domain name in any Bcc:, Cc:, From:, Reply-to:, 
Sender:, and To: header lines of the message are rewritten with the full domain
name.

This option should be turned off only when it is known that no message is ever
going to be sent outside an environment where the abbreviation makes sense.

When an MX record is looked up in the DNS and matches a wildcard record, name
servers normally return a record containing the name that has been looked up,
making it impossible to detect whether a wildcard was present or not. However,
some name servers have recently been seen to return the wildcard entry. If the
name returned by a DNS lookup begins with an asterisk, it is not used for
header rewriting.

+--------------------------------------------------------------------+
|same_domain_copy_routing|Use: dnslookup|Type: boolean|Default: false|
+--------------------------------------------------------------------+

Addresses with the same domain are normally routed by the dnslookup router to
the same list of hosts. However, this cannot be presumed, because the router
options and preconditions may refer to the local part of the address. By
default, therefore, Exim routes each address in a message independently. DNS
servers run caches, so repeated DNS lookups are not normally expensive, and in
any case, personal messages rarely have more than a few recipients.

If you are running mailing lists with large numbers of subscribers at the same
domain, and you are using a dnslookup router which is independent of the local
part, you can set same_domain_copy_routing to bypass repeated DNS lookups for
identical domains in one message. In this case, when dnslookup routes an
address to a remote transport, any other unrouted addresses in the message that
have the same domain are automatically given the same routing without
processing them independently, provided the following conditions are met:

  * No router that processed the address specified headers_add or 
    headers_remove.

  * The router did not change the address in any way, for example, by
    "widening" the domain.

+----------------------------------------------------------+
|search_parents|Use: dnslookup|Type: boolean|Default: false|
+----------------------------------------------------------+

When this option is true, the resolver option RES_DNSRCH is set for DNS
lookups. This is different from the qualify_single option in that it applies to
domains containing dots. Typically, but not standardly, it causes the resolver
to search for the name in the current domain and in parent domains. For
example, on a machine in the fict.example domain, if looking up 
teaparty.wonderland failed, the resolver would try 
teaparty.wonderland.fict.example. For details of what your resolver actually
does, consult your man pages for resolver and resolv.conf.

Setting this option true can cause problems in domains that have a wildcard MX
record, because any domain that does not have its own MX record matches the
local wildcard.

+-----------------------------------------------------------------+
|srv_fail_domains|Use: dnslookup|Type: domain list*|Default: unset|
+-----------------------------------------------------------------+

If the DNS lookup for SRV records for one of the domains in this list causes a
DNS lookup error, Exim behaves as if no SRV records were found. See section
17.1 for more discussion.

+-------------------------------------------------------------+
|widen_domains|Use: dnslookup|Type: string list|Default: unset|
+-------------------------------------------------------------+

If a DNS lookup fails and this option is set, each of its strings in turn is
added onto the end of the domain, and the lookup is tried again. For example,
if

widen_domains = fict.example:ref.example

is set and a lookup of klingon.dictionary fails, 
klingon.dictionary.fict.example is looked up, and if this fails, 
klingon.dictionary.ref.example is tried. Note that the qualify_single and 
search_parents options can cause some widening to be undertaken inside the DNS
resolver. widen_domains is not applied to sender addresses when verifying,
unless rewrite_headers is false (not the default).


17.4 Effect of qualify_single and search_parents
------------------------------------------------

When a domain from an envelope recipient is changed by the resolver as a result
of the qualify_single or search_parents options, Exim rewrites the
corresponding address in the message's header lines unless rewrite_headers is
set false. Exim then re-routes the address, using the full domain.

These two options affect only the DNS lookup that takes place inside the router
for the domain of the address that is being routed. They do not affect lookups
such as that implied by

domains = @mx_any

that may happen while processing a router precondition before the router is
entered. No widening ever takes place for these lookups.



===============================================================================
18. THE IPLITERAL ROUTER

This router has no private options. Unless it is being used purely for
verification (see verify_only) a transport is required to be defined by the
generic transport option. The router accepts the address if its domain part
takes the form of an RFC 2822 domain literal. For example, the ipliteral router
handles the address

root@[192.168.1.1]

by setting up delivery to the host with that IP address. IPv4 domain literals
consist of an IPv4 address enclosed in square brackets. IPv6 domain literals
are similar, but the address is preceded by "ipv6:". For example:

postmaster@[ipv6:fe80::a00:20ff:fe86:a061.5678]

Exim allows "ipv4:" before IPv4 addresses, for consistency, and on the grounds
that sooner or later somebody will try it.

If the IP address matches something in ignore_target_hosts, the router
declines. If an IP literal turns out to refer to the local host, the generic 
self option determines what happens.

The RFCs require support for domain literals; however, their use is
controversial in today's Internet. If you want to use this router, you must
also set the main configuration option allow_domain_literals. Otherwise, Exim
will not recognize the domain literal syntax in addresses.



===============================================================================
19. THE IPLOOKUP ROUTER

The iplookup router was written to fulfil a specific requirement in Cambridge
University (which in fact no longer exists). For this reason, it is not
included in the binary of Exim by default. If you want to include it, you must
set

ROUTER_IPLOOKUP=yes

in your Local/Makefile configuration file.

The iplookup router routes an address by sending it over a TCP or UDP
connection to one or more specific hosts. The host can then return the same or
a different address - in effect rewriting the recipient address in the
message's envelope. The new address is then passed on to subsequent routers. If
this process fails, the address can be passed on to other routers, or delivery
can be deferred. Since iplookup is just a rewriting router, a transport must
not be specified for it.

+-----------------------------------------------+
|hosts|Use: iplookup|Type: string|Default: unset|
+-----------------------------------------------+

This option must be supplied. Its value is a colon-separated list of host
names. The hosts are looked up using gethostbyname() (or getipnodebyname() when
available) and are tried in order until one responds to the query. If none
respond, what happens is controlled by optional.

+---------------------------------------------------+
|optional|Use: iplookup|Type: boolean|Default: false|
+---------------------------------------------------+

If optional is true, if no response is obtained from any host, the address is
passed to the next router, overriding no_more. If optional is false, delivery
to the address is deferred.

+-------------------------------------------+
|port|Use: iplookup|Type: integer|Default: 0|
+-------------------------------------------+

This option must be supplied. It specifies the port number for the TCP or UDP
call.

+------------------------------------------------+
|protocol|Use: iplookup|Type: string|Default: udp|
+------------------------------------------------+

This option can be set to "udp" or "tcp" to specify which of the two protocols
is to be used.

+----------------------------------------------------+
|query|Use: iplookup|Type: string*|Default: see below|
+----------------------------------------------------+

This defines the content of the query that is sent to the remote hosts. The
default value is:

$local_part@$domain $local_part@$domain

The repetition serves as a way of checking that a response is to the correct
query in the default case (see response_pattern below).

+--------------------------------------------------+
|reroute|Use: iplookup|Type: string*|Default: unset|
+--------------------------------------------------+

If this option is not set, the rerouted address is precisely the byte string
returned by the remote host, up to the first white space, if any. If set, the
string is expanded to form the rerouted address. It can include parts matched
in the response by response_pattern by means of numeric variables such as $1,
$2, etc. The variable $0 refers to the entire input string, whether or not a
pattern is in use. In all cases, the rerouted address must end up in the form 
local_part@domain.

+----------------------------------------------------------+
|response_pattern|Use: iplookup|Type: string|Default: unset|
+----------------------------------------------------------+

This option can be set to a regular expression that is applied to the string
returned from the remote host. If the pattern does not match the response, the
router declines. If response_pattern is not set, no checking of the response is
done, unless the query was defaulted, in which case there is a check that the
text returned after the first white space is the original address. This checks
that the answer that has been received is in response to the correct question.
For example, if the response is just a new domain, the following could be used:

response_pattern = ^([^@]+)$
reroute = $local_part@$1

+--------------------------------------------+
|timeout|Use: iplookup|Type: time|Default: 5s|
+--------------------------------------------+

This specifies the amount of time to wait for a response from the remote
machine. The same timeout is used for the connect() function for a TCP call. It
does not apply to UDP.



===============================================================================
20. THE MANUALROUTE ROUTER

The manualroute router is so-called because it provides a way of manually
routing an address according to its domain. It is mainly used when you want to
route addresses to remote hosts according to your own rules, bypassing the
normal DNS routing that looks up MX records. However, manualroute can also
route to local transports, a facility that may be useful if you want to save
messages for dial-in hosts in local files.

The manualroute router compares a list of domain patterns with the domain it is
trying to route. If there is no match, the router declines. Each pattern has
associated with it a list of hosts and some other optional data, which may
include a transport. The combination of a pattern and its data is called a
"routing rule". For patterns that do not have an associated transport, the
generic transport option must specify a transport, unless the router is being
used purely for verification (see verify_only).

In the case of verification, matching the domain pattern is sufficient for the
router to accept the address. When actually routing an address for delivery, an
address that matches a domain pattern is queued for the associated transport.
If the transport is not a local one, a host list must be associated with the
pattern; IP addresses are looked up for the hosts, and these are passed to the
transport along with the mail address. For local transports, a host list is
optional. If it is present, it is passed in $host as a single text string.

The list of routing rules can be provided as an inline string in route_list, or
the data can be obtained by looking up the domain in a file or database by
setting route_data. Only one of these settings may appear in any one instance
of manualroute. The format of routing rules is described below, following the
list of private options.


20.1 Private options for manualroute
------------------------------------

The private options for the manualroute router are as follows:

+-------------------------------------------------------------+
|host_all_ignored|Use: manualroute|Type: string|Default: defer|
+-------------------------------------------------------------+

See host_find_failed.

+--------------------------------------------------------------+
|host_find_failed|Use: manualroute|Type: string|Default: freeze|
+--------------------------------------------------------------+

This option controls what happens when manualroute tries to find an IP address
for a host, and the host does not exist. The option can be set to one of the
following values:

decline
defer
fail
freeze
ignore
pass

The default ("freeze") assumes that this state is a serious configuration
error. The difference between "pass" and "decline" is that the former forces
the address to be passed to the next router (or the router defined by 
pass_router), overriding no_more, whereas the latter passes the address to the
next router only if more is true.

The value "ignore" causes Exim to completely ignore a host whose IP address
cannot be found. If all the hosts in the list are ignored, the behaviour is
controlled by the host_all_ignored option. This takes the same values as 
host_find_failed, except that it cannot be set to "ignore".

The host_find_failed option applies only to a definite "does not exist" state;
if a host lookup gets a temporary error, delivery is deferred unless the
generic pass_on_timeout option is set.

+-------------------------------------------------------------+
|hosts_randomize|Use: manualroute|Type: boolean|Default: false|
+-------------------------------------------------------------+

If this option is set, the order of the items in a host list in a routing rule
is randomized each time the list is used, unless an option in the routing rule
overrides (see below). Randomizing the order of a host list can be used to do
crude load sharing. However, if more than one mail address is routed by the
same router to the same host list, the host lists are considered to be the same
(even though they may be randomized into different orders) for the purpose of
deciding whether to batch the deliveries into a single SMTP transaction.

When hosts_randomize is true, a host list may be split into groups whose order
is separately randomized. This makes it possible to set up MX-like behaviour.
The boundaries between groups are indicated by an item that is just "+" in the
host list. For example:

route_list = * host1:host2:host3:+:host4:host5

The order of the first three hosts and the order of the last two hosts is
randomized for each use, but the first three always end up before the last two.
If hosts_randomize is not set, a "+" item in the list is ignored. If a
randomized host list is passed to an smtp transport that also has 
hosts_randomize set, the list is not re-randomized.

+--------------------------------------------------------+
|route_data|Use: manualroute|Type: string*|Default: unset|
+--------------------------------------------------------+

If this option is set, it must expand to yield the data part of a routing rule.
Typically, the expansion string includes a lookup based on the domain. For
example:

route_data = ${lookup{$domain}dbm{/etc/routes}}

If the expansion is forced to fail, or the result is an empty string, the
router declines. Other kinds of expansion failure cause delivery to be
deferred.

+------------------------------------------------------------+
|route_list|Use: manualroute|Type: string list|Default: unset|
+------------------------------------------------------------+

This string is a list of routing rules, in the form defined below. Note that,
unlike most string lists, the items are separated by semicolons. This is so
that they may contain colon-separated host lists.

+----------------------------------------------------------------------+
|same_domain_copy_routing|Use: manualroute|Type: boolean|Default: false|
+----------------------------------------------------------------------+

Addresses with the same domain are normally routed by the manualroute router to
the same list of hosts. However, this cannot be presumed, because the router
options and preconditions may refer to the local part of the address. By
default, therefore, Exim routes each address in a message independently. DNS
servers run caches, so repeated DNS lookups are not normally expensive, and in
any case, personal messages rarely have more than a few recipients.

If you are running mailing lists with large numbers of subscribers at the same
domain, and you are using a manualroute router which is independent of the
local part, you can set same_domain_copy_routing to bypass repeated DNS lookups
for identical domains in one message. In this case, when manualroute routes an
address to a remote transport, any other unrouted addresses in the message that
have the same domain are automatically given the same routing without
processing them independently. However, this is only done if headers_add and 
headers_remove are unset.


20.2 Routing rules in route_list
--------------------------------

The value of route_list is a string consisting of a sequence of routing rules,
separated by semicolons. If a semicolon is needed in a rule, it can be entered
as two semicolons. Alternatively, the list separator can be changed as
described (for colon-separated lists) in section 6.20. Empty rules are ignored.
The format of each rule is

<domain pattern>  <list of hosts>  <options>

The following example contains two rules, each with a simple domain pattern and
no options:

route_list = \
  dict.ref.example  mail-1.ref.example:mail-2.ref.example ; \
  thes.ref.example  mail-3.ref.example:mail-4.ref.example

The three parts of a rule are separated by white space. The pattern and the
list of hosts can be enclosed in quotes if necessary, and if they are, the
usual quoting rules apply. Each rule in a route_list must start with a single
domain pattern, which is the only mandatory item in the rule. The pattern is in
the same format as one item in a domain list (see section 10.3), except that it
may not be the name of an interpolated file. That is, it may be wildcarded, or
a regular expression, or a file or database lookup (with semicolons doubled,
because of the use of semicolon as a separator in a route_list).

The rules in route_list are searched in order until one of the patterns matches
the domain that is being routed. The list of hosts and then options are then
used as described below. If there is no match, the router declines. When 
route_list is set, route_data must not be set.


20.3 Routing rules in route_data
--------------------------------

The use of route_list is convenient when there are only a small number of
routing rules. For larger numbers, it is easier to use a file or database to
hold the routing information, and use the route_data option instead. The value
of route_data is a list of hosts, followed by (optional) options. Most
commonly, route_data is set as a string that contains an expansion lookup. For
example, suppose we place two routing rules in a file like this:

dict.ref.example:  mail-1.ref.example:mail-2.ref.example
thes.ref.example:  mail-3.ref.example:mail-4.ref.example

This data can be accessed by setting

route_data = ${lookup{$domain}lsearch{/the/file/name}}

Failure of the lookup results in an empty string, causing the router to
decline. However, you do not have to use a lookup in route_data. The only
requirement is that the result of expanding the string is a list of hosts,
possibly followed by options, separated by white space. The list of hosts must
be enclosed in quotes if it contains white space.


20.4 Format of the list of hosts
--------------------------------

A list of hosts, whether obtained via route_data or route_list, is always
separately expanded before use. If the expansion fails, the router declines.
The result of the expansion must be a colon-separated list of names and/or IP
addresses, optionally also including ports. If the list is written with spaces,
it must be protected with quotes. The format of each item in the list is
described in the next section. The list separator can be changed as described
in section 6.21.

If the list of hosts was obtained from a route_list item, the following
variables are set during its expansion:

  * If the domain was matched against a regular expression, the numeric
    variables $1, $2, etc. may be set. For example:

    route_list = ^domain(\d+)   host-$1.text.example

  * $0 is always set to the entire domain.

  * $1 is also set when partial matching is done in a file lookup.

  * If the pattern that matched the domain was a lookup item, the data that was
    looked up is available in the expansion variable $value. For example:

    route_list = lsearch;;/some/file.routes  $value

Note the doubling of the semicolon in the pattern that is necessary because
semicolon is the default route list separator.


20.5 Format of one host item
----------------------------

Each item in the list of hosts can be either a host name or an IP address,
optionally with an attached port number, or it can be a single "+" (see 
hosts_randomize). When no port is given, an IP address is not enclosed in
brackets. When a port is specified, it overrides the port specification on the
transport. The port is separated from the name or address by a colon. This
leads to some complications:

  * Because colon is the default separator for the list of hosts, either the
    colon that specifies a port must be doubled, or the list separator must be
    changed. The following two examples have the same effect:

    route_list = * "host1.tld::1225 : host2.tld::1226"
    route_list = * "<+ host1.tld:1225 + host2.tld:1226"

  * When IPv6 addresses are involved, it gets worse, because they contain
    colons of their own. To make this case easier, it is permitted to enclose
    an IP address (either v4 or v6) in square brackets if a port number
    follows. For example:

    route_list = * "</ [10.1.1.1]:1225 / [::1]:1226"


20.6 How the list of hosts is used
----------------------------------

When an address is routed to an smtp transport by manualroute, each of the
hosts is tried, in the order specified, when carrying out the SMTP delivery.
However, the order can be changed by setting the hosts_randomize option, either
on the router (see section 20.1 above), or on the transport.

Hosts may be listed by name or by IP address. An unadorned name in the list of
hosts is interpreted as a host name. A name that is followed by "/MX" is
interpreted as an indirection to a sublist of hosts obtained by looking up MX
records in the DNS. For example:

route_list = *  x.y.z:p.q.r/MX:e.f.g

If this feature is used with a port specifier, the port must come last. For
example:

route_list = *  dom1.tld/mx::1225

If the hosts_randomize option is set, the order of the items in the list is
randomized before any lookups are done. Exim then scans the list; for any name
that is not followed by "/MX" it looks up an IP address. If this turns out to
be an interface on the local host and the item is not the first in the list,
Exim discards it and any subsequent items. If it is the first item, what
happens is controlled by the self option of the router.

A name on the list that is followed by "/MX" is replaced with the list of hosts
obtained by looking up MX records for the name. This is always a DNS lookup;
the bydns and byname options (see section 20.7 below) are not relevant here.
The order of these hosts is determined by the preference values in the MX
records, according to the usual rules. Because randomizing happens before the
MX lookup, it does not affect the order that is defined by MX preferences.

If the local host is present in the sublist obtained from MX records, but is
not the most preferred host in that list, it and any equally or less preferred
hosts are removed before the sublist is inserted into the main list.

If the local host is the most preferred host in the MX list, what happens
depends on where in the original list of hosts the "/MX" item appears. If it is
not the first item (that is, there are previous hosts in the main list), Exim
discards this name and any subsequent items in the main list.

If the MX item is first in the list of hosts, and the local host is the most
preferred host, what happens is controlled by the self option of the router.

DNS failures when lookup up the MX records are treated in the same way as DNS
failures when looking up IP addresses: pass_on_timeout and host_find_failed are
used when relevant.

The generic ignore_target_hosts option applies to all hosts in the list,
whether obtained from an MX lookup or not.


20.7 How the options are used
-----------------------------

The options are a sequence of words, space-separated. One of the words can be
the name of a transport; this overrides the transport option on the router for
this particular routing rule only. The other words (if present) control
randomization of the list of hosts on a per-rule basis, and how the IP
addresses of the hosts are to be found when routing to a remote transport.
These options are as follows:

  * randomize: randomize the order of the hosts in this list, overriding the
    setting of hosts_randomize for this routing rule only.

  * no_randomize: do not randomize the order of the hosts in this list,
    overriding the setting of hosts_randomize for this routing rule only.

  * byname: use getipnodebyname() (gethostbyname() on older systems) to find IP
    addresses. This function may ultimately cause a DNS lookup, but it may also
    look in /etc/hosts or other sources of information.

  * bydns: look up address records for the hosts directly in the DNS; fail if
    no address records are found. If there is a temporary DNS error (such as a
    timeout), delivery is deferred.

  * ipv4_only: in direct DNS lookups, look up only A records.

  * ipv4_prefer: in direct DNS lookups, sort A records before AAAA records.

For example:

route_list = domain1  host1:host2:host3  randomize bydns;\
             domain2  host4:host5

If neither byname nor bydns is given, Exim behaves as follows: First, a DNS
lookup is done. If this yields anything other than HOST_NOT_FOUND, that result
is used. Otherwise, Exim goes on to try a call to getipnodebyname() or
gethostbyname(), and the result of the lookup is the result of that call.

Warning: It has been discovered that on some systems, if a DNS lookup called
via getipnodebyname() times out, HOST_NOT_FOUND is returned instead of
TRY_AGAIN. That is why the default action is to try a DNS lookup first. Only if
that gives a definite "no such host" is the local function called.

Compatibility: From Exim 4.85 until fixed for 4.90, there was an inadvertent
constraint that a transport name as an option had to be the last option
specified.

If no IP address for a host can be found, what happens is controlled by the 
host_find_failed option.

When an address is routed to a local transport, IP addresses are not looked up.
The host list is passed to the transport in the $host variable.


20.8 Manualroute examples
-------------------------

In some of the examples that follow, the presence of the remote_smtp transport,
as defined in the default configuration file, is assumed:

  * The manualroute router can be used to forward all external mail to a smart
    host. If you have set up, in the main part of the configuration, a named
    domain list that contains your local domains, for example:

    domainlist local_domains = my.domain.example

    You can arrange for all other domains to be routed to a smart host by
    making your first router something like this:

    smart_route:
      driver = manualroute
      domains = !+local_domains
      transport = remote_smtp
      route_list = * smarthost.ref.example

    This causes all non-local addresses to be sent to the single host 
    smarthost.ref.example. If a colon-separated list of smart hosts is given,
    they are tried in order (but you can use hosts_randomize to vary the order
    each time). Another way of configuring the same thing is this:

    smart_route:
      driver = manualroute
      transport = remote_smtp
      route_list = !+local_domains  smarthost.ref.example

    There is no difference in behaviour between these two routers as they
    stand. However, they behave differently if no_more is added to them. In the
    first example, the router is skipped if the domain does not match the 
    domains precondition; the following router is always tried. If the router
    runs, it always matches the domain and so can never decline. Therefore, 
    no_more would have no effect. In the second case, the router is never
    skipped; it always runs. However, if it doesn't match the domain, it
    declines. In this case no_more would prevent subsequent routers from
    running.

  * A mail hub is a host which receives mail for a number of domains via MX
    records in the DNS and delivers it via its own private routing mechanism.
    Often the final destinations are behind a firewall, with the mail hub being
    the one machine that can connect to machines both inside and outside the
    firewall. The manualroute router is usually used on a mail hub to route
    incoming messages to the correct hosts. For a small number of domains, the
    routing can be inline, using the route_list option, but for a larger number
    a file or database lookup is easier to manage.

    If the domain names are in fact the names of the machines to which the mail
    is to be sent by the mail hub, the configuration can be quite simple. For
    example:

    hub_route:
      driver = manualroute
      transport = remote_smtp
      route_list = *.rhodes.tvs.example  $domain

    This configuration routes domains that match "*.rhodes.tvs.example" to
    hosts whose names are the same as the mail domains. A similar approach can
    be taken if the host name can be obtained from the domain name by a string
    manipulation that the expansion facilities can handle. Otherwise, a lookup
    based on the domain can be used to find the host:

    through_firewall:
      driver = manualroute
      transport = remote_smtp
      route_data = ${lookup {$domain} cdb {/internal/host/routes}}

    The result of the lookup must be the name or IP address of the host (or
    hosts) to which the address is to be routed. If the lookup fails, the route
    data is empty, causing the router to decline. The address then passes to
    the next router.

  * You can use manualroute to deliver messages to pipes or files in batched
    SMTP format for onward transportation by some other means. This is one way
    of storing mail for a dial-up host when it is not connected. The route list
    entry can be as simple as a single domain name in a configuration like
    this:

    save_in_file:
      driver = manualroute
      transport = batchsmtp_appendfile
      route_list = saved.domain.example

    though often a pattern is used to pick up more than one domain. If there
    are several domains or groups of domains with different transport
    requirements, different transports can be listed in the routing
    information:

    save_in_file:
      driver = manualroute
      route_list = \
        *.saved.domain1.example  $domain  batch_appendfile; \
        *.saved.domain2.example  \
          ${lookup{$domain}dbm{/domain2/hosts}{$value}fail} \
          batch_pipe

    The first of these just passes the domain in the $host variable, which
    doesn't achieve much (since it is also in $domain), but the second does a
    file lookup to find a value to pass, causing the router to decline to
    handle the address if the lookup fails.

  * Routing mail directly to UUCP software is a specific case of the use of 
    manualroute in a gateway to another mail environment. This is an example of
    one way it can be done:

    # Transport
    uucp:
      driver = pipe
      user = nobody
      command = /usr/local/bin/uux -r - \
        ${substr_-5:$host}!rmail ${local_part}
      return_fail_output = true

    # Router
    uucphost:
      transport = uucp
      driver = manualroute
      route_data = \
        ${lookup{$domain}lsearch{/usr/local/exim/uucphosts}}

    The file /usr/local/exim/uucphosts contains entries like

    darksite.ethereal.example:           darksite.UUCP

    It can be set up more simply without adding and removing ".UUCP" but this
    way makes clear the distinction between the domain name 
    darksite.ethereal.example and the UUCP host name darksite.



===============================================================================
21. THE QUERYPROGRAM ROUTER

The queryprogram router routes an address by running an external command and
acting on its output. This is an expensive way to route, and is intended mainly
for use in lightly-loaded systems, or for performing experiments. However, if
it is possible to use the precondition options (domains, local_parts, etc) to
skip this router for most addresses, it could sensibly be used in special
cases, even on a busy host. There are the following private options:

+------------------------------------------------------+
|command|Use: queryprogram|Type: string*|Default: unset|
+------------------------------------------------------+

This option must be set. It specifies the command that is to be run. The
command is split up into a command name and arguments, and then each is
expanded separately (exactly as for a pipe transport, described in chapter 29).

+-----------------------------------------------------------+
|command_group|Use: queryprogram|Type: string|Default: unset|
+-----------------------------------------------------------+

This option specifies a gid to be set when running the command while routing an
address for deliver. It must be set if command_user specifies a numerical uid.
If it begins with a digit, it is interpreted as the numerical value of the gid.
Otherwise it is looked up using getgrnam().

+----------------------------------------------------------+
|command_user|Use: queryprogram|Type: string|Default: unset|
+----------------------------------------------------------+

This option must be set. It specifies the uid which is set when running the
command while routing an address for delivery. If the value begins with a
digit, it is interpreted as the numerical value of the uid. Otherwise, it is
looked up using getpwnam() to obtain a value for the uid and, if command_group
is not set, a value for the gid also.

Warning: Changing uid and gid is possible only when Exim is running as root,
which it does during a normal delivery in a conventional configuration.
However, when an address is being verified during message reception, Exim is
usually running as the Exim user, not as root. If the queryprogram router is
called from a non-root process, Exim cannot change uid or gid before running
the command. In this circumstance the command runs under the current uid and
gid.

+-----------------------------------------------------------+
|current_directory|Use: queryprogram|Type: string|Default: /|
+-----------------------------------------------------------+

This option specifies an absolute path which is made the current directory
before running the command.

+------------------------------------------------+
|timeout|Use: queryprogram|Type: time|Default: 1h|
+------------------------------------------------+

If the command does not complete within the timeout period, its process group
is killed and the message is frozen. A value of zero time specifies no timeout.

The standard output of the command is connected to a pipe, which is read when
the command terminates. It should consist of a single line of output,
containing up to five fields, separated by white space. The maximum length of
the line is 1023 characters. Longer lines are silently truncated. The first
field is one of the following words (case-insensitive):

  * Accept: routing succeeded; the remaining fields specify what to do (see
    below).

  * Decline: the router declines; pass the address to the next router, unless 
    no_more is set.

  * Fail: routing failed; do not pass the address to any more routers. Any
    subsequent text on the line is an error message. If the router is run as
    part of address verification during an incoming SMTP message, the message
    is included in the SMTP response.

  * Defer: routing could not be completed at this time; try again later. Any
    subsequent text on the line is an error message which is logged. It is not
    included in any SMTP response.

  * Freeze: the same as defer, except that the message is frozen.

  * Pass: pass the address to the next router (or the router specified by 
    pass_router), overriding no_more.

  * Redirect: the message is redirected. The remainder of the line is a list of
    new addresses, which are routed independently, starting with the first
    router, or the router specified by redirect_router, if set.

When the first word is accept, the remainder of the line consists of a number
of keyed data values, as follows (split into two lines here, to fit on the
page):

ACCEPT TRANSPORT=<transport> HOSTS=<list of hosts>
LOOKUP=byname|bydns DATA=<text>

The data items can be given in any order, and all are optional. If no transport
is included, the transport specified by the generic transport option is used.
The list of hosts and the lookup type are needed only if the transport is an 
smtp transport that does not itself supply a list of hosts.

The format of the list of hosts is the same as for the manualroute router. As
well as host names and IP addresses with optional port numbers, as described in
section 20.5, it may contain names followed by "/MX" to specify sublists of
hosts that are obtained by looking up MX records (see section 20.6).

If the lookup type is not specified, Exim behaves as follows when trying to
find an IP address for each host: First, a DNS lookup is done. If this yields
anything other than HOST_NOT_FOUND, that result is used. Otherwise, Exim goes
on to try a call to getipnodebyname() or gethostbyname(), and the result of the
lookup is the result of that call.

If the DATA field is set, its value is placed in the $address_data variable.
For example, this return line

accept hosts=x1.y.example:x2.y.example data="rule1"

routes the address to the default transport, passing a list of two hosts. When
the transport runs, the string "rule1" is in $address_data.



===============================================================================
22. THE REDIRECT ROUTER

The redirect router handles several kinds of address redirection. Its most
common uses are for resolving local part aliases from a central alias file
(usually called /etc/aliases) and for handling users' personal .forward files,
but it has many other potential uses. The incoming address can be redirected in
several different ways:

  * It can be replaced by one or more new addresses which are themselves routed
    independently.

  * It can be routed to be delivered to a given file or directory.

  * It can be routed to be delivered to a specified pipe command.

  * It can cause an automatic reply to be generated.

  * It can be forced to fail, optionally with a custom error message.

  * It can be temporarily deferred, optionally with a custom message.

  * It can be discarded.

The generic transport option must not be set for redirect routers. However,
there are some private options which define transports for delivery to files
and pipes, and for generating autoreplies. See the file_transport, 
pipe_transport and reply_transport descriptions below.

If success DSNs have been requested redirection triggers one and the DSN
options are not passed any further.


22.1 Redirection data
---------------------

The router operates by interpreting a text string which it obtains either by
expanding the contents of the data option, or by reading the entire contents of
a file whose name is given in the file option. These two options are mutually
exclusive. The first is commonly used for handling system aliases, in a
configuration like this:

system_aliases:
  driver = redirect
  data = ${lookup{$local_part}lsearch{/etc/aliases}}

If the lookup fails, the expanded string in this example is empty. When the
expansion of data results in an empty string, the router declines. A forced
expansion failure also causes the router to decline; other expansion failures
cause delivery to be deferred.

A configuration using file is commonly used for handling users' .forward files,
like this:

userforward:
  driver = redirect
  check_local_user
  file = $home/.forward
  no_verify

If the file does not exist, or causes no action to be taken (for example, it is
empty or consists only of comments), the router declines. Warning: This is not
the case when the file contains syntactically valid items that happen to yield
empty addresses, for example, items containing only RFC 2822 address comments.

Tainted data may not be used for a filename.

Warning: It is unwise to use $local_part or $domain directly for redirection,
as they are provided by a potential attacker. In the examples above,
$local_part is used for looking up data held locally on the system, and not
used directly (the second example derives $home via the passsword file or
database, using $local_part).


22.2 Forward files and address verification
-------------------------------------------

It is usual to set no_verify on redirect routers which handle users' .forward
files, as in the example above. There are two reasons for this:

  * When Exim is receiving an incoming SMTP message from a remote host, it is
    running under the Exim uid, not as root. Exim is unable to change uid to
    read the file as the user, and it may not be able to read it as the Exim
    user. So in practice the router may not be able to operate.

  * However, even when the router can operate, the existence of a .forward file
    is unimportant when verifying an address. What should be checked is whether
    the local part is a valid user name or not. Cutting out the redirection
    processing saves some resources.


22.3 Interpreting redirection data
----------------------------------

The contents of the data string, whether obtained from data or file, can be
interpreted in two different ways:

  * If the allow_filter option is set true, and the data begins with the text "
    #Exim filter" or "#Sieve filter", it is interpreted as a list of filtering
    instructions in the form of an Exim or Sieve filter file, respectively.
    Details of the syntax and semantics of filter files are described in a
    separate document entitled Exim's interfaces to mail filtering; this
    document is intended for use by end users.

  * Otherwise, the data must be a comma-separated list of redirection items, as
    described in the next section.

When a message is redirected to a file (a "mail folder"), the filename given in
a non-filter redirection list must always be an absolute path. A filter may
generate a relative path - how this is handled depends on the transport's
configuration. See section 26.1 for a discussion of this issue for the 
appendfile transport.


22.4 Items in a non-filter redirection list
-------------------------------------------

When the redirection data is not an Exim or Sieve filter, for example, if it
comes from a conventional alias or forward file, it consists of a list of
addresses, filenames, pipe commands, or certain special items (see section 22.6
below). The special items can be individually enabled or disabled by means of
options whose names begin with allow_ or forbid_, depending on their default
values. The items in the list are separated by commas or newlines. If a comma
is required in an item, the entire item must be enclosed in double quotes.

Lines starting with a # character are comments, and are ignored, and # may also
appear following a comma, in which case everything between the # and the next
newline character is ignored.

If an item is entirely enclosed in double quotes, these are removed. Otherwise
double quotes are retained because some forms of mail address require their use
(but never to enclose the entire address). In the following description, "item"
refers to what remains after any surrounding double quotes have been removed.

Warning: If you use an Exim expansion to construct a redirection address, and
the expansion contains a reference to $local_part, you should make use of the 
quote_local_part expansion operator, in case the local part contains special
characters. For example, to redirect all mail for the domain obsolete.example,
retaining the existing local part, you could use this setting:

data = ${quote_local_part:$local_part}@newdomain.example


22.5 Redirecting to a local mailbox
-----------------------------------

A redirection item may safely be the same as the address currently under
consideration. This does not cause a routing loop, because a router is
automatically skipped if any ancestor of the address that is being processed is
the same as the current address and was processed by the current router. Such
an address is therefore passed to the following routers, so it is handled as if
there were no redirection. When making this loop-avoidance test, the complete
local part, including any prefix or suffix, is used.

Specifying the same local part without a domain is a common usage in personal
filter files when the user wants to have messages delivered to the local
mailbox and also forwarded elsewhere. For example, the user whose login is cleo
might have a .forward file containing this:

cleo, cleopatra@egypt.example

For compatibility with other MTAs, such unqualified local parts may be preceded
by "\", but this is not a requirement for loop prevention. However, it does
make a difference if more than one domain is being handled synonymously.

If an item begins with "\" and the rest of the item parses as a valid RFC 2822
address that does not include a domain, the item is qualified using the domain
of the incoming address. In the absence of a leading "\", unqualified addresses
are qualified using the value in qualify_recipient, but you can force the
incoming domain to be used by setting qualify_preserve_domain.

Care must be taken if there are alias names for local users. Consider an MTA
handling a single local domain where the system alias file contains:

Sam.Reman: spqr

Now suppose that Sam (whose login id is spqr) wants to save copies of messages
in the local mailbox, and also forward copies elsewhere. He creates this
forward file:

Sam.Reman, spqr@reme.elsewhere.example

With these settings, an incoming message addressed to Sam.Reman fails. The 
redirect router for system aliases does not process Sam.Reman the second time
round, because it has previously routed it, and the following routers
presumably cannot handle the alias. The forward file should really contain

spqr, spqr@reme.elsewhere.example

but because this is such a common error, the check_ancestor option (see below)
exists to provide a way to get round it. This is normally set on a redirect
router that is handling users' .forward files.


22.6 Special items in redirection lists
---------------------------------------

In addition to addresses, the following types of item may appear in redirection
lists (that is, in non-filter redirection data):

  * An item is treated as a pipe command if it begins with "|" and does not
    parse as a valid RFC 2822 address that includes a domain. A transport for
    running the command must be specified by the pipe_transport option.
    Normally, either the router or the transport specifies a user and a group
    under which to run the delivery. The default is to use the Exim user and
    group.

    Single or double quotes can be used for enclosing the individual arguments
    of the pipe command; no interpretation of escapes is done for single
    quotes. If the command contains a comma character, it is necessary to put
    the whole item in double quotes, for example:

    "|/some/command ready,steady,go"

    since items in redirection lists are terminated by commas. Do not, however,
    quote just the command. An item such as

    |"/some/command ready,steady,go"

    is interpreted as a pipe with a rather strange command name, and no
    arguments.

    Note that the above example assumes that the text comes from a lookup
    source of some sort, so that the quotes are part of the data. If composing
    a redirect router with a data option directly specifying this command, the
    quotes will be used by the configuration parser to define the extent of one
    string, but will not be passed down into the redirect router itself. There
    are two main approaches to get around this: escape quotes to be part of the
    data itself, or avoid using this mechanism and instead create a custom
    transport with the command option set and reference that transport from an 
    accept router.

  * An item is interpreted as a path name if it begins with "/" and does not
    parse as a valid RFC 2822 address that includes a domain. For example,

    /home/world/minbari

    is treated as a filename, but

    /s=molari/o=babylon/@x400gate.way

    is treated as an address. For a filename, a transport must be specified
    using the file_transport option. However, if the generated path name ends
    with a forward slash character, it is interpreted as a directory name
    rather than a filename, and directory_transport is used instead.

    Normally, either the router or the transport specifies a user and a group
    under which to run the delivery. The default is to use the Exim user and
    group.

    However, if a redirection item is the path /dev/null, delivery to it is
    bypassed at a high level, and the log entry shows "**bypassed**" instead of
    a transport name. In this case the user and group are not used.

  * If an item is of the form

    :include:<path name>

    a list of further items is taken from the given file and included at that
    point. Note: Such a file can not be a filter file; it is just an
    out-of-line addition to the list. The items in the included list are
    separated by commas or newlines and are not subject to expansion. If this
    is the first item in an alias list in an lsearch file, a colon must be used
    to terminate the alias name. This example is incorrect:

    list1    :include:/opt/lists/list1

    It must be given as

    list1:   :include:/opt/lists/list1

    Tainted data may not be used for a filename.

  * Sometimes you want to throw away mail to a particular local part. Making
    the data option expand to an empty string does not work, because that
    causes the router to decline. Instead, the alias item

    :blackhole:

    can be used. It does what its name implies. No delivery is done, and no
    error message is generated. This has the same effect as specifying /dev/
    null as a destination, but it can be independently disabled.

    Warning: If :blackhole: appears anywhere in a redirection list, no delivery
    is done for the original local part, even if other redirection items are
    present. If you are generating a multi-item list (for example, by reading a
    database) and need the ability to provide a no-op item, you must use /dev/
    null.

  * An attempt to deliver a particular address can be deferred or forced to
    fail by redirection items of the form

    :defer:
    :fail:

    respectively. When a redirection list contains such an item, it applies to
    the entire redirection; any other items in the list are ignored. Any text
    following :fail: or :defer: is placed in the error text associated with the
    failure. For example, an alias file might contain:

    X.Employee:  :fail: Gone away, no forwarding address

    In the case of an address that is being verified from an ACL or as the
    subject of a VRFY command, the text is included in the SMTP error response
    by default. The text is not included in the response to an EXPN command. In
    non-SMTP cases the text is included in the error message that Exim
    generates.

    By default for verify, Exim sends a 451 SMTP code for a :defer:, and 550
    for :fail:. However, if the message starts with three digits followed by a
    space, optionally followed by an extended code of the form n.n.n, also
    followed by a space, and the very first digit is the same as the default
    error code, the code from the message is used instead. If the very first
    digit is incorrect, a panic error is logged, and the default code is used.
    You can suppress the use of the supplied code in a redirect router by
    setting the forbid_smtp_code option true. In this case, any SMTP code is
    quietly ignored.

    In an ACL, an explicitly provided message overrides the default, but the
    default message is available in the variable $acl_verify_message and can
    therefore be included in a custom message if this is desired.

    Normally the error text is the rest of the redirection list - a comma does
    not terminate it - but a newline does act as a terminator. Newlines are not
    normally present in alias expansions. In lsearch lookups they are removed
    as part of the continuation process, but they may exist in other kinds of
    lookup and in :include: files.

    During routing for message delivery (as opposed to verification), a
    redirection containing :fail: causes an immediate failure of the incoming
    address, whereas :defer: causes the message to remain in the queue so that
    a subsequent delivery attempt can happen at a later time. If an address is
    deferred for too long, it will ultimately fail, because the normal retry
    rules still apply.

  * Sometimes it is useful to use a single-key search type with a default (see
    chapter 9) to look up aliases. However, there may be a need for exceptions
    to the default. These can be handled by aliasing them to :unknown:. This
    differs from :fail: in that it causes the redirect router to decline,
    whereas :fail: forces routing to fail. A lookup which results in an empty
    redirection list has the same effect.


22.7 Duplicate addresses
------------------------

Exim removes duplicate addresses from the list to which it is delivering, so as
to deliver just one copy to each address. This does not apply to deliveries
routed to pipes by different immediate parent addresses, but an indirect
aliasing scheme of the type

pipe:       |/some/command $local_part
localpart1: pipe
localpart2: pipe

does not work with a message that is addressed to both local parts, because
when the second is aliased to the intermediate local part "pipe" it gets
discarded as being the same as a previously handled address. However, a scheme
such as

localpart1: |/some/command $local_part
localpart2: |/some/command $local_part

does result in two different pipe deliveries, because the immediate parents of
the pipes are distinct.


22.8 Repeated redirection expansion
-----------------------------------

When a message cannot be delivered to all of its recipients immediately,
leading to two or more delivery attempts, redirection expansion is carried out
afresh each time for those addresses whose children were not all previously
delivered. If redirection is being used as a mailing list, this can lead to new
members of the list receiving copies of old messages. The one_time option can
be used to avoid this.


22.9 Errors in redirection lists
--------------------------------

If skip_syntax_errors is set, a malformed address that causes a parsing error
is skipped, and an entry is written to the main log. This may be useful for
mailing lists that are automatically managed. Otherwise, if an error is
detected while generating the list of new addresses, the original address is
deferred. See also syntax_errors_to.


22.10 Private options for the redirect router
---------------------------------------------

The private options for the redirect router are as follows:

+------------------------------------------------------+
|allow_defer|Use: redirect|Type: boolean|Default: false|
+------------------------------------------------------+

Setting this option allows the use of :defer: in non-filter redirection data,
or the defer command in an Exim filter file.

+-----------------------------------------------------+
|allow_fail|Use: redirect|Type: boolean|Default: false|
+-----------------------------------------------------+

If this option is true, the :fail: item can be used in a redirection list, and
the fail command may be used in an Exim filter file.

+-------------------------------------------------------+
|allow_filter|Use: redirect|Type: boolean|Default: false|
+-------------------------------------------------------+

Setting this option allows Exim to interpret redirection data that starts with
"#Exim filter" or "#Sieve filter" as a set of filtering instructions. There are
some features of Exim filter files that some administrators may wish to lock
out; see the forbid_filter_xxx options below.

It is also possible to lock out Exim filters or Sieve filters while allowing
the other type; see forbid_exim_filter and forbid_sieve_filter.

The filter is run using the uid and gid set by the generic user and group
options. These take their defaults from the password data if check_local_user
is set, so in the normal case of users' personal filter files, the filter is
run as the relevant user. When allow_filter is set true, Exim insists that
either check_local_user or user is set.

+-------------------------------------------------------+
|allow_freeze|Use: redirect|Type: boolean|Default: false|
+-------------------------------------------------------+

Setting this option allows the use of the freeze command in an Exim filter.
This command is more normally encountered in system filters, and is disabled by
default for redirection filters because it isn't something you usually want to
let ordinary users do.

+---------------------------------------------------------+
|check_ancestor|Use: redirect|Type: boolean|Default: false|
+---------------------------------------------------------+

This option is concerned with handling generated addresses that are the same as
some address in the list of redirection ancestors of the current address.
Although it is turned off by default in the code, it is set in the default
configuration file for handling users' .forward files. It is recommended for
this use of the redirect router.

When check_ancestor is set, if a generated address (including the domain) is
the same as any ancestor of the current address, it is replaced by a copy of
the current address. This helps in the case where local part A is aliased to B,
and B has a .forward file pointing back to A. For example, within a single
domain, the local part "Joe.Bloggs" is aliased to "jb" and  jb/.forward
contains:

\Joe.Bloggs, <other item(s)>

Without the check_ancestor setting, either local part ("jb" or "joe.bloggs")
gets processed once by each router and so ends up as it was originally. If "jb"
is the real mailbox name, mail to "jb" gets delivered (having been turned into
"joe.bloggs" by the .forward file and back to "jb" by the alias), but mail to
"joe.bloggs" fails. Setting check_ancestor on the redirect router that handles
the .forward file prevents it from turning "jb" back into "joe.bloggs" when
that was the original address. See also the repeat_use option below.

+----------------------------------------------------------+
|check_group|Use: redirect|Type: boolean|Default: see below|
+----------------------------------------------------------+

When the file option is used, the group owner of the file is checked only when
this option is set. The permitted groups are those listed in the owngroups
option, together with the user's default group if check_local_user is set. If
the file has the wrong group, routing is deferred. The default setting for this
option is true if check_local_user is set and the modemask option permits the
group write bit, or if the owngroups option is set. Otherwise it is false, and
no group check occurs.

+----------------------------------------------------------+
|check_owner|Use: redirect|Type: boolean|Default: see below|
+----------------------------------------------------------+

When the file option is used, the owner of the file is checked only when this
option is set. If check_local_user is set, the local user is permitted;
otherwise the owner must be one of those listed in the owners option. The
default value for this option is true if check_local_user or owners is set.
Otherwise the default is false, and no owner check occurs.

+-----------------------------------------------+
|data|Use: redirect|Type: string*|Default: unset|
+-----------------------------------------------+

This option is mutually exclusive with file. One or other of them must be set,
but not both. The contents of data are expanded, and then used as the list of
forwarding items, or as a set of filtering instructions. If the expansion is
forced to fail, or the result is an empty string or a string that has no effect
(consists entirely of comments), the router declines.

When filtering instructions are used, the string must begin with "#Exim
filter", and all comments in the string, including this initial one, must be
terminated with newline characters. For example:

data = #Exim filter\n\
       if $h_to: contains Exim then save $home/mail/exim endif

If you are reading the data from a database where newlines cannot be included,
you can use the ${sg} expansion item to turn the escape string of your choice
into a newline.

+--------------------------------------------------------------+
|directory_transport|Use: redirect|Type: string*|Default: unset|
+--------------------------------------------------------------+

A redirect router sets up a direct delivery to a directory when a path name
ending with a slash is specified as a new "address". The transport used is
specified by this option, which, after expansion, must be the name of a
configured transport. This should normally be an appendfile transport.

+-----------------------------------------------+
|file|Use: redirect|Type: string*|Default: unset|
+-----------------------------------------------+

This option specifies the name of a file that contains the redirection data. It
is mutually exclusive with the data option. The string is expanded before use;
if the expansion is forced to fail, the router declines. Other expansion
failures cause delivery to be deferred. The result of a successful expansion
must be an absolute path. The entire file is read and used as the redirection
data. If the data is an empty string or a string that has no effect (consists
entirely of comments), the router declines.

If the attempt to open the file fails with a "does not exist" error, Exim runs
a check on the containing directory, unless ignore_enotdir is true (see below).
If the directory does not appear to exist, delivery is deferred. This can
happen when users' .forward files are in NFS-mounted directories, and there is
a mount problem. If the containing directory does exist, but the file does not,
the router declines.

+---------------------------------------------------------+
|file_transport|Use: redirect|Type: string*|Default: unset|
+---------------------------------------------------------+

A redirect router sets up a direct delivery to a file when a path name not
ending in a slash is specified as a new "address". The transport used is
specified by this option, which, after expansion, must be the name of a
configured transport. This should normally be an appendfile transport. When it
is running, the filename is in $address_file.

+-------------------------------------------------------------+
|filter_prepend_home|Use: redirect|Type: boolean|Default: true|
+-------------------------------------------------------------+

When this option is true, if a save command in an Exim filter specifies a
relative path, and $home is defined, it is automatically prepended to the
relative path. If this option is set false, this action does not happen. The
relative path is then passed to the transport unmodified.

+-----------------------------------------------------------+
|forbid_blackhole|Use: redirect|Type: boolean|Default: false|
+-----------------------------------------------------------+

If this option is true, the :blackhole: item may not appear in a redirection
list.

+-------------------------------------------------------------+
|forbid_exim_filter|Use: redirect|Type: boolean|Default: false|
+-------------------------------------------------------------+

If this option is set true, only Sieve filters are permitted when allow_filter
is true.

+------------------------------------------------------+
|forbid_file|Use: redirect|Type: boolean|Default: false|
+------------------------------------------------------+

If this option is true, this router may not generate a new address that
specifies delivery to a local file or directory, either from a filter or from a
conventional forward file. This option is forced to be true if one_time is set.
It applies to Sieve filters as well as to Exim filters, but if true, it locks
out the Sieve's "keep" facility.

+---------------------------------------------------------------+
|forbid_filter_dlfunc|Use: redirect|Type: boolean|Default: false|
+---------------------------------------------------------------+

If this option is true, string expansions in Exim filters are not allowed to
make use of the dlfunc expansion facility to run dynamically loaded functions.

+-------------------------------------------------------------------+
|forbid_filter_existstest|Use: redirect|Type: boolean|Default: false|
+-------------------------------------------------------------------+

If this option is true, string expansions in Exim filters are not allowed to
make use of the exists condition or the stat expansion item.

+-----------------------------------------------------------------+
|forbid_filter_logwrite|Use: redirect|Type: boolean|Default: false|
+-----------------------------------------------------------------+

If this option is true, use of the logging facility in Exim filters is not
permitted. Logging is in any case available only if the filter is being run
under some unprivileged uid (which is normally the case for ordinary users'
.forward files).

+---------------------------------------------------------------+
|forbid_filter_lookup|Use: redirect|Type: boolean|Default: false|
+---------------------------------------------------------------+

If this option is true, string expansions in Exim filter files are not allowed
to make use of lookup items.

+-------------------------------------------------------------+
|forbid_filter_perl|Use: redirect|Type: boolean|Default: false|
+-------------------------------------------------------------+

This option has an effect only if Exim is built with embedded Perl support. If
it is true, string expansions in Exim filter files are not allowed to make use
of the embedded Perl support.

+-----------------------------------------------------------------+
|forbid_filter_readfile|Use: redirect|Type: boolean|Default: false|
+-----------------------------------------------------------------+

If this option is true, string expansions in Exim filter files are not allowed
to make use of readfile items.

+-------------------------------------------------------------------+
|forbid_filter_readsocket|Use: redirect|Type: boolean|Default: false|
+-------------------------------------------------------------------+

If this option is true, string expansions in Exim filter files are not allowed
to make use of readsocket items.

+--------------------------------------------------------------+
|forbid_filter_reply|Use: redirect|Type: boolean|Default: false|
+--------------------------------------------------------------+

If this option is true, this router may not generate an automatic reply
message. Automatic replies can be generated only from Exim or Sieve filter
files, not from traditional forward files. This option is forced to be true if 
one_time is set.

+------------------------------------------------------------+
|forbid_filter_run|Use: redirect|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is true, string expansions in Exim filter files are not allowed
to make use of run items.

+---------------------------------------------------------+
|forbid_include|Use: redirect|Type: boolean|Default: false|
+---------------------------------------------------------+

If this option is true, items of the form

:include:<path name>

are not permitted in non-filter redirection lists.

+------------------------------------------------------+
|forbid_pipe|Use: redirect|Type: boolean|Default: false|
+------------------------------------------------------+

If this option is true, this router may not generate a new address which
specifies delivery to a pipe, either from an Exim filter or from a conventional
forward file. This option is forced to be true if one_time is set.

+--------------------------------------------------------------+
|forbid_sieve_filter|Use: redirect|Type: boolean|Default: false|
+--------------------------------------------------------------+

If this option is set true, only Exim filters are permitted when allow_filter
is true.

+-----------------------------------------------------------+
|forbid_smtp_code|Use: redirect|Type: boolean|Default: false|
+-----------------------------------------------------------+

If this option is set true, any SMTP error codes that are present at the start
of messages specified for ":defer:" or ":fail:" are quietly ignored, and the
default codes (451 and 550, respectively) are always used.

+---------------------------------------------------------------+
|hide_child_in_errmsg|Use: redirect|Type: boolean|Default: false|
+---------------------------------------------------------------+

If this option is true, it prevents Exim from quoting a child address if it
generates a bounce or delay message for it. Instead it says "an address
generated from <the top level address>". Of course, this applies only to
bounces generated locally. If a message is forwarded to another host, its
bounce may well quote the generated address.

+--------------------------------------------------------+
|ignore_eacces|Use: redirect|Type: boolean|Default: false|
+--------------------------------------------------------+

If this option is set and an attempt to open a redirection file yields the
EACCES error (permission denied), the redirect router behaves as if the file
did not exist.

+---------------------------------------------------------+
|ignore_enotdir|Use: redirect|Type: boolean|Default: false|
+---------------------------------------------------------+

If this option is set and an attempt to open a redirection file yields the
ENOTDIR error (something on the path is not a directory), the redirect router
behaves as if the file did not exist.

Setting ignore_enotdir has another effect as well: When a redirect router that
has the file option set discovers that the file does not exist (the ENOENT
error), it tries to stat() the parent directory, as a check against unmounted
NFS directories. If the parent can not be statted, delivery is deferred.
However, it seems wrong to do this check when ignore_enotdir is set, because
that option tells Exim to ignore "something on the path is not a directory"
(the ENOTDIR error). This is a confusing area, because it seems that some
operating systems give ENOENT where others give ENOTDIR.

+-----------------------------------------------------------+
|include_directory|Use: redirect|Type: string|Default: unset|
+-----------------------------------------------------------+

If this option is set, the path names of any :include: items in a redirection
list must start with this directory.

+-------------------------------------------------------+
|modemask|Use: redirect|Type: octal integer|Default: 022|
+-------------------------------------------------------+

This specifies mode bits which must not be set for a file specified by the file
option. If any of the forbidden bits are set, delivery is deferred.

+---------------------------------------------------+
|one_time|Use: redirect|Type: boolean|Default: false|
+---------------------------------------------------+

Sometimes the fact that Exim re-evaluates aliases and reprocesses redirection
files each time it tries to deliver a message causes a problem when one or more
of the generated addresses fails be delivered at the first attempt. The problem
is not one of duplicate delivery - Exim is clever enough to handle that - but
of what happens when the redirection list changes during the time that the
message is on Exim's queue. This is particularly true in the case of mailing
lists, where new subscribers might receive copies of messages that were posted
before they subscribed.

If one_time is set and any addresses generated by the router fail to deliver at
the first attempt, the failing addresses are added to the message as "top
level" addresses, and the parent address that generated them is marked
"delivered". Thus, redirection does not happen again at the next delivery
attempt.

Warning 1: Any header line addition or removal that is specified by this router
would be lost if delivery did not succeed at the first attempt. For this
reason, the headers_add and headers_remove generic options are not permitted
when one_time is set.

Warning 2: To ensure that the router generates only addresses (as opposed to
pipe or file deliveries or auto-replies) forbid_file, forbid_pipe, and 
forbid_filter_reply are forced to be true when one_time is set.

Warning 3: The unseen generic router option may not be set with one_time.

The original top-level address is remembered with each of the generated
addresses, and is output in any log messages. However, any intermediate parent
addresses are not recorded. This makes a difference to the log only if 
all_parents log selector is set. It is expected that one_time will typically be
used for mailing lists, where there is normally just one level of expansion.

+-----------------------------------------------------+
|owners|Use: redirect|Type: string list|Default: unset|
+-----------------------------------------------------+

This specifies a list of permitted owners for the file specified by file. This
list is in addition to the local user when check_local_user is set. See 
check_owner above.

+--------------------------------------------------------+
|owngroups|Use: redirect|Type: string list|Default: unset|
+--------------------------------------------------------+

This specifies a list of permitted groups for the file specified by file. The
list is in addition to the local user's primary group when check_local_user is
set. See check_group above.

+---------------------------------------------------------+
|pipe_transport|Use: redirect|Type: string*|Default: unset|
+---------------------------------------------------------+

A redirect router sets up a direct delivery to a pipe when a string starting
with a vertical bar character is specified as a new "address". The transport
used is specified by this option, which, after expansion, must be the name of a
configured transport. This should normally be a pipe transport. When the
transport is run, the pipe command is in $address_pipe.

+---------------------------------------------------------+
|qualify_domain|Use: redirect|Type: string*|Default: unset|
+---------------------------------------------------------+

If this option is set, and an unqualified address (one without a domain) is
generated, and that address would normally be qualified by the global setting
in qualify_recipient, it is instead qualified with the domain specified by
expanding this string. If the expansion fails, the router declines. If you want
to revert to the default, you can have the expansion generate
$qualify_recipient.

This option applies to all unqualified addresses generated by Exim filters, but
for traditional .forward files, it applies only to addresses that are not
preceded by a backslash. Sieve filters cannot generate unqualified addresses.

+------------------------------------------------------------------+
|qualify_preserve_domain|Use: redirect|Type: boolean|Default: false|
+------------------------------------------------------------------+

If this option is set, the router's local qualify_domain option must not be set
(a configuration error occurs if it is). If an unqualified address (one without
a domain) is generated, it is qualified with the domain of the parent address
(the immediately preceding ancestor) instead of the global qualify_recipient
value. In the case of a traditional .forward file, this applies whether or not
the address is preceded by a backslash.

+----------------------------------------------------+
|repeat_use|Use: redirect|Type: boolean|Default: true|
+----------------------------------------------------+

If this option is set false, the router is skipped for a child address that has
any ancestor that was routed by this router. This test happens before any of
the other preconditions are tested. Exim's default anti-looping rules skip only
when the ancestor is the same as the current address. See also check_ancestor
above and the generic redirect_router option.

+----------------------------------------------------------+
|reply_transport|Use: redirect|Type: string*|Default: unset|
+----------------------------------------------------------+

A redirect router sets up an automatic reply when a mail or vacation command is
used in a filter file. The transport used is specified by this option, which,
after expansion, must be the name of a configured transport. This should
normally be an autoreply transport. Other transports are unlikely to do
anything sensible or useful.

+-------------------------------------------------+
|rewrite|Use: redirect|Type: boolean|Default: true|
+-------------------------------------------------+

If this option is set false, addresses generated by the router are not subject
to address rewriting. Otherwise, they are treated like new addresses and are
rewritten according to the global rewriting rules.

+-----------------------------------------------------------+
|sieve_subaddress|Use: redirect|Type: string*|Default: unset|
+-----------------------------------------------------------+

The value of this option is passed to a Sieve filter to specify the :subaddress
part of an address.

+------------------------------------------------------------+
|sieve_useraddress|Use: redirect|Type: string*|Default: unset|
+------------------------------------------------------------+

The value of this option is passed to a Sieve filter to specify the :user part
of an address. However, if it is unset, the entire original local part
(including any prefix or suffix) is used for :user.

+-------------------------------------------------------------------+
|sieve_vacation_directory|Use: redirect|Type: string*|Default: unset|
+-------------------------------------------------------------------+

To enable the "vacation" extension for Sieve filters, you must set 
sieve_vacation_directory to the directory where vacation databases are held (do
not put anything else in that directory), and ensure that the reply_transport
option refers to an autoreply transport. Each user needs their own directory;
Exim will create it if necessary.

+-------------------------------------------------------------+
|skip_syntax_errors|Use: redirect|Type: boolean|Default: false|
+-------------------------------------------------------------+

If skip_syntax_errors is set, syntactically malformed addresses in non-filter
redirection data are skipped, and each failing address is logged. If 
syntax_errors_to is set, a message is sent to the address it defines, giving
details of the failures. If syntax_errors_text is set, its contents are
expanded and placed at the head of the error message generated by 
syntax_errors_to. Usually it is appropriate to set syntax_errors_to to be the
same address as the generic errors_to option. The skip_syntax_errors option is
often used when handling mailing lists.

If all the addresses in a redirection list are skipped because of syntax
errors, the router declines to handle the original address, and it is passed to
the following routers.

If skip_syntax_errors is set when an Exim filter is interpreted, any syntax
error in the filter causes filtering to be abandoned without any action being
taken. The incident is logged, and the router declines to handle the address,
so it is passed to the following routers.

Syntax errors in a Sieve filter file cause the "keep" action to occur. This
action is specified by RFC 3028. The values of skip_syntax_errors, 
syntax_errors_to, and syntax_errors_text are not used.

skip_syntax_errors can be used to specify that errors in users' forward lists
or filter files should not prevent delivery. The syntax_errors_to option, used
with an address that does not get redirected, can be used to notify users of
these errors, by means of a router like this:

userforward:
  driver = redirect
  allow_filter
  check_local_user
  file = $home/.forward
  file_transport = address_file
  pipe_transport = address_pipe
  reply_transport = address_reply
  no_verify
  skip_syntax_errors
  syntax_errors_to = real-$local_part@$domain
  syntax_errors_text = \
   This is an automatically generated message. An error has\n\
   been found in your .forward file. Details of the error are\n\
   reported below. While this error persists, you will receive\n\
   a copy of this message for every message that is addressed\n\
   to you. If your .forward file is a filter file, or if it is\n\
   a non-filter file containing no valid forwarding addresses,\n\
   a copy of each incoming message will be put in your normal\n\
   mailbox. If a non-filter file contains at least one valid\n\
   forwarding address, forwarding to the valid addresses will\n\
   happen, and those will be the only deliveries that occur.

You also need a router to ensure that local addresses that are prefixed by
"real-" are recognized, but not forwarded or filtered. For example, you could
put this immediately before the userforward router:

real_localuser:
  driver = accept
  check_local_user
  local_part_prefix = real-
  transport = local_delivery

For security, it would probably be a good idea to restrict the use of this
router to locally-generated messages, using a condition such as this:

  condition = ${if match {$sender_host_address}\
                         {\N^(|127\.0\.0\.1)$\N}}

+-------------------------------------------------------------+
|syntax_errors_text|Use: redirect|Type: string*|Default: unset|
+-------------------------------------------------------------+

See skip_syntax_errors above.

+----------------------------------------------------------+
|syntax_errors_to|Use: redirect|Type: string|Default: unset|
+----------------------------------------------------------+

See skip_syntax_errors above.



===============================================================================
23. ENVIRONMENT FOR RUNNING LOCAL TRANSPORTS

Local transports handle deliveries to files and pipes. (The autoreply transport
can be thought of as similar to a pipe.) Exim always runs transports in
subprocesses, under specified uids and gids. Typical deliveries to local
mailboxes run under the uid and gid of the local user.

Exim also sets a specific current directory while running the transport; for
some transports a home directory setting is also relevant. The pipe transport
is the only one that sets up environment variables; see section 29.4 for
details.

The values used for the uid, gid, and the directories may come from several
different places. In many cases, the router that handles the address associates
settings with that address as a result of its check_local_user, group, or user
options. However, values may also be given in the transport's own
configuration, and these override anything that comes from the router.


23.1 Concurrent deliveries
--------------------------

If two different messages for the same local recipient arrive more or less
simultaneously, the two delivery processes are likely to run concurrently. When
the appendfile transport is used to write to a file, Exim applies locking rules
to stop concurrent processes from writing to the same file at the same time.

However, when you use a pipe transport, it is up to you to arrange any locking
that is needed. Here is a silly example:

my_transport:
  driver = pipe
  command = /bin/sh -c 'cat >>/some/file'

This is supposed to write the message at the end of the file. However, if two
messages arrive at the same time, the file will be scrambled. You can use the 
exim_lock utility program (see section 54.15) to lock a file using the same
algorithm that Exim itself uses.


23.2 Uids and gids
------------------

All transports have the options group and user. If group is set, it overrides
any group that the router set in the address, even if user is not set for the
transport. This makes it possible, for example, to run local mail delivery
under the uid of the recipient (set by the router), but in a special group (set
by the transport). For example:

# Routers ...
# User/group are set by check_local_user in this router
local_users:
  driver = accept
  check_local_user
  transport = group_delivery

# Transports ...
# This transport overrides the group
group_delivery:
  driver = appendfile
  file = /var/spool/mail/$local_part_data
  group = mail

If user is set for a transport, its value overrides what is set in the address
by the router. If user is non-numeric and group is not set, the gid associated
with the user is used. If user is numeric, group must be set.

When the uid is taken from the transport's configuration, the initgroups()
function is called for the groups associated with that uid if the initgroups
option is set for the transport. When the uid is not specified by the
transport, but is associated with the address by a router, the option for
calling initgroups() is taken from the router configuration.

The pipe transport contains the special option pipe_as_creator. If this is set
and user is not set, the uid of the process that called Exim to receive the
message is used, and if group is not set, the corresponding original gid is
also used.

This is the detailed preference order for obtaining a gid; the first of the
following that is set is used:

  * A group setting of the transport;

  * A group setting of the router;

  * A gid associated with a user setting of the router, either as a result of 
    check_local_user or an explicit non-numeric user setting;

  * The group associated with a non-numeric user setting of the transport;

  * In a pipe transport, the creator's gid if deliver_as_creator is set and the
    uid is the creator's uid;

  * The Exim gid if the Exim uid is being used as a default.

If, for example, the user is specified numerically on the router and there are
no group settings, no gid is available. In this situation, an error occurs.
This is different for the uid, for which there always is an ultimate default.
The first of the following that is set is used:

  * A user setting of the transport;

  * In a pipe transport, the creator's uid if deliver_as_creator is set;

  * A user setting of the router;

  * A check_local_user setting of the router;

  * The Exim uid.

Of course, an error will still occur if the uid that is chosen is on the 
never_users list.


23.3 Current and home directories
---------------------------------

Routers may set current and home directories for local transports by means of
the transport_current_directory and transport_home_directory options. However,
if the transport's current_directory or home_directory options are set, they
override the router's values. In detail, the home directory for a local
transport is taken from the first of these values that is set:

  * The home_directory option on the transport;

  * The transport_home_directory option on the router;

  * The password data if check_local_user is set on the router;

  * The router_home_directory option on the router.

The current directory is taken from the first of these values that is set:

  * The current_directory option on the transport;

  * The transport_current_directory option on the router.

If neither the router nor the transport sets a current directory, Exim uses the
value of the home directory, if it is set. Otherwise it sets the current
directory to / before running a local transport.


23.4 Expansion variables derived from the address
-------------------------------------------------

Normally a local delivery is handling a single address, and in that case the
variables such as $domain and $local_part are set during local deliveries.
However, in some circumstances more than one address may be handled at once
(for example, while writing batch SMTP for onward transmission by some other
means). In this case, the variables associated with the local part are never
set, $domain is set only if all the addresses have the same domain, and
$original_domain is never set.



===============================================================================
24. GENERIC OPTIONS FOR TRANSPORTS

The name of a transport is limited to be 64 ASCII characters long; prior to
Exim 4.95 names would be silently truncated at this length, but now it is
enforced.

The following generic options apply to all transports:

+------------------------------------------------------+
|body_only|Use: transports|Type: boolean|Default: false|
+------------------------------------------------------+

If this option is set, the message's headers are not transported. It is
mutually exclusive with headers_only. If it is used with the appendfile or pipe
transports, the settings of message_prefix and message_suffix should be
checked, because this option does not automatically suppress them.

+--------------------------------------------------------------+
|current_directory|Use: transports|Type: string*|Default: unset|
+--------------------------------------------------------------+

This specifies the current directory that is to be set while running the
transport, overriding any value that may have been set by the router. If the
expansion fails for any reason, including forced failure, an error is logged,
and delivery is deferred.

+------------------------------------------------------------+
|disable_logging|Use: transports|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is set true, nothing is logged for any deliveries by the
transport or for any transport errors. You should not set this option unless
you really, really know what you are doing.

+--------------------------------------------------------+
|debug_print|Use: transports|Type: string*|Default: unset|
+--------------------------------------------------------+

If this option is set and debugging is enabled (see the -d command line
option), the string is expanded and included in the debugging output when the
transport is run. If expansion of the string fails, the error message is
written to the debugging output, and Exim carries on processing. This facility
is provided to help with checking out the values of variables and so on when
debugging driver configurations. For example, if a headers_add option is not
working properly, debug_print could be used to output the variables it
references. A newline is added to the text if it does not end with one. The
variables $transport_name and $router_name contain the name of the transport
and the router that called it.

+--------------------------------------------------------------+
|delivery_date_add|Use: transports|Type: boolean|Default: false|
+--------------------------------------------------------------+

If this option is true, a Delivery-date: header is added to the message. This
gives the actual time the delivery was made. As this is not a standard header,
Exim has a configuration option (delivery_date_remove) which requests its
removal from incoming messages, so that delivered messages can safely be resent
to other recipients.

+--------------------------------------------------+
|driver|Use: transports|Type: string|Default: unset|
+--------------------------------------------------+

This specifies which of the available transport drivers is to be used. There is
no default, and this option must be set for every transport.

+------------------------------------------------------------+
|envelope_to_add|Use: transports|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is true, an Envelope-to: header is added to the message. This
gives the original address(es) in the incoming envelope that caused this
delivery to happen. More than one address may be present if the transport is
configured to handle several addresses at once, or if more than one original
address was redirected to the same final address. As this is not a standard
header, Exim has a configuration option (envelope_to_remove) which requests its
removal from incoming messages, so that delivered messages can safely be resent
to other recipients.

Note: If used on a transport handling multiple recipients (the smtp transport
unless max_rcpt is 1, the appendfile, pipe or lmtp transport if batch_max is
greater than 1) then information about Bcc recipients will be leaked. Doing so
is generally not advised.

+---------------------------------------------------------+
|event_action|Use: transports|Type: string*|Default: unset|
+---------------------------------------------------------+

This option declares a string to be expanded for Exim's events mechanism. For
details see chapter 61.

+-------------------------------------------------------+
|group|Use: transports|Type: string*|Default: Exim group|
+-------------------------------------------------------+

This option specifies a gid for running the transport process, overriding any
value that the router supplies, and also overriding any value associated with 
user (see below).

+------------------------------------------------------+
|headers_add|Use: transports|Type: list*|Default: unset|
+------------------------------------------------------+

This option specifies a list of text headers, newline-separated (by default,
changeable in the usual way 6.21), which are (separately) expanded and added to
the header portion of a message as it is transported, as described in section
48.6. Additional header lines can also be specified by routers. If the result
of the expansion is an empty string, or if the expansion is forced to fail, no
action is taken. Other expansion failures are treated as errors and cause the
delivery to be deferred.

Unlike most options, headers_add can be specified multiple times for a
transport; all listed headers are added.

+---------------------------------------------------------+
|headers_only|Use: transports|Type: boolean|Default: false|
+---------------------------------------------------------+

If this option is set, the message's body is not transported. It is mutually
exclusive with body_only. If it is used with the appendfile or pipe transports,
the settings of message_prefix and message_suffix should be checked, since this
option does not automatically suppress them.

+---------------------------------------------------------+
|headers_remove|Use: transports|Type: list*|Default: unset|
+---------------------------------------------------------+

This option specifies a list of text headers, colon-separated (by default,
changeable in the usual way 6.21), to be removed from the message. However, the
option has no effect when an address is just being verified. Each list item is
separately expanded. If the result of the expansion is an empty string, or if
the expansion is forced to fail, no action is taken. Other expansion failures
are treated as errors and cause the delivery to be deferred. If an item ends in
*, it will match any header with the given prefix.

Matching headers are omitted from the message as it is transported, as
described in section 48.6. Header removal can also be specified by routers.

Unlike most options, headers_remove can be specified multiple times for a
transport; all listed headers are removed.

Warning: Because of the separate expansion of the list items, items that
contain a list separator must have it doubled. To avoid this, change the list
separator (6.21).

+-----------------------------------------------------------+
|headers_rewrite|Use: transports|Type: string|Default: unset|
+-----------------------------------------------------------+

This option allows addresses in header lines to be rewritten at transport time,
that is, as the message is being copied to its destination. The contents of the
option are a colon-separated list of rewriting rules. Each rule is in exactly
the same form as one of the general rewriting rules that are applied when a
message is received. These are described in chapter 31. For example,

headers_rewrite = a@b c@d f : \
                  x@y w@z

changes a@b into c@d in From: header lines, and x@y into w@z in all
address-bearing header lines. The rules are applied to the header lines just
before they are written out at transport time, so they affect only those copies
of the message that pass through the transport. However, only the message's
original header lines, and any that were added by a system filter, are
rewritten. If a router or transport adds header lines, they are not affected by
this option. These rewriting rules are not applied to the envelope. You can
change the return path using return_path, but you cannot change envelope
recipients at this time.

+-----------------------------------------------------------+
|home_directory|Use: transports|Type: string*|Default: unset|
+-----------------------------------------------------------+

This option specifies a home directory setting for a local transport,
overriding any value that may be set by the router. The home directory is
placed in $home while expanding the transport's private options. It is also
used as the current directory if no current directory is set by the 
current_directory option on the transport or the transport_current_directory
option on the router. If the expansion fails for any reason, including forced
failure, an error is logged, and delivery is deferred.

+-------------------------------------------------------+
|initgroups|Use: transports|Type: boolean|Default: false|
+-------------------------------------------------------+

If this option is true and the uid for the delivery process is provided by the
transport, the initgroups() function is called when running the transport to
ensure that any additional groups associated with the uid are set up.

+----------------------------------------------------------+
|max_parallel|Use: transports|Type: integer*|Default: unset|
+----------------------------------------------------------+

If this option is set and expands to an integer greater than zero it limits the
number of concurrent runs of the transport. The control does not apply to
shadow transports.

Exim implements this control by means of a hints database in which a record is
incremented whenever a transport process is being created. The record is
decremented and possibly removed when the process terminates. Obviously there
is scope for records to get left lying around if there is a system or program
crash. To guard against this, Exim ignores any records that are more than six
hours old.

If you use this option, you should also arrange to delete the relevant hints
database whenever your system reboots. The names of the files start with misc
and they are kept in the spool/db directory. There may be one or two files,
depending on the type of DBM in use. The same files are used for ETRN and smtp
transport serialization.

+-----------------------------------------------------------+
|message_size_limit|Use: transports|Type: string*|Default: 0|
+-----------------------------------------------------------+

This option controls the size of messages passed through the transport. It is
expanded before use; the result of the expansion must be a sequence of decimal
digits, optionally followed by K or M. If the expansion fails for any reason,
including forced failure, or if the result is not of the required form,
delivery is deferred. If the value is greater than zero and the size of a
message exceeds this limit, the address is failed. If there is any chance that
the resulting bounce message could be routed to the same transport, you should
ensure that return_size_limit is less than the transport's message_size_limit,
as otherwise the bounce message will fail to get delivered.

+-----------------------------------------------------------------+
|rcpt_include_affixes|Use: transports|Type: boolean|Default: false|
+-----------------------------------------------------------------+

When this option is false (the default), and an address that has had any
affixes (prefixes or suffixes) removed from the local part is delivered by any
form of SMTP or LMTP, the affixes are not included. For example, if a router
that contains

local_part_prefix = *-

routes the address abc-xyz@some.domain to an SMTP transport, the envelope is
delivered with

RCPT TO:<xyz@some.domain>

This is also the case when an ACL-time callout is being used to verify a
recipient address. However, if rcpt_include_affixes is set true, the whole
local part is included in the RCPT command. This option applies to BSMTP
deliveries by the appendfile and pipe transports as well as to the lmtp and 
smtp transports.

+---------------------------------------------------------------------+
|retry_use_local_part|Use: transports|Type: boolean|Default: see below|
+---------------------------------------------------------------------+

When a delivery suffers a temporary failure, a retry record is created in
Exim's hints database. For remote deliveries, the key for the retry record is
based on the name and/or IP address of the failing remote host. For local
deliveries, the key is normally the entire address, including both the local
part and the domain. This is suitable for most common cases of local delivery
temporary failure - for example, exceeding a mailbox quota should delay only
deliveries to that mailbox, not to the whole domain.

However, in some special cases you may want to treat a temporary local delivery
as a failure associated with the domain, and not with a particular local part.
(For example, if you are storing all mail for some domain in files.) You can do
this by setting retry_use_local_part false.

For all the local transports, its default value is true. For remote transports,
the default value is false for tidiness, but changing the value has no effect
on a remote transport in the current implementation.

+--------------------------------------------------------+
|return_path|Use: transports|Type: string*|Default: unset|
+--------------------------------------------------------+

If this option is set, the string is expanded at transport time and replaces
the existing return path (envelope sender) value in the copy of the message
that is being delivered. An empty return path is permitted. This feature is
designed for remote deliveries, where the value of this option is used in the
SMTP MAIL command. If you set return_path for a local transport, the only
effect is to change the address that is placed in the Return-path: header line,
if one is added to the message (see the next option).

Note: A changed return path is not logged unless you add 
return_path_on_delivery to the log selector.

The expansion can refer to the existing value via $return_path. This is either
the message's envelope sender, or an address set by the errors_to option on a
router. If the expansion is forced to fail, no replacement occurs; if it fails
for another reason, delivery is deferred. This option can be used to support
VERP (Variable Envelope Return Paths) - see section 51.6.

Note: If a delivery error is detected locally, including the case when a remote
server rejects a message at SMTP time, the bounce message is not sent to the
value of this option. It is sent to the previously set errors address. This
defaults to the incoming sender address, but can be changed by setting 
errors_to in a router.

+------------------------------------------------------------+
|return_path_add|Use: transports|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is true, a Return-path: header is added to the message. Although
the return path is normally available in the prefix line of BSD mailboxes, this
is commonly not displayed by MUAs, and so the user does not have easy access to
it.

RFC 2821 states that the Return-path: header is added to a message "when the
delivery SMTP server makes the final delivery". This implies that this header
should not be present in incoming messages. Exim has a configuration option, 
return_path_remove, which requests removal of this header from incoming
messages, so that delivered messages can safely be resent to other recipients.

+-------------------------------------------------------------+
|shadow_condition|Use: transports|Type: string*|Default: unset|
+-------------------------------------------------------------+

See shadow_transport below.

+------------------------------------------------------------+
|shadow_transport|Use: transports|Type: string|Default: unset|
+------------------------------------------------------------+

A local transport may set the shadow_transport option to the name of another
local transport. Shadow remote transports are not supported.

Whenever a delivery to the main transport succeeds, and either shadow_condition
is unset, or its expansion does not result in the empty string or one of the
strings "0" or "no" or "false", the message is also passed to the shadow
transport, with the same delivery address or addresses. If expansion fails, no
action is taken except that non-forced expansion failures cause a log line to
be written.

The result of the shadow transport is discarded and does not affect the
subsequent processing of the message. Only a single level of shadowing is
provided; the shadow_transport option is ignored on any transport when it is
running as a shadow. Options concerned with output from pipes are also ignored.
The log line for the successful delivery has an item added on the end, of the
form

ST=<shadow transport name>

If the shadow transport did not succeed, the error message is put in
parentheses afterwards. Shadow transports can be used for a number of different
purposes, including keeping more detailed log information than Exim normally
provides, and implementing automatic acknowledgment policies based on message
headers that some sites insist on.

+-------------------------------------------------------------+
|transport_filter|Use: transports|Type: string*|Default: unset|
+-------------------------------------------------------------+

This option sets up a filtering (in the Unix shell sense) process for messages
at transport time. It should not be confused with mail filtering as set up by
individual users or via a system filter. If unset, or expanding to an empty
string, no filtering is done.

When the message is about to be written out, the command specified by 
transport_filter is started up in a separate, parallel process, and the entire
message, including the header lines, is passed to it on its standard input
(this in fact is done from a third process, to avoid deadlock). The command
must be specified as an absolute path.

The lines of the message that are written to the transport filter are
terminated by newline ("\n"). The message is passed to the filter before any
SMTP-specific processing, such as turning "\n" into "\r\n" and escaping lines
beginning with a dot, and also before any processing implied by the settings of
check_string and escape_string in the appendfile or pipe transports.

The standard error for the filter process is set to the same destination as its
standard output; this is read and written to the message's ultimate
destination. The process that writes the message to the filter, the filter
itself, and the original process that reads the result and delivers it are all
run in parallel, like a shell pipeline.

The filter can perform any transformations it likes, but of course should take
care not to break RFC 2822 syntax. Exim does not check the result, except to
test for a final newline when SMTP is in use. All messages transmitted over
SMTP must end with a newline, so Exim supplies one if it is missing.

A transport filter can be used to provide content-scanning on a per-user basis
at delivery time if the only required effect of the scan is to modify the
message. For example, a content scan could insert a new header line containing
a spam score. This could be interpreted by a filter in the user's MUA. It is
not possible to discard a message at this stage.

A problem might arise if the filter increases the size of a message that is
being sent down an SMTP connection. If the receiving SMTP server has indicated
support for the SIZE parameter, Exim will have sent the size of the message at
the start of the SMTP session. If what is actually sent is substantially more,
the server might reject the message. This can be worked round by setting the 
size_addition option on the smtp transport, either to allow for additions to
the message, or to disable the use of SIZE altogether.

The value of the transport_filter option is the command string for starting the
filter, which is run directly from Exim, not under a shell. The string is
parsed by Exim in the same way as a command string for the pipe transport: Exim
breaks it up into arguments and then expands each argument separately (see
section 29.3). Any kind of expansion failure causes delivery to be deferred.
The special argument $pipe_addresses is replaced by a number of arguments, one
for each address that applies to this delivery. (This isn't an ideal name for
this feature here, but as it was already implemented for the pipe transport, it
seemed sensible not to change it.)

The expansion variables $host and $host_address are available when the
transport is a remote one. They contain the name and IP address of the host to
which the message is being sent. For example:

transport_filter = /some/directory/transport-filter.pl \
  $host $host_address $pipe_addresses

Two problems arise if you want to use more complicated expansion items to
generate transport filter commands, both of which due to the fact that the
command is split up before expansion.

  * If an expansion item contains white space, you must quote it, so that it is
    all part of the same command item. If the entire option setting is one such
    expansion item, you have to take care what kind of quoting you use. For
    example:

    transport_filter = '/bin/cmd${if eq{$host}{a.b.c}{1}{2}}'

    This runs the command /bin/cmd1 if the host name is a.b.c, and /bin/cmd2
    otherwise.

    Option strings in general have any fully-surrounding double quote wrapping
    removed early in parsing (see 6.17). Then, for this option, quotes protect
    against whitespace being regarded as a separator while splitting into the
    command argument vector. Either double or single quotes can be used here;
    the former interprets backlash-quoted charachters and the latter does not.

    If double quotes had been used in this example, they would have been
    stripped by Exim when it read the option's value. When the value is used,
    if the single quotes were missing, the line would be split into two items,
    "/bin/cmd${if" and "eq{$host}{a.b.c}{1}{2}", and an error would occur when
    Exim tried to expand the first one.

  * Except for the special case of $pipe_addresses that is mentioned above, an
    expansion cannot generate multiple arguments, or a command name followed by
    arguments. Consider this example:

    transport_filter = ${lookup{$host}lsearch{/a/file}\
                        {$value}{/bin/cat}}

    The result of the lookup is interpreted as the name of the command, even if
    it contains white space. The simplest way round this is to use a shell:

    transport_filter = /bin/sh -c ${lookup{$host}lsearch{/a/file}\
                                   {$value}{/bin/cat}}

The filter process is run under the same uid and gid as the normal delivery.
For remote deliveries this is the Exim uid/gid by default. The command should
normally yield a zero return code. Transport filters are not supposed to fail.
A non-zero code is taken to mean that the transport filter encountered some
serious problem. Delivery of the message is deferred; the message remains on
the queue and is tried again later. It is not possible to cause a message to be
bounced from a transport filter.

If a transport filter is set on an autoreply transport, the original message is
passed through the filter as it is being copied into the newly generated
message, which happens if the return_message option is set.

+---------------------------------------------------------------+
|transport_filter_timeout|Use: transports|Type: time|Default: 5m|
+---------------------------------------------------------------+

When Exim is reading the output of a transport filter, it applies a timeout
that can be set by this option. Exceeding the timeout is normally treated as a
temporary delivery failure. However, if a transport filter is used with a pipe
transport, a timeout in the transport filter is treated in the same way as a
timeout in the pipe command itself. By default, a timeout is a hard error, but
if the pipe transport's timeout_defer option is set true, it becomes a
temporary error.

+-----------------------------------------------------+
|user|Use: transports|Type: string*|Default: Exim user|
+-----------------------------------------------------+

This option specifies the user under whose uid the delivery process is to be
run, overriding any uid that may have been set by the router. If the user is
given as a name, the uid is looked up from the password data, and the
associated group is taken as the value of the gid to be used if the group
option is not set.

For deliveries that use local transports, a user and group are normally
specified explicitly or implicitly (for example, as a result of 
check_local_user) by the router or transport.

For remote transports, you should leave this option unset unless you really are
sure you know what you are doing. When a remote transport is running, it needs
to be able to access Exim's hints databases, because each host may have its own
retry data.



===============================================================================
25. ADDRESS BATCHING IN LOCAL TRANSPORTS

The only remote transport (smtp) is normally configured to handle more than one
address at a time, so that when several addresses are routed to the same remote
host, just one copy of the message is sent. Local transports, however, normally
handle one address at a time. That is, a separate instance of the transport is
run for each address that is routed to the transport. A separate copy of the
message is delivered each time.

In special cases, it may be desirable to handle several addresses at once in a
local transport, for example:

  * In an appendfile transport, when storing messages in files for later
    delivery by some other means, a single copy of the message with multiple
    recipients saves space.

  * In an lmtp transport, when delivering over "local SMTP" to some process, a
    single copy saves time, and is the normal way LMTP is expected to work.

  * In a pipe transport, when passing the message to a scanner program or to
    some other delivery mechanism such as UUCP, multiple recipients may be
    acceptable.

These three local transports all have the same options for controlling multiple
("batched") deliveries, namely batch_max and batch_id. To save repeating the
information for each transport, these options are described here.

The batch_max option specifies the maximum number of addresses that can be
delivered together in a single run of the transport. Its default value is one
(no batching). When more than one address is routed to a transport that has a 
batch_max value greater than one, the addresses are delivered in a batch (that
is, in a single run of the transport with multiple recipients), subject to
certain conditions:

  * If any of the transport's options contain a reference to $local_part, no
    batching is possible.

  * If any of the transport's options contain a reference to $domain, only
    addresses with the same domain are batched.

  * If batch_id is set, it is expanded for each address, and only those
    addresses with the same expanded value are batched. This allows you to
    specify customized batching conditions. Failure of the expansion for any
    reason, including forced failure, disables batching, but it does not stop
    the delivery from taking place.

  * Batched addresses must also have the same errors address (where to send
    delivery errors), the same header additions and removals, the same user and
    group for the transport, and if a host list is present, the first host must
    be the same.

In the case of the appendfile and pipe transports, batching applies both when
the file or pipe command is specified in the transport, and when it is
specified by a redirect router, but all the batched addresses must of course be
routed to the same file or pipe command. These two transports have an option
called use_bsmtp, which causes them to deliver the message in "batched SMTP"
format, with the envelope represented as SMTP commands. The check_string and 
escape_string options are forced to the values

check_string = "."
escape_string = ".."

when batched SMTP is in use. A full description of the batch SMTP mechanism is
given in section 49.4. The lmtp transport does not have a use_bsmtp option,
because it always delivers using the SMTP protocol.

If the generic envelope_to_add option is set for a batching transport, the 
Envelope-to: header that is added to the message contains all the addresses
that are being processed together. If you are using a batching appendfile
transport without use_bsmtp, the only way to preserve the recipient addresses
is to set the envelope_to_add option.

If you are using a pipe transport without BSMTP, and setting the transport's 
command option, you can include $pipe_addresses as part of the command. This is
not a true variable; it is a bit of magic that causes each of the recipient
addresses to be inserted into the command as a separate argument. This provides
a way of accessing all the addresses that are being delivered in the batch. 
Note: This is not possible for pipe commands that are specified by a redirect
router.



===============================================================================
26. THE APPENDFILE TRANSPORT

The appendfile transport delivers a message by appending it to an existing
file, or by creating an entirely new file in a specified directory. Single
files to which messages are appended can be in the traditional Unix mailbox
format, or optionally in the MBX format supported by the Pine MUA and
University of Washington IMAP daemon, inter alia. When each message is being
delivered as a separate file, "maildir" format can optionally be used to give
added protection against failures that happen part-way through the delivery. A
third form of separate-file delivery known as "mailstore" is also supported.
For all file formats, Exim attempts to create as many levels of directory as
necessary, provided that create_directory is set.

The code for the optional formats is not included in the Exim binary by
default. It is necessary to set SUPPORT_MBX, SUPPORT_MAILDIR and/or
SUPPORT_MAILSTORE in Local/Makefile to have the appropriate code included.

Exim recognizes system quota errors, and generates an appropriate message. Exim
also supports its own quota control within the transport, for use when the
system facility is unavailable or cannot be used for some reason.

If there is an error while appending to a file (for example, quota exceeded or
partition filled), Exim attempts to reset the file's length and last
modification time back to what they were before. If there is an error while
creating an entirely new file, the new file is removed.

Before appending to a file, a number of security checks are made, and the file
is locked. A detailed description is given below, after the list of private
options.

The appendfile transport is most commonly used for local deliveries to users'
mailboxes. However, it can also be used as a pseudo-remote transport for
putting messages into files for remote delivery by some means other than Exim.
"Batch SMTP" format is often used in this case (see the use_bsmtp option).


26.1 The file and directory options
-----------------------------------

The file option specifies a single file, to which the message is appended; the 
directory option specifies a directory, in which a new file containing the
message is created. Only one of these two options can be set, and for normal
deliveries to mailboxes, one of them must be set.

However, appendfile is also used for delivering messages to files or
directories whose names (or parts of names) are obtained from alias,
forwarding, or filtering operations (for example, a save command in a user's
Exim filter). When such a transport is running, $local_part contains the local
part that was aliased or forwarded, and $address_file contains the name (or
partial name) of the file or directory generated by the redirection operation.
There are two cases:

  * If neither file nor directory is set, the redirection operation must
    specify an absolute path (one that begins with "/"). This is the most
    common case when users with local accounts use filtering to sort mail into
    different folders. See for example, the address_file transport in the
    default configuration. If the path ends with a slash, it is assumed to be
    the name of a directory. A delivery to a directory can also be forced by
    setting maildir_format or mailstore_format.

  * If file or directory is set for a delivery from a redirection, it is used
    to determine the file or directory name for the delivery. Normally, the
    contents of $address_file are used in some way in the string expansion.

If the create_file option is set to a path which matches (see the option
definition below for details) a file or directory name for the delivery, that
name becomes de-tainted.

Tainted data may not be used for a file or directory name. This means that, for
instance, $local_part cannot be used directly as a component of a path. It can
however be used as the key for a lookup which returns a path (or component).

As an example of the second case, consider an environment where users do not
have home directories. They may be permitted to use Exim filter commands of the
form:

save folder23

or Sieve filter commands of the form:

require "fileinto";
fileinto "folder23";

In this situation, the expansion of file or directory in the transport must
transform the relative path into an appropriate absolute filename. In the case
of Sieve filters, the name inbox must be handled. It is the name that is used
as a result of a "keep" action in the filter. This example shows one way of
handling this requirement:

file = ${if eq{$address_file}{inbox} \
            {/var/mail/$local_part_data} \
            {${if eq{${substr_0_1:$address_file}}{/} \
                  {$address_file} \
                  {$home/mail/$address_file} \
            }} \
       }

With this setting of file, inbox refers to the standard mailbox location,
absolute paths are used without change, and other folders are in the mail
directory within the home directory.

Note 1: While processing an Exim filter, a relative path such as folder23 is
turned into an absolute path if a home directory is known to the router. In
particular, this is the case if check_local_user is set. If you want to prevent
this happening at routing time, you can set router_home_directory empty. This
forces the router to pass the relative path to the transport.

Note 2: An absolute path in $address_file is not treated specially; the file or
directory option is still used if it is set.


26.2 Private options for appendfile
-----------------------------------

+-------------------------------------------------------+
|allow_fifo|Use: appendfile|Type: boolean|Default: false|
+-------------------------------------------------------+

Setting this option permits delivery to named pipes (FIFOs) as well as to
regular files. If no process is reading the named pipe at delivery time, the
delivery is deferred.

+----------------------------------------------------------+
|allow_symlink|Use: appendfile|Type: boolean|Default: false|
+----------------------------------------------------------+

By default, appendfile will not deliver if the path name for the file is that
of a symbolic link. Setting this option relaxes that constraint, but there are
security issues involved in the use of symbolic links. Be sure you know what
you are doing if you set this. Details of exactly what this option affects are
included in the discussion which follows this list of options.

+-----------------------------------------------------+
|batch_id|Use: appendfile|Type: string*|Default: unset|
+-----------------------------------------------------+

See the description of local delivery batching in chapter 25. However, batching
is automatically disabled for appendfile deliveries that happen as a result of
forwarding or aliasing or other redirection directly to a file.

+--------------------------------------------------+
|batch_max|Use: appendfile|Type: integer|Default: 1|
+--------------------------------------------------+

See the description of local delivery batching in chapter 25.

+--------------------------------------------------------+
|check_group|Use: appendfile|Type: boolean|Default: false|
+--------------------------------------------------------+

When this option is set, the group owner of the file defined by the file option
is checked to see that it is the same as the group under which the delivery
process is running. The default setting is false because the default file mode
is 0600, which means that the group is irrelevant.

+-------------------------------------------------------+
|check_owner|Use: appendfile|Type: boolean|Default: true|
+-------------------------------------------------------+

When this option is set, the owner of the file defined by the file option is
checked to ensure that it is the same as the user under which the delivery
process is running.

+------------------------------------------------------------+
|check_string|Use: appendfile|Type: string|Default: see below|
+------------------------------------------------------------+

As appendfile writes the message, the start of each line is tested for matching
check_string, and if it does, the initial matching characters are replaced by
the contents of escape_string. The value of check_string is a literal string,
not a regular expression, and the case of any letters it contains is
significant.

If use_bsmtp is set the values of check_string and escape_string are forced to
"." and ".." respectively, and any settings in the configuration are ignored.
Otherwise, they default to "From " and ">From " when the file option is set,
and unset when any of the directory, maildir, or mailstore options are set.

The default settings, along with message_prefix and message_suffix, are
suitable for traditional "BSD" mailboxes, where a line beginning with "From "
indicates the start of a new message. All four options need changing if another
format is used. For example, to deliver to mailboxes in MMDF format:

check_string = "\1\1\1\1\n"
escape_string = "\1\1\1\1 \n"
message_prefix = "\1\1\1\1\n"
message_suffix = "\1\1\1\1\n"

+------------------------------------------------------------+
|create_directory|Use: appendfile|Type: boolean|Default: true|
+------------------------------------------------------------+

When this option is true, Exim attempts to create any missing superior
directories for the file that it is about to write. A created directory's mode
is given by the directory_mode option.

The group ownership of a newly created directory is highly dependent on the
operating system (and possibly the file system) that is being used. For
example, in Solaris, if the parent directory has the setgid bit set, its group
is propagated to the child; if not, the currently set group is used. However,
in FreeBSD, the parent's group is always used.

+----------------------------------------------------------+
|create_file|Use: appendfile|Type: string|Default: anywhere|
+----------------------------------------------------------+

This option constrains the location of files and directories that are created
by this transport. It applies to files defined by the file option and
directories defined by the directory option. In the case of maildir delivery,
it applies to the top level directory, not the maildir directories beneath.

The option must be set to one of the words "anywhere", "inhome", or
"belowhome", or to an absolute path.

In the second and third cases, a home directory must have been set for the
transport, and the file or directory being created must reside within it. The
"belowhome" checking additionally checks for attempts to use "../" to evade the
testing. This option is not useful when an explicit filename is given for
normal mailbox deliveries. It is intended for the case when filenames are
generated from users' .forward files. These are usually handled by an 
appendfile transport called address_file. See also file_must_exist.

In the fourth case, the value given for this option must be an absolute path
for an existing directory. The value is used for checking instead of a home
directory; checking is done in "belowhome" mode.

If "belowhome" checking is used, the file or directory path becomes de-tainted.

+------------------------------------------------------+
|directory|Use: appendfile|Type: string*|Default: unset|
+------------------------------------------------------+

This option is mutually exclusive with the file option, but one of file or 
directory must be set, unless the delivery is the direct result of a
redirection (see section 26.1).

When directory is set, the string is expanded, and the message is delivered
into a new file or files in or below the given directory, instead of being
appended to a single mailbox file. A number of different formats are provided
(see maildir_format and mailstore_format), and see section 26.4 for further
details of this form of delivery.

The result of expansion must not be tainted, unless the create_file option
specifies a path.

+---------------------------------------------------------------+
|directory_file|Use: appendfile|Type: string*|Default: see below|
+---------------------------------------------------------------+

When directory is set, but neither maildir_format nor mailstore_format is set, 
appendfile delivers each message into a file whose name is obtained by
expanding this string. The default value is:

q${base62:$tod_epoch}-$inode

This generates a unique name from the current time, in base 62 form, and the
inode of the file. The variable $inode is available only when expanding this
option.

+----------------------------------------------------------------+
|directory_mode|Use: appendfile|Type: octal integer|Default: 0700|
+----------------------------------------------------------------+

If appendfile creates any directories as a result of the create_directory
option, their mode is specified by this option.

+-------------------------------------------------------------------+
|escape_string|Use: appendfile|Type: string|Default: see description|
+-------------------------------------------------------------------+

See check_string above.

+-------------------------------------------------+
|file|Use: appendfile|Type: string*|Default: unset|
+-------------------------------------------------+

This option is mutually exclusive with the directory option, but one of file or
directory must be set, unless the delivery is the direct result of a
redirection (see section 26.1). The file option specifies a single file, to
which the message is appended. One or more of use_fcntl_lock, use_flock_lock,
or use_lockfile must be set with file.

The result of expansion must not be tainted, unless the create_file option
specifies a path.

If you are using more than one host to deliver over NFS into the same
mailboxes, you should always use lock files.

The string value is expanded for each delivery, and must yield an absolute
path. The most common settings of this option are variations on one of these
examples:

file = /var/spool/mail/$local_part_data
file = /home/$local_part_data/inbox
file = $home/inbox

In the first example, all deliveries are done into the same directory. If Exim
is configured to use lock files (see use_lockfile below) it must be able to
create a file in the directory, so the "sticky" bit must be turned on for
deliveries to be possible, or alternatively the group option can be used to run
the delivery under a group id which has write access to the directory.

+-------------------------------------------------------+
|file_format|Use: appendfile|Type: string|Default: unset|
+-------------------------------------------------------+

This option requests the transport to check the format of an existing file
before adding to it. The check consists of matching a specific string at the
start of the file. The value of the option consists of an even number of
colon-separated strings. The first of each pair is the test string, and the
second is the name of a transport. If the transport associated with a matched
string is not the current transport, control is passed over to the other
transport. For example, suppose the standard local_delivery transport has this
added to it:

file_format = "From       : local_delivery :\
               \1\1\1\1\n : local_mmdf_delivery"

Mailboxes that begin with "From" are still handled by this transport, but if a
mailbox begins with four binary ones followed by a newline, control is passed
to a transport called local_mmdf_delivery, which presumably is configured to do
the delivery in MMDF format. If a mailbox does not exist or is empty, it is
assumed to match the current transport. If the start of a mailbox doesn't match
any string, or if the transport named for a given string is not defined,
delivery is deferred.

+------------------------------------------------------------+
|file_must_exist|Use: appendfile|Type: boolean|Default: false|
+------------------------------------------------------------+

If this option is true, the file specified by the file option must exist. A
temporary error occurs if it does not, causing delivery to be deferred. If this
option is false, the file is created if it does not exist.

+---------------------------------------------------------+
|lock_fcntl_timeout|Use: appendfile|Type: time|Default: 0s|
+---------------------------------------------------------+

By default, the appendfile transport uses non-blocking calls to fcntl() when
locking an open mailbox file. If the call fails, the delivery process sleeps
for lock_interval and tries again, up to lock_retries times. Non-blocking calls
are used so that the file is not kept open during the wait for the lock; the
reason for this is to make it as safe as possible for deliveries over NFS in
the case when processes might be accessing an NFS mailbox without using a lock
file. This should not be done, but misunderstandings and hence
misconfigurations are not unknown.

On a busy system, however, the performance of a non-blocking lock approach is
not as good as using a blocking lock with a timeout. In this case, the waiting
is done inside the system call, and Exim's delivery process acquires the lock
and can proceed as soon as the previous lock holder releases it.

If lock_fcntl_timeout is set to a non-zero time, blocking locks, with that
timeout, are used. There may still be some retrying: the maximum number of
retries is

(lock_retries * lock_interval) / lock_fcntl_timeout

rounded up to the next whole number. In other words, the total time during
which appendfile is trying to get a lock is roughly the same, unless 
lock_fcntl_timeout is set very large.

You should consider setting this option if you are getting a lot of delayed
local deliveries because of errors of the form

failed to lock mailbox /some/file (fcntl)

+---------------------------------------------------------+
|lock_flock_timeout|Use: appendfile|Type: time|Default: 0s|
+---------------------------------------------------------+

This timeout applies to file locking when using flock() (see use_flock); the
timeout operates in a similar manner to lock_fcntl_timeout.

+----------------------------------------------------+
|lock_interval|Use: appendfile|Type: time|Default: 3s|
+----------------------------------------------------+

This specifies the time to wait between attempts to lock the file. See below
for details of locking.

+------------------------------------------------------+
|lock_retries|Use: appendfile|Type: integer|Default: 10|
+------------------------------------------------------+

This specifies the maximum number of attempts to lock the file. A value of zero
is treated as 1. See below for details of locking.

+---------------------------------------------------------------+
|lockfile_mode|Use: appendfile|Type: octal integer|Default: 0600|
+---------------------------------------------------------------+

This specifies the mode of the created lock file, when a lock file is being
used (see use_lockfile and use_mbx_lock).

+--------------------------------------------------------+
|lockfile_timeout|Use: appendfile|Type: time|Default: 30m|
+--------------------------------------------------------+

When a lock file is being used (see use_lockfile), if a lock file already
exists and is older than this value, it is assumed to have been left behind by
accident, and Exim attempts to remove it.

+--------------------------------------------------------------+
|mailbox_filecount|Use: appendfile|Type: string*|Default: unset|
+--------------------------------------------------------------+

If this option is set, it is expanded, and the result is taken as the current
number of files in the mailbox. It must be a decimal number, optionally
followed by K or M. This provides a way of obtaining this information from an
external source that maintains the data.

+---------------------------------------------------------+
|mailbox_size|Use: appendfile|Type: string*|Default: unset|
+---------------------------------------------------------+

If this option is set, it is expanded, and the result is taken as the current
size the mailbox. It must be a decimal number, optionally followed by K or M.
This provides a way of obtaining this information from an external source that
maintains the data. This is likely to be helpful for maildir deliveries where
it is computationally expensive to compute the size of a mailbox.

+-----------------------------------------------------------+
|maildir_format|Use: appendfile|Type: boolean|Default: false|
+-----------------------------------------------------------+

If this option is set with the directory option, the delivery is into a new
file, in the "maildir" format that is used by other mail software. When the
transport is activated directly from a redirect router (for example, the 
address_file transport in the default configuration), setting maildir_format
causes the path received from the router to be treated as a directory, whether
or not it ends with "/". This option is available only if SUPPORT_MAILDIR is
present in Local/Makefile. See section 26.5 below for further details.

+-----------------------------------------------------------------------------+
|maildir_quota_directory_regex|Use: appendfile|Type: string|Default: See below|
+-----------------------------------------------------------------------------+

This option is relevant only when maildir_use_size_file is set. It defines a
regular expression for specifying directories, relative to the quota directory
(see quota_directory), that should be included in the quota calculation. The
default value is:

maildir_quota_directory_regex = ^(?:cur|new|\..*)$

This includes the cur and new directories, and any maildir++ folders
(directories whose names begin with a dot). If you want to exclude the Trash
folder from the count (as some sites do), you need to change this setting to

maildir_quota_directory_regex = ^(?:cur|new|\.(?!Trash).*)$

This uses a negative lookahead in the regular expression to exclude the
directory whose name is .Trash. When a directory is excluded from quota
calculations, quota processing is bypassed for any messages that are delivered
directly into that directory.

+---------------------------------------------------------+
|maildir_retries|Use: appendfile|Type: integer|Default: 10|
+---------------------------------------------------------+

This option specifies the number of times to retry when writing a file in
"maildir" format. See section 26.5 below.

+--------------------------------------------------------+
|maildir_tag|Use: appendfile|Type: string*|Default: unset|
+--------------------------------------------------------+

This option applies only to deliveries in maildir format, and is described in
section 26.5 below.

+-------------------------------------------------------------------+
|maildir_use_size_file|Use: appendfile*|Type: boolean|Default: false|
+-------------------------------------------------------------------+

The result of string expansion for this option must be a valid boolean value.
If it is true, it enables support for maildirsize files. Exim creates a
maildirsize file in a maildir if one does not exist, taking the quota from the 
quota option of the transport. If quota is unset, the value is zero. See 
maildir_quota_directory_regex above and section 26.5 below for further details.

+----------------------------------------------------------------------+
|maildirfolder_create_regex|Use: appendfile|Type: string|Default: unset|
+----------------------------------------------------------------------+

The value of this option is a regular expression. If it is unset, it has no
effect. Otherwise, before a maildir delivery takes place, the pattern is
matched against the name of the maildir directory, that is, the directory
containing the new and tmp subdirectories that will be used for the delivery.
If there is a match, Exim checks for the existence of a file called
maildirfolder in the directory, and creates it if it does not exist. See
section 26.5 for more details.

+-------------------------------------------------------------+
|mailstore_format|Use: appendfile|Type: boolean|Default: false|
+-------------------------------------------------------------+

If this option is set with the directory option, the delivery is into two new
files in "mailstore" format. The option is available only if SUPPORT_MAILSTORE
is present in Local/Makefile. See section 26.4 below for further details.

+-------------------------------------------------------------+
|mailstore_prefix|Use: appendfile|Type: string*|Default: unset|
+-------------------------------------------------------------+

This option applies only to deliveries in mailstore format, and is described in
section 26.4 below.

+-------------------------------------------------------------+
|mailstore_suffix|Use: appendfile|Type: string*|Default: unset|
+-------------------------------------------------------------+

This option applies only to deliveries in mailstore format, and is described in
section 26.4 below.

+-------------------------------------------------------+
|mbx_format|Use: appendfile|Type: boolean|Default: false|
+-------------------------------------------------------+

This option is available only if Exim has been compiled with SUPPORT_MBX set in
Local/Makefile. If mbx_format is set with the file option, the message is
appended to the mailbox file in MBX format instead of traditional Unix format.
This format is supported by Pine4 and its associated IMAP and POP daemons, by
means of the c-client library that they all use.

Note: The message_prefix and message_suffix options are not automatically
changed by the use of mbx_format. They should normally be set empty when using
MBX format, so this option almost always appears in this combination:

mbx_format = true
message_prefix =
message_suffix =

If none of the locking options are mentioned in the configuration, use_mbx_lock
is assumed and the other locking options default to false. It is possible to
specify the other kinds of locking with mbx_format, but use_fcntl_lock and 
use_mbx_lock are mutually exclusive. MBX locking interworks with c-client,
providing for shared access to the mailbox. It should not be used if any
program that does not use this form of locking is going to access the mailbox,
nor should it be used if the mailbox file is NFS mounted, because it works only
when the mailbox is accessed from a single host.

If you set use_fcntl_lock with an MBX-format mailbox, you cannot use the
standard version of c-client, because as long as it has a mailbox open (this
means for the whole of a Pine or IMAP session), Exim will not be able to append
messages to it.

+---------------------------------------------------------------+
|message_prefix|Use: appendfile|Type: string*|Default: see below|
+---------------------------------------------------------------+

The string specified here is expanded and output at the start of every message.
The default is unset unless file is specified and use_bsmtp is not set, in
which case it is:

message_prefix = "From ${if def:return_path{$return_path}\
  {MAILER-DAEMON}} $tod_bsdinbox\n"

Note: If you set use_crlf true, you must change any occurrences of "\n" to "\r\
n" in message_prefix.

+---------------------------------------------------------------+
|message_suffix|Use: appendfile|Type: string*|Default: see below|
+---------------------------------------------------------------+

The string specified here is expanded and output at the end of every message.
The default is unset unless file is specified and use_bsmtp is not set, in
which case it is a single newline character. The suffix can be suppressed by
setting

message_suffix =

Note: If you set use_crlf true, you must change any occurrences of "\n" to "\r\
n" in message_suffix.

+------------------------------------------------------+
|mode|Use: appendfile|Type: octal integer|Default: 0600|
+------------------------------------------------------+

If the output file is created, it is given this mode. If it already exists and
has wider permissions, they are reduced to this mode. If it has narrower
permissions, an error occurs unless mode_fail_narrower is false. However, if
the delivery is the result of a save command in a filter file specifying a
particular mode, the mode of the output file is always forced to take that
value, and this option is ignored.

+--------------------------------------------------------------+
|mode_fail_narrower|Use: appendfile|Type: boolean|Default: true|
+--------------------------------------------------------------+

This option applies in the case when an existing mailbox file has a narrower
mode than that specified by the mode option. If mode_fail_narrower is true, the
delivery is deferred ("mailbox has the wrong mode"); otherwise Exim continues
with the delivery attempt, using the existing mode of the file.

+----------------------------------------------------------+
|notify_comsat|Use: appendfile|Type: boolean|Default: false|
+----------------------------------------------------------+

If this option is true, the comsat daemon is notified after every successful
delivery to a user mailbox. This is the daemon that notifies logged on users
about incoming mail.

+--------------------------------------------------+
|quota|Use: appendfile|Type: string*|Default: unset|
+--------------------------------------------------+

This option imposes a limit on the size of the file to which Exim is appending,
or to the total space used in the directory tree when the directory option is
set. In the latter case, computation of the space used is expensive, because
all the files in the directory (and any sub-directories) have to be
individually inspected and their sizes summed. (See quota_size_regex and 
maildir_use_size_file for ways to avoid this in environments where users have
no shell access to their mailboxes).

As there is no interlock against two simultaneous deliveries into a multi-file
mailbox, it is possible for the quota to be overrun in this case. For
single-file mailboxes, of course, an interlock is a necessity.

A file's size is taken as its used value. Because of blocking effects, this may
be a lot less than the actual amount of disk space allocated to the file. If
the sizes of a number of files are being added up, the rounding effect can
become quite noticeable, especially on systems that have large block sizes.
Nevertheless, it seems best to stick to the used figure, because this is the
obvious value which users understand most easily.

The value of the option is expanded, and must then be a numerical value
(decimal point allowed), optionally followed by one of the letters K, M, or G,
for kilobytes, megabytes, or gigabytes, optionally followed by a slash and
further option modifiers. If Exim is running on a system with large file
support (Linux and FreeBSD have this), mailboxes larger than 2G can be handled.

The option modifier no_check can be used to force delivery even if the over
quota condition is met. The quota gets updated as usual.

Note: A value of zero is interpreted as "no quota".

The expansion happens while Exim is running as root, before it changes uid for
the delivery. This means that files that are inaccessible to the end user can
be used to hold quota values that are looked up in the expansion. When delivery
fails because this quota is exceeded, the handling of the error is as for
system quota failures.

By default, Exim's quota checking mimics system quotas, and restricts the
mailbox to the specified maximum size, though the value is not accurate to the
last byte, owing to separator lines and additional headers that may get added
during message delivery. When a mailbox is nearly full, large messages may get
refused even though small ones are accepted, because the size of the current
message is added to the quota when the check is made. This behaviour can be
changed by setting quota_is_inclusive false. When this is done, the check for
exceeding the quota does not include the current message. Thus, deliveries
continue until the quota has been exceeded; thereafter, no further messages are
delivered. See also quota_warn_threshold.

+------------------------------------------------------------+
|quota_directory|Use: appendfile|Type: string*|Default: unset|
+------------------------------------------------------------+

This option defines the directory to check for quota purposes when delivering
into individual files. The default is the delivery directory, or, if a file
called maildirfolder exists in a maildir directory, the parent of the delivery
directory.

+--------------------------------------------------------+
|quota_filecount|Use: appendfile|Type: string*|Default: 0|
+--------------------------------------------------------+

This option applies when the directory option is set. It limits the total
number of files in the directory (compare the inode limit in system quotas). It
can only be used if quota is also set. The value is expanded; an expansion
failure causes delivery to be deferred. A value of zero is interpreted as "no
quota".

The option modifier no_check can be used to force delivery even if the over
quota condition is met. The quota gets updated as usual.

+--------------------------------------------------------------+
|quota_is_inclusive|Use: appendfile|Type: boolean|Default: true|
+--------------------------------------------------------------+

See quota above.

+------------------------------------------------------------+
|quota_size_regex|Use: appendfile|Type: string|Default: unset|
+------------------------------------------------------------+

This option applies when one of the delivery modes that writes a separate file
for each message is being used. When Exim wants to find the size of one of
these files in order to test the quota, it first checks quota_size_regex. If
this is set to a regular expression that matches the filename, and it captures
one string, that string is interpreted as a representation of the file's size.
The value of quota_size_regex is not expanded.

This feature is useful only when users have no shell access to their mailboxes
- otherwise they could defeat the quota simply by renaming the files. This
facility can be used with maildir deliveries, by setting maildir_tag to add the
file length to the filename. For example:

maildir_tag = ,S=$message_size
quota_size_regex = ,S=(\d+)

An alternative to $message_size is $message_linecount, which contains the
number of lines in the message.

The regular expression should not assume that the length is at the end of the
filename (even though maildir_tag puts it there) because maildir MUAs sometimes
add other information onto the ends of message filenames.

Section 26.7 contains further information.

This option should not be used when other message-handling software may
duplicate messages by making hardlinks to the files. When that is done Exim
will count the message size once for each filename, in contrast with the actual
disk usage. When the option is not set, calculating total usage requires a
system-call per file to get the size; the number of links is then available
also as is used to adjust the effective size.

+-------------------------------------------------------------------+
|quota_warn_message|Use: appendfile|Type: string*|Default: see below|
+-------------------------------------------------------------------+

See below for the use of this option. If it is not set when 
quota_warn_threshold is set, it defaults to

quota_warn_message = "\
  To: $local_part@$domain\n\
  Subject: Your mailbox\n\n\
  This message is automatically created \
  by mail delivery software.\n\n\
  The size of your mailbox has exceeded \
  a warning threshold that is\n\
  set by the system administrator.\n"

+-------------------------------------------------------------+
|quota_warn_threshold|Use: appendfile|Type: string*|Default: 0|
+-------------------------------------------------------------+

This option is expanded in the same way as quota (see above). If the resulting
value is greater than zero, and delivery of the message causes the size of the
file or total space in the directory tree to cross the given threshold, a
warning message is sent. If quota is also set, the threshold may be specified
as a percentage of it by following the value with a percent sign. For example:

quota = 10M
quota_warn_threshold = 75%

If quota is not set, a setting of quota_warn_threshold that ends with a percent
sign is ignored.

The warning message itself is specified by the quota_warn_message option, and
it must start with a To: header line containing the recipient(s) of the warning
message. These do not necessarily have to include the recipient(s) of the
original message. A Subject: line should also normally be supplied. You can
include any other header lines that you want. If you do not include a From:
line, the default is:

From: Mail Delivery System <mailer-daemon@$qualify_domain_sender>

If you supply a Reply-To: line, it overrides the global errors_reply_to option.

The quota option does not have to be set in order to use this option; they are
independent of one another except when the threshold is specified as a
percentage.

+------------------------------------------------------+
|use_bsmtp|Use: appendfile|Type: boolean|Default: false|
+------------------------------------------------------+

If this option is set true, appendfile writes messages in "batch SMTP" format,
with the envelope sender and recipient(s) included as SMTP commands. If you
want to include a leading HELO command with such messages, you can do so by
setting the message_prefix option. See section 49.4 for details of batch SMTP.

+-----------------------------------------------------+
|use_crlf|Use: appendfile|Type: boolean|Default: false|
+-----------------------------------------------------+

This option causes lines to be terminated with the two-character CRLF sequence
(carriage return, linefeed) instead of just a linefeed character. In the case
of batched SMTP, the byte sequence written to the file is then an exact image
of what would be sent down a real SMTP connection.

Note: The contents of the message_prefix and message_suffix options (which are
used to supply the traditional "From " and blank line separators in
Berkeley-style mailboxes) are written verbatim, so must contain their own
carriage return characters if these are needed. In cases where these options
have non-empty defaults, the values end with a single linefeed, so they must be
changed to end with "\r\n" if use_crlf is set.

+---------------------------------------------------------------+
|use_fcntl_lock|Use: appendfile|Type: boolean|Default: see below|
+---------------------------------------------------------------+

This option controls the use of the fcntl() function to lock a file for
exclusive use when a message is being appended. It is set by default unless 
use_flock_lock is set. Otherwise, it should be turned off only if you know that
all your MUAs use lock file locking. When both use_fcntl_lock and 
use_flock_lock are unset, use_lockfile must be set.

+-----------------------------------------------------------+
|use_flock_lock|Use: appendfile|Type: boolean|Default: false|
+-----------------------------------------------------------+

This option is provided to support the use of flock() for file locking, for the
few situations where it is needed. Most modern operating systems support fcntl
() and lockf() locking, and these two functions interwork with each other. Exim
uses fcntl() locking by default.

This option is required only if you are using an operating system where flock()
is used by programs that access mailboxes (typically MUAs), and where flock()
does not correctly interwork with fcntl(). You can use both fcntl() and flock()
locking simultaneously if you want.

Not all operating systems provide flock(). Some versions of Solaris do not have
it (and some, I think, provide a not quite right version built on top of lockf
()). If the OS does not have flock(), Exim will be built without the ability to
use it, and any attempt to do so will cause a configuration error.

Warning: flock() locks do not work on NFS files (unless flock() is just being
mapped onto fcntl() by the OS).

+-------------------------------------------------------------+
|use_lockfile|Use: appendfile|Type: boolean|Default: see below|
+-------------------------------------------------------------+

If this option is turned off, Exim does not attempt to create a lock file when
appending to a mailbox file. In this situation, the only locking is by fcntl().
You should only turn use_lockfile off if you are absolutely sure that every MUA
that is ever going to look at your users' mailboxes uses fcntl() rather than a
lock file, and even then only when you are not delivering over NFS from more
than one host.

In order to append to an NFS file safely from more than one host, it is
necessary to take out a lock before opening the file, and the lock file
achieves this. Otherwise, even with fcntl() locking, there is a risk of file
corruption.

The use_lockfile option is set by default unless use_mbx_lock is set. It is not
possible to turn both use_lockfile and use_fcntl_lock off, except when 
mbx_format is set.

+-------------------------------------------------------------+
|use_mbx_lock|Use: appendfile|Type: boolean|Default: see below|
+-------------------------------------------------------------+

This option is available only if Exim has been compiled with SUPPORT_MBX set in
Local/Makefile. Setting the option specifies that special MBX locking rules be
used. It is set by default if mbx_format is set and none of the locking options
are mentioned in the configuration. The locking rules are the same as are used
by the c-client library that underlies Pine and the IMAP4 and POP daemons that
come with it (see the discussion below). The rules allow for shared access to
the mailbox. However, this kind of locking does not work when the mailbox is
NFS mounted.

You can set use_mbx_lock with either (or both) of use_fcntl_lock and 
use_flock_lock to control what kind of locking is used in implementing the MBX
locking rules. The default is to use fcntl() if use_mbx_lock is set without 
use_fcntl_lock or use_flock_lock.


26.3 Operational details for appending
--------------------------------------

Before appending to a file, the following preparations are made:

  * If the name of the file is /dev/null, no action is taken, and a success
    return is given.

  * If any directories on the file's path are missing, Exim creates them if the
    create_directory option is set. A created directory's mode is given by the 
    directory_mode option.

  * If file_format is set, the format of an existing file is checked. If this
    indicates that a different transport should be used, control is passed to
    that transport.

  * If use_lockfile is set, a lock file is built in a way that will work
    reliably over NFS, as follows:

     1. Create a "hitching post" file whose name is that of the lock file with
        the current time, primary host name, and process id added, by opening
        for writing as a new file. If this fails with an access error, delivery
        is deferred.

     2. Close the hitching post file, and hard link it to the lock filename.

     3. If the call to link() succeeds, creation of the lock file has
        succeeded. Unlink the hitching post name.

     4. Otherwise, use stat() to get information about the hitching post file,
        and then unlink hitching post name. If the number of links is exactly
        two, creation of the lock file succeeded but something (for example, an
        NFS server crash and restart) caused this fact not to be communicated
        to the link() call.

     5. If creation of the lock file failed, wait for lock_interval and try
        again, up to lock_retries times. However, since any program that writes
        to a mailbox should complete its task very quickly, it is reasonable to
        time out old lock files that are normally the result of user agent and
        system crashes. If an existing lock file is older than lockfile_timeout
        Exim attempts to unlink it before trying again.

  * A call is made to lstat() to discover whether the main file exists, and if
    so, what its characteristics are. If lstat() fails for any reason other
    than non-existence, delivery is deferred.

  * If the file does exist and is a symbolic link, delivery is deferred, unless
    the allow_symlink option is set, in which case the ownership of the link is
    checked, and then stat() is called to find out about the real file, which
    is then subjected to the checks below. The check on the top-level link
    ownership prevents one user creating a link for another's mailbox in a
    sticky directory, though allowing symbolic links in this case is definitely
    not a good idea. If there is a chain of symbolic links, the intermediate
    ones are not checked.

  * If the file already exists but is not a regular file, or if the file's
    owner and group (if the group is being checked - see check_group above) are
    different from the user and group under which the delivery is running,
    delivery is deferred.

  * If the file's permissions are more generous than specified, they are
    reduced. If they are insufficient, delivery is deferred, unless 
    mode_fail_narrower is set false, in which case the delivery is tried using
    the existing permissions.

  * The file's inode number is saved, and the file is then opened for
    appending. If this fails because the file has vanished, appendfile behaves
    as if it hadn't existed (see below). For any other failures, delivery is
    deferred.

  * If the file is opened successfully, check that the inode number hasn't
    changed, that it is still a regular file, and that the owner and
    permissions have not changed. If anything is wrong, defer delivery and
    freeze the message.

  * If the file did not exist originally, defer delivery if the file_must_exist
    option is set. Otherwise, check that the file is being created in a
    permitted directory if the create_file option is set (deferring on
    failure), and then open for writing as a new file, with the O_EXCL and
    O_CREAT options, except when dealing with a symbolic link (the 
    allow_symlink option must be set). In this case, which can happen if the
    link points to a non-existent file, the file is opened for writing using
    O_CREAT but not O_EXCL, because that prevents link following.

  * If opening fails because the file exists, obey the tests given above for
    existing files. However, to avoid looping in a situation where the file is
    being continuously created and destroyed, the exists/not-exists loop is
    broken after 10 repetitions, and the message is then frozen.

  * If opening fails with any other error, defer delivery.

  * Once the file is open, unless both use_fcntl_lock and use_flock_lock are
    false, it is locked using fcntl() or flock() or both. If use_mbx_lock is
    false, an exclusive lock is requested in each case. However, if 
    use_mbx_lock is true, Exim takes out a shared lock on the open file, and an
    exclusive lock on the file whose name is

    /tmp/.<device-number>.<inode-number>

    using the device and inode numbers of the open mailbox file, in accordance
    with the MBX locking rules. This file is created with a mode that is
    specified by the lockfile_mode option.

    If Exim fails to lock the file, there are two possible courses of action,
    depending on the value of the locking timeout. This is obtained from 
    lock_fcntl_timeout or lock_flock_timeout, as appropriate.

    If the timeout value is zero, the file is closed, Exim waits for 
    lock_interval, and then goes back and re-opens the file as above and tries
    to lock it again. This happens up to lock_retries times, after which the
    delivery is deferred.

    If the timeout has a value greater than zero, blocking calls to fcntl() or
    flock() are used (with the given timeout), so there has already been some
    waiting involved by the time locking fails. Nevertheless, Exim does not
    give up immediately. It retries up to

    (lock_retries * lock_interval) / <timeout>

    times (rounded up).

At the end of delivery, Exim closes the file (which releases the fcntl() and/or
flock() locks) and then deletes the lock file if one was created.


26.4 Operational details for delivery to a new file
---------------------------------------------------

When the directory option is set instead of file, each message is delivered
into a newly-created file or set of files. When appendfile is activated
directly from a redirect router, neither file nor directory is normally set,
because the path for delivery is supplied by the router. (See for example, the 
address_file transport in the default configuration.) In this case, delivery is
to a new file if either the path name ends in "/", or the maildir_format or 
mailstore_format option is set.

No locking is required while writing the message to a new file, so the various
locking options of the transport are ignored. The "From" line that by default
separates messages in a single file is not normally needed, nor is the escaping
of message lines that start with "From", and there is no need to ensure a
newline at the end of each message. Consequently, the default values for 
check_string, message_prefix, and message_suffix are all unset when any of 
directory, maildir_format, or mailstore_format is set.

If Exim is required to check a quota setting, it adds up the sizes of all the
files in the delivery directory by default. However, you can specify a
different directory by setting quota_directory. Also, for maildir deliveries
(see below) the maildirfolder convention is honoured.

There are three different ways in which delivery to individual files can be
done, controlled by the settings of the maildir_format and mailstore_format
options. Note that code to support maildir or mailstore formats is not included
in the binary unless SUPPORT_MAILDIR or SUPPORT_MAILSTORE, respectively, is set
in Local/Makefile.

In all three cases an attempt is made to create the directory and any necessary
sub-directories if they do not exist, provided that the create_directory option
is set (the default). The location of a created directory can be constrained by
setting create_file. A created directory's mode is given by the directory_mode
option. If creation fails, or if the create_directory option is not set when
creation is required, delivery is deferred.


26.5 Maildir delivery
---------------------

If the maildir_format option is true, Exim delivers each message by writing it
to a file whose name is tmp/<stime>.H<mtime>P<pid>.<host> in the directory that
is defined by the directory option (the "delivery directory"). If the delivery
is successful, the file is renamed into the new subdirectory.

In the filename, <stime> is the current time of day in seconds, and <mtime> is
the microsecond fraction of the time. After a maildir delivery, Exim checks
that the time-of-day clock has moved on by at least one microsecond before
terminating the delivery process. This guarantees uniqueness for the filename.
However, as a precaution, Exim calls stat() for the file before opening it. If
any response other than ENOENT (does not exist) is given, Exim waits 2 seconds
and tries again, up to maildir_retries times.

Before Exim carries out a maildir delivery, it ensures that subdirectories
called new, cur, and tmp exist in the delivery directory. If they do not exist,
Exim tries to create them and any superior directories in their path, subject
to the create_directory and create_file options. If the 
maildirfolder_create_regex option is set, and the regular expression it
contains matches the delivery directory, Exim also ensures that a file called
maildirfolder exists in the delivery directory. If a missing directory or
maildirfolder file cannot be created, delivery is deferred.

These features make it possible to use Exim to create all the necessary files
and directories in a maildir mailbox, including subdirectories for maildir++
folders. Consider this example:

maildir_format = true
directory = /var/mail/$local_part_data\
           ${if eq{$local_part_suffix}{}{}\
           {/.${substr_1:$local_part_suffix}}}
maildirfolder_create_regex = /\.[^/]+$

If $local_part_suffix is empty (there was no suffix for the local part),
delivery is into a toplevel maildir with a name like /var/mail/pimbo (for the
user called pimbo). The pattern in maildirfolder_create_regex does not match
this name, so Exim will not look for or create the file /var/mail/pimbo/
maildirfolder, though it will create /var/mail/pimbo/{cur,new,tmp} if
necessary.

However, if $local_part_suffix contains "-eximusers" (for example), delivery is
into the maildir++ folder /var/mail/pimbo/.eximusers, which does match 
maildirfolder_create_regex. In this case, Exim will create /var/mail/pimbo
/.eximusers/maildirfolder as well as the three maildir directories /var/mail/
pimbo/.eximusers/{cur,new,tmp}.

Warning: Take care when setting maildirfolder_create_regex that it does not
inadvertently match the toplevel maildir directory, because a maildirfolder
file at top level would completely break quota calculations.

If Exim is required to check a quota setting before a maildir delivery, and 
quota_directory is not set, it looks for a file called maildirfolder in the
maildir directory (alongside new, cur, tmp). If this exists, Exim assumes the
directory is a maildir++ folder directory, which is one level down from the
user's top level mailbox directory. This causes it to start at the parent
directory instead of the current directory when calculating the amount of space
used.

One problem with delivering into a multi-file mailbox is that it is
computationally expensive to compute the size of the mailbox for quota
checking. Various approaches have been taken to reduce the amount of work
needed. The next two sections describe two of them. A third alternative is to
use some external process for maintaining the size data, and use the expansion
of the mailbox_size option as a way of importing it into Exim.


26.6 Using tags to record message sizes
---------------------------------------

If maildir_tag is set, the string is expanded for each delivery. When the
maildir file is renamed into the new sub-directory, the tag is added to its
name. However, if adding the tag takes the length of the name to the point
where the test stat() call fails with ENAMETOOLONG, the tag is dropped and the
maildir file is created with no tag.

Tags can be used to encode the size of files in their names; see 
quota_size_regex above for an example. The expansion of maildir_tag happens
after the message has been written. The value of the $message_size variable is
set to the number of bytes actually written. If the expansion is forced to
fail, the tag is ignored, but a non-forced failure causes delivery to be
deferred. The expanded tag may contain any printing characters except "/".
Non-printing characters in the string are ignored; if the resulting string is
empty, it is ignored. If it starts with an alphanumeric character, a leading
colon is inserted; this default has not proven to be the path that popular
maildir implementations have chosen (but changing it in Exim would break
backwards compatibility).

For one common implementation, you might set:

maildir_tag = ,S=${message_size}

but you should check the documentation of the other software to be sure.

It is advisable to also set quota_size_regex when setting maildir_tag as this
allows Exim to extract the size from your tag, instead of having to stat() each
message file.


26.7 Using a maildirsize file
-----------------------------

If maildir_use_size_file is true, Exim implements the maildir++ rules for
storing quota and message size information in a file called maildirsize within
the toplevel maildir directory. If this file does not exist, Exim creates it,
setting the quota from the quota option of the transport. If the maildir
directory itself does not exist, it is created before any attempt to write a
maildirsize file.

The maildirsize file is used to hold information about the sizes of messages in
the maildir, thus speeding up quota calculations. The quota value in the file
is just a cache; if the quota is changed in the transport, the new value
overrides the cached value when the next message is delivered. The cache is
maintained for the benefit of other programs that access the maildir and need
to know the quota.

If the quota option in the transport is unset or zero, the maildirsize file is
maintained (with a zero quota setting), but no quota is imposed.

A regular expression is available for controlling which directories in the
maildir participate in quota calculations when a maildirsizefile is in use. See
the description of the maildir_quota_directory_regex option above for details.


26.8 Mailstore delivery
-----------------------

If the mailstore_format option is true, each message is written as two files in
the given directory. A unique base name is constructed from the message id and
the current delivery process, and the files that are written use this base name
plus the suffixes .env and .msg. The .env file contains the message's envelope,
and the .msg file contains the message itself. The base name is placed in the
variable $mailstore_basename.

During delivery, the envelope is first written to a file with the suffix .tmp.
The .msg file is then written, and when it is complete, the .tmp file is
renamed as the .env file. Programs that access messages in mailstore format
should wait for the presence of both a .msg and a .env file before accessing
either of them. An alternative approach is to wait for the absence of a .tmp
file.

The envelope file starts with any text defined by the mailstore_prefix option,
expanded and terminated by a newline if there isn't one. Then follows the
sender address on one line, then all the recipient addresses, one per line.
There can be more than one recipient only if the batch_max option is set
greater than one. Finally, mailstore_suffix is expanded and the result appended
to the file, followed by a newline if it does not end with one.

If expansion of mailstore_prefix or mailstore_suffix ends with a forced
failure, it is ignored. Other expansion errors are treated as serious
configuration errors, and delivery is deferred. The variable
$mailstore_basename is available for use during these expansions.


26.9 Non-special new file delivery
----------------------------------

If neither maildir_format nor mailstore_format is set, a single new file is
created directly in the named directory. For example, when delivering messages
into files in batched SMTP format for later delivery to some host (see section
49.4), a setting such as

directory = /var/bsmtp/$host

might be used. A message is written to a file with a temporary name, which is
then renamed when the delivery is complete. The final name is obtained by
expanding the contents of the directory_file option.



===============================================================================
27. THE AUTOREPLY TRANSPORT

The autoreply transport is not a true transport in that it does not cause the
message to be transmitted. Instead, it generates a new mail message as an
automatic reply to the incoming message. References: and Auto-Submitted: header
lines are included. These are constructed according to the rules in RFCs 2822
and 3834, respectively.

If the router that passes the message to this transport does not have the 
unseen option set, the original message (for the current recipient) is not
delivered anywhere. However, when the unseen option is set on the router that
passes the message to this transport, routing of the address continues, so
another router can set up a normal message delivery.

The autoreply transport is usually run as the result of mail filtering, a
"vacation" message being the standard example. However, it can also be run
directly from a router like any other transport. To reduce the possibility of
message cascades, messages created by the autoreply transport always have empty
envelope sender addresses, like bounce messages.

The parameters of the message to be sent can be specified in the configuration
by options described below. However, these are used only when the address
passed to the transport does not contain its own reply information. When the
transport is run as a consequence of a mail or vacation command in a filter
file, the parameters of the message are supplied by the filter, and passed with
the address. The transport's options that define the message are then ignored
(so they are not usually set in this case). The message is specified entirely
by the filter or by the transport; it is never built from a mixture of options.
However, the file_optional, mode, and return_message options apply in all
cases.

Autoreply is implemented as a local transport. When used as a result of a
command in a user's filter file, autoreply normally runs under the uid and gid
of the user, and with appropriate current and home directories (see chapter 23
).

There is a subtle difference between routing a message to a pipe transport that
generates some text to be returned to the sender, and routing it to an 
autoreply transport. This difference is noticeable only if more than one
address from the same message is so handled. In the case of a pipe, the
separate outputs from the different addresses are gathered up and returned to
the sender in a single message, whereas if autoreply is used, a separate
message is generated for each address that is passed to it.

Non-printing characters are not permitted in the header lines generated for the
message that autoreply creates, with the exception of newlines that are
immediately followed by white space. If any non-printing characters are found,
the transport defers. Whether characters with the top bit set count as printing
characters or not is controlled by the print_topbitchars global option.

If any of the generic options for manipulating headers (for example, 
headers_add) are set on an autoreply transport, they apply to the copy of the
original message that is included in the generated message when return_message
is set. They do not apply to the generated message itself.

If the autoreply transport receives return code 2 from Exim when it submits the
message, indicating that there were no recipients, it does not treat this as an
error. This means that autoreplies sent to $sender_address when this is empty
(because the incoming message is a bounce message) do not cause problems. They
are just discarded.


27.1 Private options for autoreply
----------------------------------

+-----------------------------------------------+
|bcc|Use: autoreply|Type: string*|Default: unset|
+-----------------------------------------------+

This specifies the addresses that are to receive "blind carbon copies" of the
message when the message is specified by the transport.

+----------------------------------------------+
|cc|Use: autoreply|Type: string*|Default: unset|
+----------------------------------------------+

This specifies recipients of the message and the contents of the Cc: header
when the message is specified by the transport.

+------------------------------------------------+
|file|Use: autoreply|Type: string*|Default: unset|
+------------------------------------------------+

The contents of the file are sent as the body of the message when the message
is specified by the transport. If both file and text are set, the text string
comes first.

+-------------------------------------------------------+
|file_expand|Use: autoreply|Type: boolean|Default: false|
+-------------------------------------------------------+

If this is set, the contents of the file named by the file option are subjected
to string expansion as they are added to the message.

+---------------------------------------------------------+
|file_optional|Use: autoreply|Type: boolean|Default: false|
+---------------------------------------------------------+

If this option is true, no error is generated if the file named by the file
option or passed with the address does not exist or cannot be read.

+------------------------------------------------+
|from|Use: autoreply|Type: string*|Default: unset|
+------------------------------------------------+

This specifies the contents of the From: header when the message is specified
by the transport.

+---------------------------------------------------+
|headers|Use: autoreply|Type: string*|Default: unset|
+---------------------------------------------------+

This specifies additional RFC 2822 headers that are to be added to the message
when the message is specified by the transport. Several can be given by using "
\n" to separate them. There is no check on the format.

+-----------------------------------------------+
|log|Use: autoreply|Type: string*|Default: unset|
+-----------------------------------------------+

This option names a file in which a record of every message sent is logged when
the message is specified by the transport.

+-----------------------------------------------------+
|mode|Use: autoreply|Type: octal integer|Default: 0600|
+-----------------------------------------------------+

If either the log file or the "once" file has to be created, this mode is used.

+------------------------------------------------------------+
|never_mail|Use: autoreply|Type: address list*|Default: unset|
+------------------------------------------------------------+

If any run of the transport creates a message with a recipient that matches any
item in the list, that recipient is quietly discarded. If all recipients are
discarded, no message is created. This applies both when the recipients are
generated by a filter and when they are specified in the transport.

+------------------------------------------------+
|once|Use: autoreply|Type: string*|Default: unset|
+------------------------------------------------+

This option names a file or DBM database in which a record of each To:
recipient is kept when the message is specified by the transport. Note: This
does not apply to Cc: or Bcc: recipients.

If once is unset, or is set to an empty string, the message is always sent. By
default, if once is set to a non-empty filename, the message is not sent if a
potential recipient is already listed in the database. However, if the 
once_repeat option specifies a time greater than zero, the message is sent if
that much time has elapsed since a message was last sent to this recipient. A
setting of zero time for once_repeat (the default) prevents a message from
being sent a second time - in this case, zero means infinity.

If once_file_size is zero, a DBM database is used to remember recipients, and
it is allowed to grow as large as necessary. If once_file_size is set greater
than zero, it changes the way Exim implements the once option. Instead of using
a DBM file to record every recipient it sends to, it uses a regular file, whose
size will never get larger than the given value.

In the file, Exim keeps a linear list of recipient addresses and the times at
which they were sent messages. If the file is full when a new address needs to
be added, the oldest address is dropped. If once_repeat is not set, this means
that a given recipient may receive multiple messages, but at unpredictable
intervals that depend on the rate of turnover of addresses in the file. If 
once_repeat is set, it specifies a maximum time between repeats.

+------------------------------------------------------+
|once_file_size|Use: autoreply|Type: integer|Default: 0|
+------------------------------------------------------+

See once above.

+--------------------------------------------------+
|once_repeat|Use: autoreply|Type: time*|Default: 0s|
+--------------------------------------------------+

See once above. After expansion, the value of this option must be a valid time
value.

+----------------------------------------------------+
|reply_to|Use: autoreply|Type: string*|Default: unset|
+----------------------------------------------------+

This specifies the contents of the Reply-To: header when the message is
specified by the transport.

+----------------------------------------------------------+
|return_message|Use: autoreply|Type: boolean|Default: false|
+----------------------------------------------------------+

If this is set, a copy of the original message is returned with the new
message, subject to the maximum size set in the return_size_limit global
configuration option.

+---------------------------------------------------+
|subject|Use: autoreply|Type: string*|Default: unset|
+---------------------------------------------------+

This specifies the contents of the Subject: header when the message is
specified by the transport. It is tempting to quote the original subject in
automatic responses. For example:

subject = Re: $h_subject:

There is a danger in doing this, however. It may allow a third party to
subscribe your users to an opt-in mailing list, provided that the list accepts
bounce messages as subscription confirmations. Well-managed lists require a
non-bounce message to confirm a subscription, so the danger is relatively
small.

+------------------------------------------------+
|text|Use: autoreply|Type: string*|Default: unset|
+------------------------------------------------+

This specifies a single string to be used as the body of the message when the
message is specified by the transport. If both text and file are set, the text
comes first.

+----------------------------------------------+
|to|Use: autoreply|Type: string*|Default: unset|
+----------------------------------------------+

This specifies recipients of the message and the contents of the To: header
when the message is specified by the transport.



===============================================================================
28. THE LMTP TRANSPORT

The lmtp transport runs the LMTP protocol (RFC 2033) over a pipe to a specified
command or by interacting with a Unix domain socket. This transport is
something of a cross between the pipe and smtp transports. Exim also has
support for using LMTP over TCP/IP; this is implemented as an option for the 
smtp transport. Because LMTP is expected to be of minority interest, the
default build-time configure in src/EDITME has it commented out. You need to
ensure that

TRANSPORT_LMTP=yes

is present in your Local/Makefile in order to have the lmtp transport included
in the Exim binary. The private options of the lmtp transport are as follows:

+-----------------------------------------------+
|batch_id|Use: lmtp|Type: string*|Default: unset|
+-----------------------------------------------+

See the description of local delivery batching in chapter 25.

+--------------------------------------------+
|batch_max|Use: lmtp|Type: integer|Default: 1|
+--------------------------------------------+

This limits the number of addresses that can be handled in a single delivery.
Most LMTP servers can handle several addresses at once, so it is normally a
good idea to increase this value. See the description of local delivery
batching in chapter 25.

+----------------------------------------------+
|command|Use: lmtp|Type: string*|Default: unset|
+----------------------------------------------+

This option must be set if socket is not set. The string is a command which is
run in a separate process. It is split up into a command name and list of
arguments, each of which is separately expanded (so expansion cannot change the
number of arguments). The command is run directly, not via a shell. The message
is passed to the new process using the standard input and output to operate the
LMTP protocol.

+---------------------------------------------------+
|ignore_quota|Use: lmtp|Type: boolean|Default: false|
+---------------------------------------------------+

If this option is set true, the string "IGNOREQUOTA" is added to RCPT commands,
provided that the LMTP server has advertised support for IGNOREQUOTA in its
response to the LHLO command.

+---------------------------------------------+
|socket|Use: lmtp|Type: string*|Default: unset|
+---------------------------------------------+

This option must be set if command is not set. The result of expansion must be
the name of a Unix domain socket. The transport connects to the socket and
delivers the message to it using the LMTP protocol.

+----------------------------------------+
|timeout|Use: lmtp|Type: time|Default: 5m|
+----------------------------------------+

The transport is aborted if the created process or Unix domain socket does not
respond to LMTP commands or message input within this timeout. Delivery is
deferred, and will be tried again later. Here is an example of a typical LMTP
transport:

lmtp:
  driver = lmtp
  command = /some/local/lmtp/delivery/program
  batch_max = 20
  user = exim

This delivers up to 20 addresses at a time, in a mixture of domains if
necessary, running as the user exim.



===============================================================================
29. THE PIPE TRANSPORT

The pipe transport is used to deliver messages via a pipe to a command running
in another process. One example is the use of pipe as a pseudo-remote transport
for passing messages to some other delivery mechanism (such as UUCP). Another
is the use by individual users to automatically process their incoming
messages. The pipe transport can be used in one of the following ways:

  * A router routes one address to a transport in the normal way, and the
    transport is configured as a pipe transport. In this case, $local_part
    contains the local part of the address (as usual), and the command that is
    run is specified by the command option on the transport.

  * If the batch_max option is set greater than 1 (the default is 1), the
    transport can handle more than one address in a single run. In this case,
    when more than one address is routed to the transport, $local_part is not
    set (because it is not unique). However, the pseudo-variable
    $pipe_addresses (described in section 29.3 below) contains all the
    addresses that are routed to the transport.

  * A router redirects an address directly to a pipe command (for example, from
    an alias or forward file). In this case, $address_pipe contains the text of
    the pipe command, and the command option on the transport is ignored unless
    force_command is set. If only one address is being transported (batch_max
    is not greater than one, or only one address was redirected to this pipe
    command), $local_part contains the local part that was redirected.

The pipe transport is a non-interactive delivery method. Exim can also deliver
messages over pipes using the LMTP interactive protocol. This is implemented by
the lmtp transport.

In the case when pipe is run as a consequence of an entry in a local user's
.forward file, the command runs under the uid and gid of that user. In other
cases, the uid and gid have to be specified explicitly, either on the transport
or on the router that handles the address. Current and "home" directories are
also controllable. See chapter 23 for details of the local delivery environment
and chapter 25 for a discussion of local delivery batching.

Tainted data may not be used for the command name.


29.1 Concurrent delivery
------------------------

If two messages arrive at almost the same time, and both are routed to a pipe
delivery, the two pipe transports may be run concurrently. You must ensure that
any pipe commands you set up are robust against this happening. If the commands
write to a file, the exim_lock utility might be of use. Alternatively the 
max_parallel option could be used with a value of "1" to enforce serialization.


29.2 Returned status and data
-----------------------------

If the command exits with a non-zero return code, the delivery is deemed to
have failed, unless either the ignore_status option is set (in which case the
return code is treated as zero), or the return code is one of those listed in
the temp_errors option, which are interpreted as meaning "try again later". In
this case, delivery is deferred. Details of a permanent failure are logged, but
are not included in the bounce message, which merely contains "local delivery
failed".

If the command exits on a signal and the freeze_signal option is set then the
message will be frozen in the queue. If that option is not set, a bounce will
be sent as normal.

If the return code is greater than 128 and the command being run is a shell
script, it normally means that the script was terminated by a signal whose
value is the return code minus 128. The freeze_signal option does not apply in
this case.

If Exim is unable to run the command (that is, if execve() fails), the return
code is set to 127. This is the value that a shell returns if it is asked to
run a non-existent command. The wording for the log line suggests that a
non-existent command may be the problem.

The return_output option can affect the result of a pipe delivery. If it is set
and the command produces any output on its standard output or standard error
streams, the command is considered to have failed, even if it gave a zero
return code or if ignore_status is set. The output from the command is included
as part of the bounce message. The return_fail_output option is similar, except
that output is returned only when the command exits with a failure return code,
that is, a value other than zero or a code that matches temp_errors.


29.3 How the command is run
---------------------------

The command line is (by default) broken down into a command name and arguments
by the pipe transport itself. The allow_commands and restrict_to_path options
can be used to restrict the commands that may be run.

Unquoted arguments are delimited by white space. If an argument appears in
double quotes, backslash is interpreted as an escape character in the usual
way. If an argument appears in single quotes, no escaping is done.

String expansion is applied to the command line except when it comes from a
traditional .forward file (commands from a filter file are expanded). The
expansion is applied to each argument in turn rather than to the whole line.
For this reason, any string expansion item that contains white space must be
quoted so as to be contained within a single argument. A setting such as

command = /some/path ${if eq{$local_part}{postmaster}{xx}{yy}}

will not work, because the expansion item gets split between several arguments.
You have to write

command = /some/path "${if eq{$local_part}{postmaster}{xx}{yy}}"

to ensure that it is all in one argument. The expansion is done in this way,
argument by argument, so that the number of arguments cannot be changed as a
result of expansion, and quotes or backslashes in inserted variables do not
interact with external quoting. However, this leads to problems if you want to
generate multiple arguments (or the command name plus arguments) from a single
expansion. In this situation, the simplest solution is to use a shell. For
example:

command = /bin/sh -c ${lookup{$local_part}lsearch{/some/file}}

Special handling takes place when an argument consists of precisely the text
"$pipe_addresses" (no quotes). This is not a general expansion variable; the
only place this string is recognized is when it appears as an argument for a
pipe or transport filter command. It causes each address that is being handled
to be inserted in the argument list at that point as a separate argument. This
avoids any problems with spaces or shell metacharacters, and is of use when a 
pipe transport is handling groups of addresses in a batch.

If force_command is enabled on the transport, special handling takes place for
an argument that consists of precisely the text "$address_pipe". It is handled
similarly to $pipe_addresses above. It is expanded and each argument is
inserted in the argument list at that point as a separate argument. The
"$address_pipe" item does not need to be the only item in the argument; in
fact, if it were then force_command should behave as a no-op. Rather, it should
be used to adjust the command run while preserving the argument vector
separation.

After splitting up into arguments and expansion, the resulting command is run
in a subprocess directly from the transport, not under a shell. The message
that is being delivered is supplied on the standard input, and the standard
output and standard error are both connected to a single pipe that is read by
Exim. The max_output option controls how much output the command may produce,
and the return_output and return_fail_output options control what is done with
it.

Not running the command under a shell (by default) lessens the security risks
in cases when a command from a user's filter file is built out of data that was
taken from an incoming message. If a shell is required, it can of course be
explicitly specified as the command to be run. However, there are circumstances
where existing commands (for example, in .forward files) expect to be run under
a shell and cannot easily be modified. To allow for these cases, there is an
option called use_shell, which changes the way the pipe transport works.
Instead of breaking up the command line as just described, it expands it as a
single string and passes the result to /bin/sh. The restrict_to_path option and
the $pipe_addresses facility cannot be used with use_shell, and the whole
mechanism is inherently less secure.


29.4 Environment variables
--------------------------

The environment variables listed below are set up when the command is invoked.
This list is a compromise for maximum compatibility with other MTAs. Note that
the environment option can be used to add additional variables to this
environment. The environment for the pipe transport is not subject to the 
add_environment and keep_environment main config options. Note: Using
enviroment variables loses track of tainted data. Writers of pipe transport
commands should be wary of data supplied by potential attackers.

DOMAIN               the domain of the address
HOME                 the home directory, if set
HOST                 the host name when called from a router (see below)
LOCAL_PART           see below
LOCAL_PART_PREFIX    see below
LOCAL_PART_SUFFIX    see below
LOGNAME              see below
MESSAGE_ID           Exim's local ID for the message
PATH                 as specified by the path option below
QUALIFY_DOMAIN       the sender qualification domain
RECIPIENT            the complete recipient address
SENDER               the sender of the message (empty if a bounce)
SHELL                /bin/sh
TZ                   the value of the timezone option, if set
USER                 see below

When a pipe transport is called directly from (for example) an accept router,
LOCAL_PART is set to the local part of the address. When it is called as a
result of a forward or alias expansion, LOCAL_PART is set to the local part of
the address that was expanded. In both cases, any affixes are removed from the
local part, and made available in LOCAL_PART_PREFIX and LOCAL_PART_SUFFIX,
respectively. LOGNAME and USER are set to the same value as LOCAL_PART for
compatibility with other MTAs.

HOST is set only when a pipe transport is called from a router that associates
hosts with an address, typically when using pipe as a pseudo-remote transport.
HOST is set to the first host name specified by the router.

If the transport's generic home_directory option is set, its value is used for
the HOME environment variable. Otherwise, a home directory may be set by the
router's transport_home_directory option, which defaults to the user's home
directory if check_local_user is set.


29.5 Private options for pipe
-----------------------------

+----------------------------------------------------------+
|allow_commands|Use: pipe|Type: string list*|Default: unset|
+----------------------------------------------------------+

The string is expanded, and is then interpreted as a colon-separated list of
permitted commands. If restrict_to_path is not set, the only commands permitted
are those in the allow_commands list. They need not be absolute paths; the path
option is still used for relative paths. If restrict_to_path is set with 
allow_commands, the command must either be in the allow_commands list, or a
name without any slashes that is found on the path. In other words, if neither 
allow_commands nor restrict_to_path is set, there is no restriction on the
command, but otherwise only commands that are permitted by one or the other are
allowed. For example, if

allow_commands = /usr/bin/vacation

and restrict_to_path is not set, the only permitted command is /usr/bin/
vacation. The allow_commands option may not be set if use_shell is set.

+-----------------------------------------------+
|batch_id|Use: pipe|Type: string*|Default: unset|
+-----------------------------------------------+

See the description of local delivery batching in chapter 25.

+--------------------------------------------+
|batch_max|Use: pipe|Type: integer|Default: 1|
+--------------------------------------------+

This limits the number of addresses that can be handled in a single delivery.
See the description of local delivery batching in chapter 25.

+--------------------------------------------------+
|check_string|Use: pipe|Type: string|Default: unset|
+--------------------------------------------------+

As pipe writes the message, the start of each line is tested for matching 
check_string, and if it does, the initial matching characters are replaced by
the contents of escape_string, provided both are set. The value of check_string
is a literal string, not a regular expression, and the case of any letters it
contains is significant. When use_bsmtp is set, the contents of check_string
and escape_string are forced to values that implement the SMTP escaping
protocol. Any settings made in the configuration file are ignored.

+----------------------------------------------+
|command|Use: pipe|Type: string*|Default: unset|
+----------------------------------------------+

This option need not be set when pipe is being used to deliver to pipes
obtained directly from address redirections. In other cases, the option must be
set, to provide a command to be run. It need not yield an absolute path (see
the path option below). The command is split up into separate arguments by
Exim, and each argument is separately expanded, as described in section 29.3
above.

No part of the resulting command may be tainted.

+--------------------------------------------------+
|environment|Use: pipe|Type: string*|Default: unset|
+--------------------------------------------------+

This option is used to add additional variables to the environment in which the
command runs (see section 29.4 for the default list). Its value is a string
which is expanded, and then interpreted as a colon-separated list of
environment settings of the form <name>=<value>.

+---------------------------------------------------+
|escape_string|Use: pipe|Type: string|Default: unset|
+---------------------------------------------------+

See check_string above.

+-------------------------------------------------------+
|freeze_exec_fail|Use: pipe|Type: boolean|Default: false|
+-------------------------------------------------------+

Failure to exec the command in a pipe transport is by default treated like any
other failure while running the command. However, if freeze_exec_fail is set,
failure to exec is treated specially, and causes the message to be frozen,
whatever the setting of ignore_status.

+----------------------------------------------------+
|freeze_signal|Use: pipe|Type: boolean|Default: false|
+----------------------------------------------------+

Normally if the process run by a command in a pipe transport exits on a signal,
a bounce message is sent. If freeze_signal is set, the message will be frozen
in Exim's queue instead.

+----------------------------------------------------+
|force_command|Use: pipe|Type: boolean|Default: false|
+----------------------------------------------------+

Normally when a router redirects an address directly to a pipe command the 
command option on the transport is ignored. If force_command is set, the 
command option will used. This is especially useful for forcing a wrapper or
additional argument to be added to the command. For example:

command = /usr/bin/remote_exec myhost -- $address_pipe
force_command

Note that $address_pipe is handled specially in command when force_command is
set, expanding out to the original argument vector as separate items, similarly
to a Unix shell ""$@"" construct.

+----------------------------------------------------+
|ignore_status|Use: pipe|Type: boolean|Default: false|
+----------------------------------------------------+

If this option is true, the status returned by the subprocess that is set up to
run the command is ignored, and Exim behaves as if zero had been returned.
Otherwise, a non-zero status or termination by signal causes an error return
from the transport unless the status value is one of those listed in 
temp_errors; these cause the delivery to be deferred and tried again later.

Note: This option does not apply to timeouts, which do not return a status. See
the timeout_defer option for how timeouts are handled.

+-------------------------------------------------------+
|log_defer_output|Use: pipe|Type: boolean|Default: false|
+-------------------------------------------------------+

If this option is set, and the status returned by the command is one of the
codes listed in temp_errors (that is, delivery was deferred), and any output
was produced on stdout or stderr, the first line of it is written to the main
log.

+------------------------------------------------------+
|log_fail_output|Use: pipe|Type: boolean|Default: false|
+------------------------------------------------------+

If this option is set, and the command returns any output on stdout or stderr,
and also ends with a return code that is neither zero nor one of the return
codes listed in temp_errors (that is, the delivery failed), the first line of
output is written to the main log. This option and log_output are mutually
exclusive. Only one of them may be set.

+-------------------------------------------------+
|log_output|Use: pipe|Type: boolean|Default: false|
+-------------------------------------------------+

If this option is set and the command returns any output on stdout or stderr,
the first line of output is written to the main log, whatever the return code.
This option and log_fail_output are mutually exclusive. Only one of them may be
set.

+-----------------------------------------------+
|max_output|Use: pipe|Type: integer|Default: 20K|
+-----------------------------------------------+

This specifies the maximum amount of output that the command may produce on its
standard output and standard error file combined. If the limit is exceeded, the
process running the command is killed. This is intended as a safety measure to
catch runaway processes. The limit is applied independently of the settings of
the options that control what is done with such output (for example, 
return_output). Because of buffering effects, the amount of output may exceed
the limit by a small amount before Exim notices.

+---------------------------------------------------------+
|message_prefix|Use: pipe|Type: string*|Default: see below|
+---------------------------------------------------------+

The string specified here is expanded and output at the start of every message.
The default is unset if use_bsmtp is set. Otherwise it is

message_prefix = \
  From ${if def:return_path{$return_path}{MAILER-DAEMON}}\
  ${tod_bsdinbox}\n

This is required by the commonly used /usr/bin/vacation program. However, it
must not be present if delivery is to the Cyrus IMAP server, or to the tmail
local delivery agent. The prefix can be suppressed by setting

message_prefix =

Note: If you set use_crlf true, you must change any occurrences of "\n" to "\r\
n" in message_prefix.

+---------------------------------------------------------+
|message_suffix|Use: pipe|Type: string*|Default: see below|
+---------------------------------------------------------+

The string specified here is expanded and output at the end of every message.
The default is unset if use_bsmtp is set. Otherwise it is a single newline. The
suffix can be suppressed by setting

message_suffix =

Note: If you set use_crlf true, you must change any occurrences of "\n" to "\r\
n" in message_suffix.

+---------------------------------------------------+
|path|Use: pipe|Type: string*|Default: /bin:/usr/bin|
+---------------------------------------------------+

This option is expanded and specifies the string that is set up in the PATH
environment variable of the subprocess. If the command option does not yield an
absolute path name, the command is sought in the PATH directories, in the usual
way. Warning: This does not apply to a command specified as a transport filter.

+------------------------------------------------------+
|permit_coredump|Use: pipe|Type: boolean|Default: false|
+------------------------------------------------------+

Normally Exim inhibits core-dumps during delivery. If you have a need to get a
core-dump of a pipe command, enable this command. This enables core-dumps
during delivery and affects both the Exim binary and the pipe command run. It
is recommended that this option remain off unless and until you have a need for
it and that this only be enabled when needed, as the risk of excessive resource
consumption can be quite high. Note also that Exim is typically installed as a
setuid binary and most operating systems will inhibit coredumps of these by
default, so further OS-specific action may be required.

+------------------------------------------------------+
|pipe_as_creator|Use: pipe|Type: boolean|Default: false|
+------------------------------------------------------+

If the generic user option is not set and this option is true, the delivery
process is run under the uid that was in force when Exim was originally called
to accept the message. If the group id is not otherwise set (via the generic 
group option), the gid that was in force when Exim was originally called to
accept the message is used.

+-------------------------------------------------------+
|restrict_to_path|Use: pipe|Type: boolean|Default: false|
+-------------------------------------------------------+

When this option is set, any command name not listed in allow_commands must
contain no slashes. The command is searched for only in the directories listed
in the path option. This option is intended for use in the case when a pipe
command has been generated from a user's .forward file. This is usually handled
by a pipe transport called address_pipe.

+---------------------------------------------------------+
|return_fail_output|Use: pipe|Type: boolean|Default: false|
+---------------------------------------------------------+

If this option is true, and the command produced any output and ended with a
return code other than zero or one of the codes listed in temp_errors (that is,
the delivery failed), the output is returned in the bounce message. However, if
the message has a null sender (that is, it is itself a bounce message), output
from the command is discarded. This option and return_output are mutually
exclusive. Only one of them may be set.

+----------------------------------------------------+
|return_output|Use: pipe|Type: boolean|Default: false|
+----------------------------------------------------+

If this option is true, and the command produced any output, the delivery is
deemed to have failed whatever the return code from the command, and the output
is returned in the bounce message. Otherwise, the output is just discarded.
However, if the message has a null sender (that is, it is a bounce message),
output from the command is always discarded, whatever the setting of this
option. This option and return_fail_output are mutually exclusive. Only one of
them may be set.

+----------------------------------------------------------+
|temp_errors|Use: pipe|Type: string list|Default: see below|
+----------------------------------------------------------+

This option contains either a colon-separated list of numbers, or a single
asterisk. If ignore_status is false and return_output is not set, and the
command exits with a non-zero return code, the failure is treated as temporary
and the delivery is deferred if the return code matches one of the numbers, or
if the setting is a single asterisk. Otherwise, non-zero return codes are
treated as permanent errors. The default setting contains the codes defined by
EX_TEMPFAIL and EX_CANTCREAT in sysexits.h. If Exim is compiled on a system
that does not define these macros, it assumes values of 75 and 73,
respectively.

+----------------------------------------+
|timeout|Use: pipe|Type: time|Default: 1h|
+----------------------------------------+

If the command fails to complete within this time, it is killed. This normally
causes the delivery to fail (but see timeout_defer). A zero time interval
specifies no timeout. In order to ensure that any subprocesses created by the
command are also killed, Exim makes the initial process a process group leader,
and kills the whole process group on a timeout. However, this can be defeated
if one of the processes starts a new process group.

+----------------------------------------------------+
|timeout_defer|Use: pipe|Type: boolean|Default: false|
+----------------------------------------------------+

A timeout in a pipe transport, either in the command that the transport runs,
or in a transport filter that is associated with it, is by default treated as a
hard error, and the delivery fails. However, if timeout_defer is set true, both
kinds of timeout become temporary errors, causing the delivery to be deferred.

+------------------------------------------------+
|umask|Use: pipe|Type: octal integer|Default: 022|
+------------------------------------------------+

This specifies the umask setting for the subprocess that runs the command.

+------------------------------------------------+
|use_bsmtp|Use: pipe|Type: boolean|Default: false|
+------------------------------------------------+

If this option is set true, the pipe transport writes messages in "batch SMTP"
format, with the envelope sender and recipient(s) included as SMTP commands. If
you want to include a leading HELO command with such messages, you can do so by
setting the message_prefix option. See section 49.4 for details of batch SMTP.

+---------------------------------------------------------+
|use_classresources|Use: pipe|Type: boolean|Default: false|
+---------------------------------------------------------+

This option is available only when Exim is running on FreeBSD, NetBSD, or BSD/
OS. If it is set true, the setclassresources() function is used to set resource
limits when a pipe transport is run to perform a delivery. The limits for the
uid under which the pipe is to run are obtained from the login class database.

+-----------------------------------------------+
|use_crlf|Use: pipe|Type: boolean|Default: false|
+-----------------------------------------------+

This option causes lines to be terminated with the two-character CRLF sequence
(carriage return, linefeed) instead of just a linefeed character. In the case
of batched SMTP, the byte sequence written to the pipe is then an exact image
of what would be sent down a real SMTP connection.

The contents of the message_prefix and message_suffix options are written
verbatim, so must contain their own carriage return characters if these are
needed. When use_bsmtp is not set, the default values for both message_prefix
and message_suffix end with a single linefeed, so their values must be changed
to end with "\r\n" if use_crlf is set.

+------------------------------------------------+
|use_shell|Use: pipe|Type: boolean|Default: false|
+------------------------------------------------+

If this option is set, it causes the command to be passed to /bin/sh instead of
being run directly from the transport, as described in section 29.3. This is
less secure, but is needed in some situations where the command is expected to
be run under a shell and cannot easily be modified. The allow_commands and 
restrict_to_path options, and the "$pipe_addresses" facility are incompatible
with use_shell. The command is expanded as a single string, and handed to /bin/
sh as data for its -c option.


29.6 Using an external local delivery agent
-------------------------------------------

The pipe transport can be used to pass all messages that require local delivery
to a separate local delivery agent such as procmail. When doing this, care must
be taken to ensure that the pipe is run under an appropriate uid and gid. In
some configurations one wants this to be a uid that is trusted by the delivery
agent to supply the correct sender of the message. It may be necessary to
recompile or reconfigure the delivery agent so that it trusts an appropriate
user. The following is an example transport and router configuration for 
procmail:

# transport
procmail_pipe:
  driver = pipe
  command = /usr/local/bin/procmail -d $local_part_data
  return_path_add
  delivery_date_add
  envelope_to_add
  check_string = "From "
  escape_string = ">From "
  umask = 077
  user = $local_part_data
  group = mail

# router
procmail:
  driver = accept
  check_local_user
  transport = procmail_pipe

In this example, the pipe is run as the local user, but with the group set to 
mail. An alternative is to run the pipe as a specific user such as mail or exim
, but in this case you must arrange for procmail to trust that user to supply a
correct sender address. If you do not specify either a group or a user option,
the pipe command is run as the local user. The home directory is the user's
home directory by default.

Note: The command that the pipe transport runs does not begin with

IFS=" "

as shown in some procmail documentation, because Exim does not by default use a
shell to run pipe commands.

The next example shows a transport and a router for a system where local
deliveries are handled by the Cyrus IMAP server.

# transport
local_delivery_cyrus:
  driver = pipe
  command = /usr/cyrus/bin/deliver \
            -- $local_part_data
  user = cyrus
  group = mail
  return_output
  log_output
  message_prefix =
  message_suffix =

# router
local_user_cyrus:
  driver = accept
  check_local_user
  transport = local_delivery_cyrus

Note the unsetting of message_prefix and message_suffix, and the use of 
return_output to cause any text written by Cyrus to be returned to the sender.



===============================================================================
30. THE SMTP TRANSPORT

The smtp transport delivers messages over TCP/IP connections using the SMTP or
LMTP protocol. The list of hosts to try can either be taken from the address
that is being processed (having been set up by the router), or specified
explicitly for the transport. Timeout and retry processing (see chapter 32) is
applied to each IP address independently.


30.1 Multiple messages on a single connection
---------------------------------------------

The sending of multiple messages over a single TCP/IP connection can arise in
two ways:

  * If a message contains more than max_rcpt (see below) addresses that are
    routed to the same host, more than one copy of the message has to be sent
    to that host. In this situation, multiple copies may be sent in a single
    run of the smtp transport over a single TCP/IP connection. (What Exim
    actually does when it has too many addresses to send in one message also
    depends on the value of the global remote_max_parallel option. Details are
    given in section 49.1.)

  * When a message has been successfully delivered over a TCP/IP connection,
    Exim looks in its hints database to see if there are any other messages
    awaiting a connection to the same host. If there are, a new delivery
    process is started for one of them, and the current TCP/IP connection is
    passed on to it. The new process may in turn send multiple copies and
    possibly create yet another process.

For each copy sent over the same TCP/IP connection, a sequence counter is
incremented, and if it ever gets to the value of connection_max_messages, no
further messages are sent over that connection.


30.2 Use of the $host and $host_address variables
-------------------------------------------------

At the start of a run of the smtp transport, the values of $host and
$host_address are the name and IP address of the first host on the host list
passed by the router. However, when the transport is about to connect to a
specific host, and while it is connected to that host, $host and $host_address
are set to the values for that host. These are the values that are in force
when the helo_data, hosts_try_auth, interface, serialize_hosts, and the various
TLS options are expanded.


30.3 Use of $tls_cipher and $tls_peerdn
---------------------------------------

At the start of a run of the smtp transport, the values of $tls_bits,
$tls_cipher, $tls_peerdn and $tls_sni are the values that were set when the
message was received. These are the values that are used for options that are
expanded before any SMTP connections are made. Just before each connection is
made, these four variables are emptied. If TLS is subsequently started, they
are set to the appropriate values for the outgoing connection, and these are
the values that are in force when any authenticators are run and when the 
authenticated_sender option is expanded.

These variables are deprecated in favour of $tls_in_cipher et. al. and will be
removed in a future release.


30.4 Private options for smtp
-----------------------------

The private options of the smtp transport are as follows:

+------------------------------------------------------------------+
|address_retry_include_sender|Use: smtp|Type: boolean|Default: true|
+------------------------------------------------------------------+

When an address is delayed because of a 4xx response to a RCPT command, it is
the combination of sender and recipient that is delayed in subsequent queue
runs until the retry time is reached. You can delay the recipient without
reference to the sender (which is what earlier versions of Exim did), by
setting address_retry_include_sender false. However, this can lead to problems
with servers that regularly issue 4xx responses to RCPT commands.

+------------------------------------------------------+
|allow_localhost|Use: smtp|Type: boolean|Default: false|
+------------------------------------------------------+

When a host specified in hosts or fallback_hosts (see below) turns out to be
the local host, or is listed in hosts_treat_as_local, delivery is deferred by
default. However, if allow_localhost is set, Exim goes on to do the delivery
anyway. This should be used only in special cases when the configuration
ensures that no looping will result (for example, a differently configured Exim
is listening on the port to which the message is sent).

+-----------------------------------------------------------+
|authenticated_sender|Use: smtp|Type: string*|Default: unset|
+-----------------------------------------------------------+

When Exim has authenticated as a client, or if authenticated_sender_force is
true, this option sets a value for the AUTH= item on outgoing MAIL commands,
overriding any existing authenticated sender value. If the string expansion is
forced to fail, the option is ignored. Other expansion failures cause delivery
to be deferred. If the result of expansion is an empty string, that is also
ignored.

The expansion happens after the outgoing connection has been made and TLS
started, if required. This means that the $host, $host_address, $tls_out_cipher
, and $tls_out_peerdn variables are set according to the particular connection.

If the SMTP session is not authenticated, the expansion of authenticated_sender
still happens (and can cause the delivery to be deferred if it fails), but no
AUTH= item is added to MAIL commands unless authenticated_sender_force is true.

This option allows you to use the smtp transport in LMTP mode to deliver mail
to Cyrus IMAP and provide the proper local part as the "authenticated sender",
via a setting such as:

authenticated_sender = $local_part

This removes the need for IMAP subfolders to be assigned special ACLs to allow
direct delivery to those subfolders.

Because of expected uses such as that just described for Cyrus (when no domain
is involved), there is no checking on the syntax of the provided value.

+-----------------------------------------------------------------+
|authenticated_sender_force|Use: smtp|Type: boolean|Default: false|
+-----------------------------------------------------------------+

If this option is set true, the authenticated_sender option's value is used for
the AUTH= item on outgoing MAIL commands, even if Exim has not authenticated as
a client.

+------------------------------------------------+
|command_timeout|Use: smtp|Type: time|Default: 5m|
+------------------------------------------------+

This sets a timeout for receiving a response to an SMTP command that has been
sent out. It is also used when waiting for the initial banner line from the
remote host. Its value must not be zero.

+------------------------------------------------+
|connect_timeout|Use: smtp|Type: time|Default: 5m|
+------------------------------------------------+

This sets a timeout for the connect() function, which sets up a TCP/IP call to
a remote host. A setting of zero allows the system timeout (typically several
minutes) to act. To have any effect, the value of this option must be less than
the system timeout. However, it has been observed that on some systems there is
no system timeout, which is why the default value for this option is 5 minutes,
a value recommended by RFC 1123.

+------------------------------------------------------------+
|connection_max_messages|Use: smtp|Type: integer|Default: 500|
+------------------------------------------------------------+

This controls the maximum number of separate message deliveries that are sent
over a single TCP/IP connection. If the value is zero, there is no limit. For
testing purposes, this value can be overridden by the -oB command line option.

+---------------------------------------------------------------+
|dane_require_tls_ciphers|Use: smtp|Type: string*|Default: unset|
+---------------------------------------------------------------+

This option may be used to override tls_require_ciphers for connections where
DANE has been determined to be in effect. If not set, then tls_require_ciphers
will be used. Normal SMTP delivery is not able to make strong demands of TLS
cipher configuration, because delivery will fall back to plaintext. Once DANE
has been determined to be in effect, there is no plaintext fallback and making
the TLS cipherlist configuration stronger will increase security, rather than
counter-intuitively decreasing it. If the option expands to be empty or is
forced to fail, then it will be treated as unset and tls_require_ciphers will
be used instead.

+---------------------------------------------+
|data_timeout|Use: smtp|Type: time|Default: 5m|
+---------------------------------------------+

This sets a timeout for the transmission of each block in the data portion of
the message. As a result, the overall timeout for a message depends on the size
of the message. Its value must not be zero. See also final_timeout.

+-------------------------------------------------+
|dkim_canon|Use: smtp|Type: string*|Default: unset|
+-------------------------------------------------+

DKIM signing option. For details see section 58.1.1.

+-------------------------------------------------------+
|dkim_domain|Use: smtp|Type: string list*|Default: unset|
+-------------------------------------------------------+

DKIM signing option. For details see section 58.1.1.

+-------------------------------------------------+
|dkim_hash|Use: smtp|Type: string*|Default: sha256|
+-------------------------------------------------+

DKIM signing option. For details see section 58.1.1.

+----------------------------------------------------+
|dkim_identity|Use: smtp|Type: string*|Default: unset|
+----------------------------------------------------+

DKIM signing option. For details see section 58.1.1.

+-------------------------------------------------------+
|dkim_private_key|Use: smtp|Type: string*|Default: unset|
+-------------------------------------------------------+

DKIM signing option. For details see section 58.1.1.

+----------------------------------------------------+
|dkim_selector|Use: smtp|Type: string*|Default: unset|
+----------------------------------------------------+

DKIM signing option. For details see section 58.1.1.

+--------------------------------------------------+
|dkim_strict|Use: smtp|Type: string*|Default: unset|
+--------------------------------------------------+

DKIM signing option. For details see section 58.1.1.

+----------------------------------------------------------+
|dkim_sign_headers|Use: smtp|Type: string*|Default: per RFC|
+----------------------------------------------------------+

DKIM signing option. For details see section 58.1.1.

+------------------------------------------------------+
|dkim_timestamps|Use: smtp|Type: string*|Default: unset|
+------------------------------------------------------+

DKIM signing option. For details see section 58.1.1.

+--------------------------------------------------------+
|delay_after_cutoff|Use: smtp|Type: boolean|Default: true|
+--------------------------------------------------------+

This option controls what happens when all remote IP addresses for a given
domain have been inaccessible for so long that they have passed their retry
cutoff times.

In the default state, if the next retry time has not been reached for any of
them, the address is bounced without trying any deliveries. In other words,
Exim delays retrying an IP address after the final cutoff time until a new
retry time is reached, and can therefore bounce an address without ever trying
a delivery, when machines have been down for a long time. Some people are
unhappy at this prospect, so...

If delay_after_cutoff is set false, Exim behaves differently. If all IP
addresses are past their final cutoff time, Exim tries to deliver to those IP
addresses that have not been tried since the message arrived. If there are
none, of if they all fail, the address is bounced. In other words, it does not
delay when a new message arrives, but immediately tries those expired IP
addresses that haven't been tried since the message arrived. If there is a
continuous stream of messages for the dead hosts, unsetting delay_after_cutoff
means that there will be many more attempts to deliver to them.

+--------------------------------------------------------+
|dns_qualify_single|Use: smtp|Type: boolean|Default: true|
+--------------------------------------------------------+

If the hosts or fallback_hosts option is being used, and the gethostbyname
option is false, the RES_DEFNAMES resolver option is set. See the 
qualify_single option in chapter 17 for more details.

+---------------------------------------------------------+
|dns_search_parents|Use: smtp|Type: boolean|Default: false|
+---------------------------------------------------------+

If the hosts or fallback_hosts option is being used, and the gethostbyname
option is false, the RES_DNSRCH resolver option is set. See the search_parents
option in chapter 17 for more details.

+--------------------------------------------------------------+
|dnssec_request_domains|Use: smtp|Type: domain list*|Default: *|
+--------------------------------------------------------------+

DNS lookups for domains matching dnssec_request_domains will be done with the
DNSSEC request bit set. Setting this transport option is only useful if the
transport overrides or sets the host names. See the dnssec_request_domains
router option.

+------------------------------------------------------------------+
|dnssec_require_domains|Use: smtp|Type: domain list*|Default: unset|
+------------------------------------------------------------------+

DNS lookups for domains matching dnssec_require_domains will be done with the
DNSSEC request bit set. Setting this transport option is only useful if the
transport overrides or sets the host names. See the dnssec_require_domains
router option.

+-------------------------------------------+
|dscp|Use: smtp|Type: string*|Default: unset|
+-------------------------------------------+

This option causes the DSCP value associated with a socket to be set to one of
a number of fixed strings or to numeric value. The -bI:dscp option may be used
to ask Exim which names it knows of. Common values include "throughput",
"mincost", and on newer systems "ef", "af41", etc. Numeric values may be in the
range 0 to 0x3F.

The outbound packets from Exim will be marked with this value in the header
(for IPv4, the TOS field; for IPv6, the TCLASS field); there is no guarantee
that these values will have any effect, not be stripped by networking
equipment, or do much of anything without cooperation with your Network
Engineer and those of all network operators between the source and destination.

+---------------------------------------------------------+
|fallback_hosts|Use: smtp|Type: string list|Default: unset|
+---------------------------------------------------------+

String expansion is not applied to this option. The argument must be a
colon-separated list of host names or IP addresses, optionally also including
port numbers, though the separator can be changed, as described in section 6.20
. Each individual item in the list is the same as an item in a route_list
setting for the manualroute router, as described in section 20.5.

Fallback hosts can also be specified on routers, which associate them with the
addresses they process. As for the hosts option without hosts_override, 
fallback_hosts specified on the transport is used only if the address does not
have its own associated fallback host list. Unlike hosts, a setting of 
fallback_hosts on an address is not overridden by hosts_override. However, 
hosts_randomize does apply to fallback host lists.

If Exim is unable to deliver to any of the hosts for a particular address, and
the errors are not permanent rejections, the address is put on a separate
transport queue with its host list replaced by the fallback hosts, unless the
address was routed via MX records and the current host was in the original MX
list. In that situation, the fallback host list is not used.

Once normal deliveries are complete, the fallback queue is delivered by
re-running the same transports with the new host lists. If several failing
addresses have the same fallback hosts (and max_rcpt permits it), a single copy
of the message is sent.

The resolution of the host names on the fallback list is controlled by the 
gethostbyname option, as for the hosts option. Fallback hosts apply both to
cases when the host list comes with the address and when it is taken from hosts
. This option provides a "use a smart host only if delivery fails" facility.

+-----------------------------------------------+
|final_timeout|Use: smtp|Type: time|Default: 10m|
+-----------------------------------------------+

This is the timeout that applies while waiting for the response to the final
line containing just "." that terminates a message. Its value must not be zero.

+----------------------------------------------------+
|gethostbyname|Use: smtp|Type: boolean|Default: false|
+----------------------------------------------------+

If this option is true when the hosts and/or fallback_hosts options are being
used, names are looked up using gethostbyname() (or getipnodebyname() when
available) instead of using the DNS. Of course, that function may in fact use
the DNS, but it may also consult other sources of information such as /etc/
hosts.

+---------------------------------------------------------+
|gnutls_compat_mode|Use: smtp|Type: boolean|Default: unset|
+---------------------------------------------------------+

This option controls whether GnuTLS is used in compatibility mode in an Exim
server. This reduces security slightly, but improves interworking with older
implementations of TLS.

+----------------------------------------------------+
|helo_data|Use: smtp|Type: string*|Default: see below|
+----------------------------------------------------+

The value of this option is expanded after a connection to a another host has
been set up. The result is used as the argument for the EHLO, HELO, or LHLO
command that starts the outgoing SMTP or LMTP session. The default value of the
option is:

$primary_hostname

During the expansion, the variables $host and $host_address are set to the
identity of the remote host, and the variables $sending_ip_address and
$sending_port are set to the local IP address and port number that are being
used. These variables can be used to generate different values for different
servers or different local IP addresses. For example, if you want the string
that is used for helo_data to be obtained by a DNS lookup of the outgoing
interface address, you could use this:

helo_data = ${lookup dnsdb{ptr=$sending_ip_address} \
  {${listextract{1}{<\n $value}}} \
  {$primary_hostname}}

The use of helo_data applies both to sending messages and when doing callouts.

+-----------------------------------------------------------------+
|host_name_extract|Use: smtp|Type: string list*|Default: see below|
+-----------------------------------------------------------------+

Some mail-accepting sites (notably Microsoft) operate many servers behind a
network load-balancer. When this is done, with separated TLS session caches,
TLS session resuption becomes problematic. It will only succeed when the same
server happens to be selected by the load-balancer, matching the session stored
in the client's cache.

Exim can pull out a server name, if there is one, from the response to the
client's SMTP EHLO command. The default value of this option:

    ${if and { {match {$host} {.outlook.com\$}} \
               {match {$item} {\N^250-([\w.]+)\s\N}} \
             } {$1}}

suffices for one known case. During the expansion of this option the $item
variable will have the server's EHLO response. The result of the option
expansion is included in the key used to store and retrieve the TLS session,
for session resumption.

Operators of high-load sites may wish to evaluate their logs for indications of
other destination sites operating load-balancers, and develop a suitable
expression for this option. The smtp:ehlo event and the $tls_out_resumption
variable will be useful for such work.

+-------------------------------------------------+
|hosts|Use: smtp|Type: string list*|Default: unset|
+-------------------------------------------------+

Hosts are associated with an address by a router such as dnslookup, which finds
the hosts by looking up the address domain in the DNS, or by manualroute, which
has lists of hosts in its configuration. However, email addresses can be passed
to the smtp transport by any router, and not all of them can provide an
associated list of hosts.

The hosts option specifies a list of hosts to be used if the address being
processed does not have any hosts associated with it. The hosts specified by 
hosts are also used, whether or not the address has its own hosts, if 
hosts_override is set.

The string is first expanded, before being interpreted as a colon-separated
list of host names or IP addresses, possibly including port numbers. The
separator may be changed to something other than colon, as described in section
6.20. Each individual item in the list is the same as an item in a route_list
setting for the manualroute router, as described in section 20.5. However, note
that the "/MX" facility of the manualroute router is not available here.

If the expansion fails, delivery is deferred. Unless the failure was caused by
the inability to complete a lookup, the error is logged to the panic log as
well as the main log. Host names are looked up either by searching directly for
address records in the DNS or by calling gethostbyname() (or getipnodebyname()
when available), depending on the setting of the gethostbyname option. When
Exim is compiled with IPv6 support, if a host that is looked up in the DNS has
both IPv4 and IPv6 addresses, both types of address are used.

During delivery, the hosts are tried in order, subject to their retry status,
unless hosts_randomize is set.

+-----------------------------------------------------------+
|hosts_avoid_esmtp|Use: smtp|Type: host list*|Default: unset|
+-----------------------------------------------------------+

This option is for use with broken hosts that announce ESMTP facilities (for
example, PIPELINING) and then fail to implement them properly. When a host
matches hosts_avoid_esmtp, Exim sends HELO rather than EHLO at the start of the
SMTP session. This means that it cannot use any of the ESMTP facilities such as
AUTH, PIPELINING, SIZE, and STARTTLS.

+----------------------------------------------------------------+
|hosts_avoid_pipelining|Use: smtp|Type: host list*|Default: unset|
+----------------------------------------------------------------+

Exim will not use the ESMTP PIPELINING extension when delivering to any host
that matches this list, even if the server host advertises PIPELINING support.

+------------------------------------------------------------+
|hosts_pipe_connect|Use: smtp|Type: host list*|Default: unset|
+------------------------------------------------------------+

If Exim is built with the SUPPORT_PIPE_CONNECT build option this option
controls which to hosts the facility watched for and recorded, and used for
subsequent connections.

The retry hints database is used for the record, and records are subject to the
retry_data_expire option. When used, the pipelining saves on roundtrip times.
It also turns SMTP into a client-first protocol so combines well with TCP Fast
Open.

See also the pipelining_connect_advertise_hosts main option.

Note: When the facility is used, if the transport interface option is unset the
helo_data option will be expanded before the $sending_ip_address variable is
filled in. A check is made for the use of that variable, without the presence
of a "def:" test on it, but suitably complex coding can avoid the check and
produce unexpected results. You have been warned.

+---------------------------------------------------------+
|hosts_avoid_tls|Use: smtp|Type: host list*|Default: unset|
+---------------------------------------------------------+

Exim will not try to start a TLS session when delivering to any host that
matches this list. See chapter 43 for details of TLS.

+----------------------------------------------------------------+
|hosts_verify_avoid_tls|Use: smtp|Type: host list*|Default: unset|
+----------------------------------------------------------------+

Exim will not try to start a TLS session for a verify callout, or when
delivering in cutthrough mode, to any host that matches this list.

+------------------------------------------------+
|hosts_max_try|Use: smtp|Type: integer|Default: 5|
+------------------------------------------------+

This option limits the number of IP addresses that are tried for any one
delivery in cases where there are temporary delivery errors. Section 30.5
describes in detail how the value of this option is used.

+-----------------------------------------------------------+
|hosts_max_try_hardlimit|Use: smtp|Type: integer|Default: 50|
+-----------------------------------------------------------+

This is an additional check on the maximum number of IP addresses that Exim
tries for any one delivery. Section 30.5 describes its use and why it exists.

+----------------------------------------------------------+
|hosts_nopass_tls|Use: smtp|Type: host list*|Default: unset|
+----------------------------------------------------------+

For any host that matches this list, a connection on which a TLS session has
been started will not be passed to a new delivery process for sending another
message on the same connection. See section 43.9 for an explanation of when
this might be needed.

+-----------------------------------------------------------+
|hosts_noproxy_tls|Use: smtp|Type: host list*|Default: unset|
+-----------------------------------------------------------+

For any host that matches this list, a TLS session which has been started will
not be passed to a new delivery process for sending another message on the same
session.

The traditional implementation closes down TLS and re-starts it in the new
process, on the same open TCP connection, for each successive message sent. If
permitted by this option a pipe to to the new process is set up instead, and
the original process maintains the TLS connection and proxies the SMTP
connection from and to the new process and any subsequents. The new process has
no access to TLS information, so cannot include it in logging.

+-----------------------------------------------------+
|hosts_override|Use: smtp|Type: boolean|Default: false|
+-----------------------------------------------------+

If this option is set and the hosts option is also set, any hosts that are
attached to the address are ignored, and instead the hosts specified by the 
hosts option are always used. This option does not apply to fallback_hosts.

+------------------------------------------------------+
|hosts_randomize|Use: smtp|Type: boolean|Default: false|
+------------------------------------------------------+

If this option is set, and either the list of hosts is taken from the hosts or
the fallback_hosts option, or the hosts supplied by the router were not
obtained from MX records (this includes fallback hosts from the router), and
were not randomized by the router, the order of trying the hosts is randomized
each time the transport runs. Randomizing the order of a host list can be used
to do crude load sharing.

When hosts_randomize is true, a host list may be split into groups whose order
is separately randomized. This makes it possible to set up MX-like behaviour.
The boundaries between groups are indicated by an item that is just "+" in the
host list. For example:

hosts = host1:host2:host3:+:host4:host5

The order of the first three hosts and the order of the last two hosts is
randomized for each use, but the first three always end up before the last two.
If hosts_randomize is not set, a "+" item in the list is ignored.

+------------------------------------------------------------+
|hosts_require_auth|Use: smtp|Type: host list*|Default: unset|
+------------------------------------------------------------+

This option provides a list of servers for which authentication must succeed
before Exim will try to transfer a message. If authentication fails for servers
which are not in this list, Exim tries to send unauthenticated. If
authentication fails for one of these servers, delivery is deferred. This
temporary error is detectable in the retry rules, so it can be turned into a
hard failure if required. See also hosts_try_auth, and chapter 33 for details
of authentication.

+----------------------------------------------------------------+
|hosts_request_ocsp|Use: smtp|Type: host list*|Default: see below|
+----------------------------------------------------------------+

Exim will request a Certificate Status on a TLS session for any host that
matches this list. tls_verify_certificates should also be set for the
transport.

The default is "**" if DANE is not in use for the connection, or if DANE-TA us
used. It is empty if DANE-EE is used.

+------------------------------------------------------------+
|hosts_require_alpn|Use: smtp|Type: host list*|Default: unset|
+------------------------------------------------------------+

If the TLS library supports ALPN then a successful negotiation of ALPN will be
required for any host matching the list, for TLS to be used. See also the 
tls_alpn option.

Note: prevention of fallback to in-clear connection is not managed by this
option; see hosts_require_tls.

+------------------------------------------------------------+
|hosts_require_dane|Use: smtp|Type: host list*|Default: unset|
+------------------------------------------------------------+

If built with DANE support, Exim will require that a DNSSEC-validated TLSA
record is present for any host matching the list, and that a DANE-verified TLS
connection is made. There will be no fallback to in-clear communication. See
the dnssec_request_domains router and transport options. See section 43.12.

+------------------------------------------------------------+
|hosts_require_ocsp|Use: smtp|Type: host list*|Default: unset|
+------------------------------------------------------------+

Exim will request, and check for a valid Certificate Status being given, on a
TLS session for any host that matches this list. tls_verify_certificates should
also be set for the transport.

+-----------------------------------------------------------+
|hosts_require_tls|Use: smtp|Type: host list*|Default: unset|
+-----------------------------------------------------------+

Exim will insist on using a TLS session when delivering to any host that
matches this list. See chapter 43 for details of TLS. Note: This option affects
outgoing mail only. To insist on TLS for incoming messages, use an appropriate
ACL.

+--------------------------------------------------------+
|hosts_try_auth|Use: smtp|Type: host list*|Default: unset|
+--------------------------------------------------------+

This option provides a list of servers to which, provided they announce
authentication support, Exim will attempt to authenticate as a client when it
connects. If authentication fails and hosts_require_auth permits, Exim will try
to transfer the message unauthenticated. See also chapter 33 for details of
authentication.

+--------------------------------------------------------+
|hosts_try_chunking|Use: smtp|Type: host list*|Default: *|
+--------------------------------------------------------+

This option provides a list of servers to which, provided they announce
CHUNKING support, Exim will attempt to use BDAT commands rather than DATA.
Unless DKIM signing is being done, BDAT will not be used in conjunction with a
transport filter.

+----------------------------------------------------+
|hosts_try_dane|Use: smtp|Type: host list*|Default: *|
+----------------------------------------------------+

If built with DANE support, Exim will look up a TLSA record for any host
matching the list, If one is found and that lookup was DNSSEC-validated, then
Exim requires that a DANE-verified TLS connection is made for that host; there
will be no fallback to in-clear communication. See the dnssec_request_domains
router and transport options. See section 43.12.

+--------------------------------------------------------+
|hosts_try_fastopen|Use: smtp|Type: host list*|Default: *|
+--------------------------------------------------------+

This option provides a list of servers to which, provided the facility is
supported by this system, Exim will attempt to perform a TCP Fast Open. No data
is sent on the SYN segment but, if the remote server also supports the
facility, it can send its SMTP banner immediately after the SYN,ACK segment.
This can save up to one round-trip time.

The facility is only active for previously-contacted servers, as the initiator
must present a cookie in the SYN segment.

On (at least some) current Linux distributions the facility must be enabled in
the kernel by the sysadmin before the support is usable. There is no option for
control of the server side; if the system supports it it is always enabled.
Note that lengthy operations in the connect ACL, such as DNSBL lookups, will
still delay the emission of the SMTP banner.

+----------------------------------------------------+
|hosts_try_prdr|Use: smtp|Type: host list*|Default: *|
+----------------------------------------------------+

This option provides a list of servers to which, provided they announce PRDR
support, Exim will attempt to negotiate PRDR for multi-recipient messages. The
option can usually be left as default.

+-----------------------------------------------------+
|interface|Use: smtp|Type: string list*|Default: unset|
+-----------------------------------------------------+

This option specifies which interface to bind to when making an outgoing SMTP
call. The value is an IP address, not an interface name such as "eth0". Do not
confuse this with the interface address that was used when a message was
received, which is in $received_ip_address, formerly known as
$interface_address. The name was changed to minimize confusion with the
outgoing interface address. There is no variable that contains an outgoing
interface address because, unless it is set by this option, its value is
unknown.

During the expansion of the interface option the variables $host and
$host_address refer to the host to which a connection is about to be made
during the expansion of the string. Forced expansion failure, or an empty
string result causes the option to be ignored. Otherwise, after expansion, the
string must be a list of IP addresses, colon-separated by default, but the
separator can be changed in the usual way (6.21). For example:

interface = <; 192.168.123.123 ; 3ffe:ffff:836f::fe86:a061

The first interface of the correct type (IPv4 or IPv6) is used for the outgoing
connection. If none of them are the correct type, the option is ignored. If 
interface is not set, or is ignored, the system's IP functions choose which
interface to use if the host has more than one.

+-----------------------------------------------+
|keepalive|Use: smtp|Type: boolean|Default: true|
+-----------------------------------------------+

This option controls the setting of SO_KEEPALIVE on outgoing TCP/IP socket
connections. When set, it causes the kernel to probe idle connections
periodically, by sending packets with "old" sequence numbers. The other end of
the connection should send a acknowledgment if the connection is still okay or
a reset if the connection has been aborted. The reason for doing this is that
it has the beneficial effect of freeing up certain types of connection that can
get stuck when the remote host is disconnected without tidying up the TCP/IP
call properly. The keepalive mechanism takes several hours to detect
unreachable hosts.

+--------------------------------------------------------+
|lmtp_ignore_quota|Use: smtp|Type: boolean|Default: false|
+--------------------------------------------------------+

If this option is set true when the protocol option is set to "lmtp", the
string "IGNOREQUOTA" is added to RCPT commands, provided that the LMTP server
has advertised support for IGNOREQUOTA in its response to the LHLO command.

+----------------------------------------------+
|max_rcpt|Use: smtp|Type: integer*|Default: 100|
+----------------------------------------------+

This option,

after expansion,

limits the number of RCPT commands that are sent in a single SMTP message
transaction. A value setting of zero disables the limit.

If a constant is given,

each set of addresses is treated independently, and so can cause parallel
connections to the same host if remote_max_parallel permits this.

+-------------------------------------------------------------+
|message_linelength_limit|Use: smtp|Type: integer|Default: 998|
+-------------------------------------------------------------+

This option sets the maximum line length, in bytes, that the transport will
send. Any messages with lines exceeding the given value will fail and a
failure-DSN ("bounce") message will if possible be returned to the sender. The
default value is that defined by the SMTP standards.

It is generally wise to also check in the data ACL so that messages received
via SMTP can be refused without producing a bounce.

+---------------------------------------------------+
|multi_domain|Use: smtp|Type: boolean*|Default: true|
+---------------------------------------------------+

When this option is set, the smtp transport can handle a number of addresses
containing a mixture of different domains provided they all resolve to the same
list of hosts. Turning the option off restricts the transport to handling only
one domain at a time. This is useful if you want to use $domain in an expansion
for the transport, because it is set only when there is a single domain
involved in a remote delivery.

It is expanded per-address and can depend on any of $address_data, $domain_data
, $local_part_data, $host, $host_address and $host_port.

If the connection is DANE-enabled then this option is ignored; only messages
having the domain used for the DANE TLSA lookup are sent on the connection.

+-----------------------------------------------+
|port|Use: smtp|Type: string*|Default: see below|
+-----------------------------------------------+

This option specifies the TCP/IP port on the server to which Exim connects. 
Note: Do not confuse this with the port that was used when a message was
received, which is in $received_port, formerly known as $interface_port. The
name was changed to minimize confusion with the outgoing port. There is no
variable that contains an outgoing port.

If the value of this option begins with a digit it is taken as a port number;
otherwise it is looked up using getservbyname(). The default value is normally
"smtp", but if protocol is set to "lmtp" the default is "lmtp" and if protocol
is set to "smtps" the default is "smtps". If the expansion fails, or if a port
number cannot be found, delivery is deferred.

Note that at least one Linux distribution has been seen failing to put "smtps"
in its "/etc/services" file, resulting is such deferrals.

+---------------------------------------------+
|protocol|Use: smtp|Type: string|Default: smtp|
+---------------------------------------------+

If this option is set to "lmtp" instead of "smtp", the default value for the 
port option changes to "lmtp", and the transport operates the LMTP protocol
(RFC 2033) instead of SMTP. This protocol is sometimes used for local
deliveries into closed message stores. Exim also has support for running LMTP
over a pipe to a local process - see chapter 28.

If this option is set to "smtps", the default value for the port option changes
to "smtps", and the transport initiates TLS immediately after connecting, as an
outbound SSL-on-connect, instead of using STARTTLS to upgrade. The Internet
standards bodies used to strongly discourage use of this mode, but as of RFC
8314 it is preferred over STARTTLS for message submission (as distinct from
MTA-MTA communication).

+---------------------------------------------------------------+
|retry_include_ip_address|Use: smtp|Type: boolean*|Default: true|
+---------------------------------------------------------------+

Exim normally includes both the host name and the IP address in the key it
constructs for indexing retry data after a temporary delivery failure. This
means that when one of several IP addresses for a host is failing, it gets
tried periodically (controlled by the retry rules), but use of the other IP
addresses is not affected.

However, in some dialup environments hosts are assigned a different IP address
each time they connect. In this situation the use of the IP address as part of
the retry key leads to undesirable behaviour. Setting this option false causes
Exim to use only the host name. Since it is expanded it can be made to depend
on the host or domain.

+---------------------------------------------------------+
|serialize_hosts|Use: smtp|Type: host list*|Default: unset|
+---------------------------------------------------------+

Because Exim operates in a distributed manner, if several messages for the same
host arrive at around the same time, more than one simultaneous connection to
the remote host can occur. This is not usually a problem except when there is a
slow link between the hosts. In that situation it may be helpful to restrict
Exim to one connection at a time. This can be done by setting serialize_hosts
to match the relevant hosts.

Exim implements serialization by means of a hints database in which a record is
written whenever a process connects to one of the restricted hosts. The record
is deleted when the connection is completed. Obviously there is scope for
records to get left lying around if there is a system or program crash. To
guard against this, Exim ignores any records that are more than six hours old.

If you set up this kind of serialization, you should also arrange to delete the
relevant hints database whenever your system reboots. The names of the files
start with misc and they are kept in the spool/db directory. There may be one
or two files, depending on the type of DBM in use. The same files are used for
ETRN serialization.

See also the max_parallel generic transport option.

+---------------------------------------------------+
|size_addition|Use: smtp|Type: integer|Default: 1024|
+---------------------------------------------------+

If a remote SMTP server indicates that it supports the SIZE option of the MAIL
command, Exim uses this to pass over the message size at the start of an SMTP
transaction. It adds the value of size_addition to the value it sends, to allow
for headers and other text that may be added during delivery by configuration
options or in a transport filter. It may be necessary to increase this if a lot
of text is added to messages.

Alternatively, if the value of size_addition is set negative, it disables the
use of the SIZE option altogether.

+--------------------------------------------------+
|socks_proxy|Use: smtp|Type: string*|Default: unset|
+--------------------------------------------------+

This option enables use of SOCKS proxies for connections made by the transport.
For details see section 59.2.

+-----------------------------------------------+
|tls_alpn|Use: smtp|Type: string*|Default: unset|
+-----------------------------------------------+

If this option is set and the TLS library supports ALPN, the value given is
used.

As of writing no value has been standardised for email use. The authors suggest
using "smtp".

+------------------------------------------------------+
|tls_certificate|Use: smtp|Type: string*|Default: unset|
+------------------------------------------------------+

The value of this option must be the absolute path to a file which contains the
client's certificate, for possible use when sending a message over an encrypted
connection. The values of $host and $host_address are set to the name and
address of the server during the expansion. See chapter 43 for details of TLS.

Note: This option must be set if you want Exim to be able to use a TLS
certificate when sending messages as a client. The global option of the same
name specifies the certificate for Exim as a server; it is not automatically
assumed that the same certificate should be used when Exim is operating as a
client.

+----------------------------------------------+
|tls_crl|Use: smtp|Type: string*|Default: unset|
+----------------------------------------------+

This option specifies a certificate revocation list. The expanded value must be
the name of a file that contains a CRL in PEM format.

+-----------------------------------------------------+
|tls_dh_min_bits|Use: smtp|Type: integer|Default: 1024|
+-----------------------------------------------------+

When establishing a TLS session, if a ciphersuite which uses Diffie-Hellman key
agreement is negotiated, the server will provide a large prime number for use.
This option establishes the minimum acceptable size of that number. If the
parameter offered by the server is too small, then the TLS handshake will fail.

Only supported when using GnuTLS.

+-----------------------------------------------------+
|tls_privatekey|Use: smtp|Type: string*|Default: unset|
+-----------------------------------------------------+

The value of this option must be the absolute path to a file which contains the
client's private key. This is used when sending a message over an encrypted
connection using a client certificate. The values of $host and $host_address
are set to the name and address of the server during the expansion. If this
option is unset, or the expansion is forced to fail, or the result is an empty
string, the private key is assumed to be in the same file as the certificate.
See chapter 43 for details of TLS.

+----------------------------------------------------------+
|tls_require_ciphers|Use: smtp|Type: string*|Default: unset|
+----------------------------------------------------------+

The value of this option must be a list of permitted cipher suites, for use
when setting up an outgoing encrypted connection. (There is a global option of
the same name for controlling incoming connections.) The values of $host and
$host_address are set to the name and address of the server during the
expansion. See chapter 43 for details of TLS; note that this option is used in
different ways by OpenSSL and GnuTLS (see sections 43.4 and 43.5). For GnuTLS,
the order of the ciphers is a preference order.

+--------------------------------------------------------------+
|tls_resumption_hosts|Use: smtp|Type: host list*|Default: unset|
+--------------------------------------------------------------+

This option controls which connections to use the TLS resumption feature. See
43.11 for details.

+----------------------------------------------+
|tls_sni|Use: smtp|Type: string*|Default: unset|
+----------------------------------------------+

If this option is set and the connection is not DANE-validated then it sets the
$tls_out_sni variable and causes any TLS session to pass this value as the
Server Name Indication extension to the remote side, which can be used by the
remote side to select an appropriate certificate and private key for the
session.

See 43.8 for more information.

Note that for OpenSSL, this feature requires a build of OpenSSL that supports
TLS extensions.

+-----------------------------------------------------------+
|tls_tempfail_tryclear|Use: smtp|Type: boolean|Default: true|
+-----------------------------------------------------------+

When the server host is not in hosts_require_tls, and there is a problem in
setting up a TLS session, this option determines whether or not Exim should try
to deliver the message unencrypted. If it is set false, delivery to the current
host is deferred; if there are other hosts, they are tried. If this option is
set true, Exim attempts to deliver unencrypted after a 4xx response to
STARTTLS. Also, if STARTTLS is accepted, but the subsequent TLS negotiation
fails, Exim closes the current connection (because it is in an unknown state),
opens a new one to the same host, and then tries the delivery in clear.

+----------------------------------------------------------+
|tls_try_verify_hosts|Use: smtp|Type: host list*|Default: *|
+----------------------------------------------------------+

This option gives a list of hosts for which, on encrypted connections,
certificate verification will be tried but need not succeed. The 
tls_verify_certificates option must also be set. Note that unless the host is
in this list TLS connections will be denied to hosts using self-signed
certificates when tls_verify_certificates is matched. The
$tls_out_certificate_verified variable is set when certificate verification
succeeds.

+---------------------------------------------------------------+
|tls_verify_cert_hostnames|Use: smtp|Type: host list*|Default: *|
+---------------------------------------------------------------+

This option give a list of hosts for which, while verifying the server
certificate, checks will be included on the host name (note that this will
generally be the result of a DNS MX lookup) versus the Subject-Alternate-Name
(or, if none, Subject-Name) fields. Wildcard names are permitted, limited to
being the initial component of a 3-or-more component FQDN.

There is no equivalent checking on client certificates.

+---------------------------------------------------------------+
|tls_verify_certificates|Use: smtp|Type: string*|Default: system|
+---------------------------------------------------------------+

The value of this option must be either the word "system" or the absolute path
to a file or directory containing permitted certificates for servers, for use
when setting up an encrypted connection.

The "system" value for the option will use a location compiled into the SSL
library. This is not available for GnuTLS versions preceding 3.0.20; a value of
"system" is taken as empty and an explicit location must be specified.

The use of a directory for the option value is not available for GnuTLS
versions preceding 3.3.6 and a single file must be used.

With OpenSSL the certificates specified explicitly either by file or directory
are added to those given by the system default location.

The values of $host and $host_address are set to the name and address of the
server during the expansion of this option. See chapter 43 for details of TLS.

For back-compatibility, if neither tls_verify_hosts nor tls_try_verify_hosts
are set (a single-colon empty list counts as being set) and certificate
verification fails the TLS connection is closed.

+----------------------------------------------------------+
|tls_verify_hosts|Use: smtp|Type: host list*|Default: unset|
+----------------------------------------------------------+

This option gives a list of hosts for which, on encrypted connections,
certificate verification must succeed. The tls_verify_certificates option must
also be set. If both this option and tls_try_verify_hosts are unset operation
is as if this option selected all hosts. Warning: Including a host in 
tls_verify_hosts does not require that connections use TLS. Fallback to
in-clear communication will be done unless restricted by the hosts_require_tls
option.

+-----------------------------------------------------+
|utf8_downconvert|Use: smtp|Type: integer*|Default: -1|
+-----------------------------------------------------+

If built with internationalization support, this option controls conversion of
UTF-8 in message envelope addresses to a-label form. If, after expansion, the
value is 1, 0, or -1 then this value overrides any value previously set for the
message. Otherwise, any previously set value is used. To permit use of a
previous value, set this option to an empty string. For details on the values
see section 60.1.


30.5 How the limits for the number of hosts to try are used
-----------------------------------------------------------

There are two options that are concerned with the number of hosts that are
tried when an SMTP delivery takes place. They are hosts_max_try and 
hosts_max_try_hardlimit.

The hosts_max_try option limits the number of hosts that are tried for a single
delivery. However, despite the term "host" in its name, the option actually
applies to each IP address independently. In other words, a multihomed host is
treated as several independent hosts, just as it is for retrying.

Many of the larger ISPs have multiple MX records which often point to
multihomed hosts. As a result, a list of a dozen or more IP addresses may be
created as a result of routing one of these domains.

Trying every single IP address on such a long list does not seem sensible; if
several at the top of the list fail, it is reasonable to assume there is some
problem that is likely to affect all of them. Roughly speaking, the value of 
hosts_max_try is the maximum number that are tried before deferring the
delivery. However, the logic cannot be quite that simple.

Firstly, IP addresses that are skipped because their retry times have not
arrived do not count, and in addition, addresses that are past their retry
limits are also not counted, even when they are tried. This means that when
some IP addresses are past their retry limits, more than the value of 
hosts_max_retry may be tried. The reason for this behaviour is to ensure that
all IP addresses are considered before timing out an email address (but see
below for an exception).

Secondly, when the hosts_max_try limit is reached, Exim looks down the host
list to see if there is a subsequent host with a different (higher valued) MX.
If there is, that host is considered next, and the current IP address is used
but not counted. This behaviour helps in the case of a domain with a retry rule
that hardly ever delays any hosts, as is now explained:

Consider the case of a long list of hosts with one MX value, and a few with a
higher MX value. If hosts_max_try is small (the default is 5) only a few hosts
at the top of the list are tried at first. With the default retry rule, which
specifies increasing retry times, the higher MX hosts are eventually tried when
those at the top of the list are skipped because they have not reached their
retry times.

However, it is common practice to put a fixed short retry time on domains for
large ISPs, on the grounds that their servers are rarely down for very long.
Unfortunately, these are exactly the domains that tend to resolve to long lists
of hosts. The short retry time means that the lowest MX hosts are tried every
time. The attempts may be in a different order because of random sorting, but
without the special MX check, the higher MX hosts would never be tried until
all the lower MX hosts had timed out (which might be several days), because
there are always some lower MX hosts that have reached their retry times. With
the special check, Exim considers at least one IP address from each MX value at
every delivery attempt, even if the hosts_max_try limit has already been
reached.

The above logic means that hosts_max_try is not a hard limit, and in
particular, Exim normally eventually tries all the IP addresses before timing
out an email address. When hosts_max_try was implemented, this seemed a
reasonable thing to do. Recently, however, some lunatic DNS configurations have
been set up with hundreds of IP addresses for some domains. It can take a very
long time indeed for an address to time out in these cases.

The hosts_max_try_hardlimit option was added to help with this problem. Exim
never tries more than this number of IP addresses; if it hits this limit and
they are all timed out, the email address is bounced, even though not all
possible IP addresses have been tried.



===============================================================================
31. ADDRESS REWRITING

There are some circumstances in which Exim automatically rewrites domains in
addresses. The two most common are when an address is given without a domain
(referred to as an "unqualified address") or when an address contains an
abbreviated domain that is expanded by DNS lookup.

Unqualified envelope addresses are accepted only for locally submitted
messages, or for messages that are received from hosts matching 
sender_unqualified_hosts or recipient_unqualified_hosts, as appropriate.
Unqualified addresses in header lines are qualified if they are in locally
submitted messages, or messages from hosts that are permitted to send
unqualified envelope addresses. Otherwise, unqualified addresses in header
lines are neither qualified nor rewritten.

One situation in which Exim does not automatically rewrite a domain is when it
is the name of a CNAME record in the DNS. The older RFCs suggest that such a
domain should be rewritten using the "canonical" name, and some MTAs do this.
The new RFCs do not contain this suggestion.


31.1 Explicitly configured address rewriting
--------------------------------------------

This chapter describes the rewriting rules that can be used in the main rewrite
section of the configuration file, and also in the generic headers_rewrite
option that can be set on any transport.

Some people believe that configured address rewriting is a Mortal Sin. Others
believe that life is not possible without it. Exim provides the facility; you
do not have to use it.

The main rewriting rules that appear in the "rewrite" section of the
configuration file are applied to addresses in incoming messages, both envelope
addresses and addresses in header lines. Each rule specifies the types of
address to which it applies.

Whether or not addresses in header lines are rewritten depends on the origin of
the headers and the type of rewriting. Global rewriting, that is, rewriting
rules from the rewrite section of the configuration file, is applied only to
those headers that were received with the message. Header lines that are added
by ACLs or by a system filter or by individual routers or transports (which are
specific to individual recipient addresses) are not rewritten by the global
rules.

Rewriting at transport time, by means of the headers_rewrite option, applies
all headers except those added by routers and transports. That is, as well as
the headers that were received with the message, it also applies to headers
that were added by an ACL or a system filter.

In general, rewriting addresses from your own system or domain has some
legitimacy. Rewriting other addresses should be done only with great care and
in special circumstances. The author of Exim believes that rewriting should be
used sparingly, and mainly for "regularizing" addresses in your own domains.
Although it can sometimes be used as a routing tool, this is very strongly
discouraged.

There are two commonly encountered circumstances where rewriting is used, as
illustrated by these examples:

  * The company whose domain is hitch.fict.example has a number of hosts that
    exchange mail with each other behind a firewall, but there is only a single
    gateway to the outer world. The gateway rewrites *.hitch.fict.example as 
    hitch.fict.example when sending mail off-site.

  * A host rewrites the local parts of its own users so that, for example, 
    fp42@hitch.fict.example becomes Ford.Prefect@hitch.fict.example.


31.2 When does rewriting happen?
--------------------------------

Configured address rewriting can take place at several different stages of a
message's processing.

At the start of an ACL for MAIL, the sender address may have been rewritten by
a special SMTP-time rewrite rule (see section 31.6.3), but no ordinary rewrite
rules have yet been applied. If, however, the sender address is verified in the
ACL, it is rewritten before verification, and remains rewritten thereafter. The
subsequent value of $sender_address is the rewritten address. This also applies
if sender verification happens in a RCPT ACL. Otherwise, when the sender
address is not verified, it is rewritten as soon as a message's header lines
have been received.

Similarly, at the start of an ACL for RCPT, the current recipient's address may
have been rewritten by a special SMTP-time rewrite rule, but no ordinary
rewrite rules have yet been applied to it. However, the behaviour is different
from the sender address when a recipient is verified. The address is rewritten
for the verification, but the rewriting is not remembered at this stage. The
value of $local_part and $domain after verification are always the same as they
were before (that is, they contain the unrewritten - except for SMTP-time
rewriting - address).

As soon as a message's header lines have been received, all the envelope
recipient addresses are permanently rewritten, and rewriting is also applied to
the addresses in the header lines (if configured). This happens before adding
any header lines that were specified in MAIL or RCPT ACLs, and before the DATA
ACL and local_scan() functions are run.

When an address is being routed, either for delivery or for verification,
rewriting is applied immediately to child addresses that are generated by
redirection, unless no_rewrite is set on the router.

At transport time, additional rewriting of addresses in header lines can be
specified by setting the generic headers_rewrite option on a transport. This
option contains rules that are identical in form to those in the rewrite
section of the configuration file. They are applied to the original message
header lines and any that were added by ACLs or a system filter. They are not
applied to header lines that are added by routers or the transport.

The outgoing envelope sender can be rewritten by means of the return_path
transport option. However, it is not possible to rewrite envelope recipients at
transport time.


31.3 Testing the rewriting rules that apply on input
----------------------------------------------------

Exim's input rewriting configuration appears in a part of the runtime
configuration file headed by "begin rewrite". It can be tested by the -brw
command line option. This takes an address (which can be a full RFC 2822
address) as its argument. The output is a list of how the address would be
transformed by the rewriting rules for each of the different places it might
appear in an incoming message, that is, for each different header and for the
envelope sender and recipient fields. For example,

exim -brw ph10@exim.workshop.example

might produce the output

sender: Philip.Hazel@exim.workshop.example
from: Philip.Hazel@exim.workshop.example
to: ph10@exim.workshop.example
cc: ph10@exim.workshop.example
bcc: ph10@exim.workshop.example
reply-to: Philip.Hazel@exim.workshop.example
env-from: Philip.Hazel@exim.workshop.example
env-to: ph10@exim.workshop.example

which shows that rewriting has been set up for that address when used in any of
the source fields, but not when it appears as a recipient address. At the
present time, there is no equivalent way of testing rewriting rules that are
set for a particular transport.


31.4 Rewriting rules
--------------------

The rewrite section of the configuration file consists of lines of rewriting
rules in the form

<source pattern>  <replacement>  <flags>

Rewriting rules that are specified for the headers_rewrite generic transport
option are given as a colon-separated list. Each item in the list takes the
same form as a line in the main rewriting configuration (except that any colons
must be doubled, of course).

The formats of source patterns and replacement strings are described below.
Each is terminated by white space, unless enclosed in double quotes, in which
case normal quoting conventions apply inside the quotes. The flags are single
characters which may appear in any order. Spaces and tabs between them are
ignored.

For each address that could potentially be rewritten, the rules are scanned in
order, and replacements for the address from earlier rules can themselves be
replaced by later rules (but see the "q" and "R" flags).

The order in which addresses are rewritten is undefined, may change between
releases, and must not be relied on, with one exception: when a message is
received, the envelope sender is always rewritten first, before any header
lines are rewritten. For example, the replacement string for a rewrite of an
address in To: must not assume that the message's address in From: has (or has
not) already been rewritten. However, a rewrite of From: may assume that the
envelope sender has already been rewritten.

The variables $local_part and $domain can be used in the replacement string to
refer to the address that is being rewritten. Note that lookup-driven rewriting
can be done by a rule of the form

*@*   ${lookup ...

where the lookup key uses $1 and $2 or $local_part and $domain to refer to the
address that is being rewritten.


31.5 Rewriting patterns
-----------------------

The source pattern in a rewriting rule is any item which may appear in an
address list (see section 10.5). It is in fact processed as a single-item
address list, which means that it is expanded before being tested against the
address. As always, if you use a regular expression as a pattern, you must take
care to escape dollar and backslash characters, or use the "\N" facility to
suppress string expansion within the regular expression.

Domains in patterns should be given in lower case. Local parts in patterns are
case-sensitive. If you want to do case-insensitive matching of local parts, you
can use a regular expression that starts with "^(?i)".

After matching, the numerical variables $1, $2, etc. may be set, depending on
the type of match which occurred. These can be used in the replacement string
to insert portions of the incoming address. $0 always refers to the complete
incoming address. When a regular expression is used, the numerical variables
are set from its capturing subexpressions. For other types of pattern they are
set as follows:

  * If a local part or domain starts with an asterisk, the numerical variables
    refer to the character strings matched by asterisks, with $1 associated
    with the first asterisk, and $2 with the second, if present. For example,
    if the pattern

    *queen@*.fict.example

    is matched against the address hearts-queen@wonderland.fict.example then

    $0 = hearts-queen@wonderland.fict.example
    $1 = hearts-
    $2 = wonderland

    Note that if the local part does not start with an asterisk, but the domain
    does, it is $1 that contains the wild part of the domain.

  * If the domain part of the pattern is a partial lookup, the wild and fixed
    parts of the domain are placed in the next available numerical variables.
    Suppose, for example, that the address foo@bar.baz.example is processed by
    a rewriting rule of the form

    *@partial-dbm;/some/dbm/file    <replacement string>

    and the key in the file that matches the domain is "*.baz.example". Then

    $1 = foo
    $2 = bar
    $3 = baz.example

    If the address foo@baz.example is looked up, this matches the same wildcard
    file entry, and in this case $2 is set to the empty string, but $3 is still
    set to baz.example. If a non-wild key is matched in a partial lookup, $2 is
    again set to the empty string and $3 is set to the whole domain. For
    non-partial domain lookups, no numerical variables are set.


31.6 Rewriting replacements
---------------------------

If the replacement string for a rule is a single asterisk, addresses that match
the pattern and the flags are not rewritten, and no subsequent rewriting rules
are scanned. For example,

hatta@lookingglass.fict.example  *  f

specifies that hatta@lookingglass.fict.example is never to be rewritten in 
From: headers.

If the replacement string is not a single asterisk, it is expanded, and must
yield a fully qualified address. Within the expansion, the variables
$local_part and $domain refer to the address that is being rewritten. Any
letters they contain retain their original case - they are not lower cased. The
numerical variables are set up according to the type of pattern that matched
the address, as described above. If the expansion is forced to fail by the
presence of "fail" in a conditional or lookup item, rewriting by the current
rule is abandoned, but subsequent rules may take effect. Any other expansion
failure causes the entire rewriting operation to be abandoned, and an entry
written to the panic log.

31.6.1 Rewriting flags

There are three different kinds of flag that may appear on rewriting rules:

  * Flags that specify which headers and envelope addresses to rewrite: E, F,
    T, b, c, f, h, r, s, t.

  * A flag that specifies rewriting at SMTP time: S.

  * Flags that control the rewriting process: Q, q, R, w.

For rules that are part of the headers_rewrite generic transport option, E, F,
T, and S are not permitted.

31.6.2 Flags specifying which headers and envelope addresses to rewrite

If none of the following flag letters, nor the "S" flag (see section 31.6.3)
are present, a main rewriting rule applies to all headers and to both the
sender and recipient fields of the envelope, whereas a transport-time rewriting
rule just applies to all headers. Otherwise, the rewriting rule is skipped
unless the relevant addresses are being processed.

E       rewrite all envelope fields
F       rewrite the envelope From field
T       rewrite the envelope To field
b       rewrite the Bcc: header
c       rewrite the Cc: header
f       rewrite the From: header
h       rewrite all headers
r       rewrite the Reply-To: header
s       rewrite the Sender: header
t       rewrite the To: header

"All headers" means all of the headers listed above that can be selected
individually, plus their Resent- versions. It does not include other headers
such as Subject: etc.

You should be particularly careful about rewriting Sender: headers, and
restrict this to special known cases in your own domains.

31.6.3 The SMTP-time rewriting flag

The rewrite flag "S" specifies a rewrite of incoming envelope addresses at SMTP
time, as soon as an address is received in a MAIL or RCPT command, and before
any other processing; even before syntax checking. The pattern is required to
be a regular expression, and it is matched against the whole of the data for
the command, including any surrounding angle brackets.

This form of rewrite rule allows for the handling of addresses that are not
compliant with RFCs 2821 and 2822 (for example, "bang paths" in batched SMTP
input). Because the input is not required to be a syntactically valid address,
the variables $local_part and $domain are not available during the expansion of
the replacement string. The result of rewriting replaces the original address
in the MAIL or RCPT command.

31.6.4 Flags controlling the rewriting process

There are four flags which control the way the rewriting process works. These
take effect only when a rule is invoked, that is, when the address is of the
correct type (matches the flags) and matches the pattern:

  * If the "Q" flag is set on a rule, the rewritten address is permitted to be
    an unqualified local part. It is qualified with qualify_recipient. In the
    absence of "Q" the rewritten address must always include a domain.

  * If the "q" flag is set on a rule, no further rewriting rules are
    considered, even if no rewriting actually takes place because of a "fail"
    in the expansion. The "q" flag is not effective if the address is of the
    wrong type (does not match the flags) or does not match the pattern.

  * The "R" flag causes a successful rewriting rule to be re-applied to the new
    address, up to ten times. It can be combined with the "q" flag, to stop
    rewriting once it fails to match (after at least one successful rewrite).

  * When an address in a header is rewritten, the rewriting normally applies
    only to the working part of the address, with any comments and RFC 2822
    "phrase" left unchanged. For example, rewriting might change

    From: Ford Prefect <fp42@restaurant.hitch.fict.example>

    into

    From: Ford Prefect <prefectf@hitch.fict.example>

    Sometimes there is a need to replace the whole address item, and this can
    be done by adding the flag letter "w" to a rule. If this is set on a rule
    that causes an address in a header line to be rewritten, the entire address
    is replaced, not just the working part. The replacement must be a complete
    RFC 2822 address, including the angle brackets if necessary. If text
    outside angle brackets contains a character whose value is greater than 126
    or less than 32 (except for tab), the text is encoded according to RFC
    2047. The character set is taken from headers_charset, which gets its
    default at build time.

    When the "w" flag is set on a rule that causes an envelope address to be
    rewritten, all but the working part of the replacement address is
    discarded.


31.7 Rewriting examples
-----------------------

Here is an example of the two common rewriting paradigms:

*@*.hitch.fict.example  $1@hitch.fict.example
*@hitch.fict.example    ${lookup{$1}dbm{/etc/realnames}\
                     {$value}fail}@hitch.fict.example bctfrF

Note the use of "fail" in the lookup expansion in the second rule, forcing the
string expansion to fail if the lookup does not succeed. In this context it has
the effect of leaving the original address unchanged, but Exim goes on to
consider subsequent rewriting rules, if any, because the "q" flag is not
present in that rule. An alternative to "fail" would be to supply $1
explicitly, which would cause the rewritten address to be the same as before,
at the cost of a small bit of processing. Not supplying either of these is an
error, since the rewritten address would then contain no local part.

The first example above replaces the domain with a superior, more general
domain. This may not be desirable for certain local parts. If the rule

root@*.hitch.fict.example  *

were inserted before the first rule, rewriting would be suppressed for the
local part root at any domain ending in hitch.fict.example.

Rewriting can be made conditional on a number of tests, by making use of ${if
in the expansion item. For example, to apply a rewriting rule only to messages
that originate outside the local host:

*@*.hitch.fict.example  "${if !eq {$sender_host_address}{}\
                         {$1@hitch.fict.example}fail}"

The replacement string is quoted in this example because it contains white
space.

Exim does not handle addresses in the form of "bang paths". If it sees such an
address it treats it as an unqualified local part which it qualifies with the
local qualification domain (if the source of the message is local or if the
remote host is permitted to send unqualified addresses). Rewriting can
sometimes be used to handle simple bang paths with a fixed number of
components. For example, the rule

\N^([^!]+)!(.*)@your.domain.example$\N   $2@$1

rewrites a two-component bang path host.name!user as the domain address 
user@host.name. However, there is a security implication in using this as a
global rewriting rule for envelope addresses. It can provide a backdoor method
for using your system as a relay, because the incoming addresses appear to be
local. If the bang path addresses are received via SMTP, it is safer to use the
"S" flag to rewrite them as they are received, so that relay checking can be
done on the rewritten addresses.



===============================================================================
32. RETRY CONFIGURATION

The "retry" section of the runtime configuration file contains a list of retry
rules that control how often Exim tries to deliver messages that cannot be
delivered at the first attempt. If there are no retry rules (the section is
empty or not present), there are no retries. In this situation, temporary
errors are treated as permanent. The default configuration contains a single,
general-purpose retry rule (see section 7.6). The -brt command line option can
be used to test which retry rule will be used for a given address, domain and
error.

The most common cause of retries is temporary failure to deliver to a remote
host because the host is down, or inaccessible because of a network problem.
Exim's retry processing in this case is applied on a per-host (strictly, per IP
address) basis, not on a per-message basis. Thus, if one message has recently
been delayed, delivery of a new message to the same host is not immediately
tried, but waits for the host's retry time to arrive. If the retry_defer log
selector is set, the message "retry time not reached" is written to the main
log whenever a delivery is skipped for this reason. Section 49.1.1 contains
more details of the handling of errors during remote deliveries.

Retry processing applies to routing as well as to delivering, except as covered
in the next paragraph. The retry rules do not distinguish between these
actions. It is not possible, for example, to specify different behaviour for
failures to route the domain snark.fict.example and failures to deliver to the
host snark.fict.example. I didn't think anyone would ever need this added
complication, so did not implement it. However, although they share the same
retry rule, the actual retry times for routing and transporting a given domain
are maintained independently.

When a delivery is not part of a queue run (typically an immediate delivery on
receipt of a message), the routers are always run, and local deliveries are
always attempted, even if retry times are set for them. This makes for better
behaviour if one particular message is causing problems (for example, causing
quota overflow, or provoking an error in a filter file). If such a delivery
suffers a temporary failure, the retry data is updated as normal, and
subsequent delivery attempts from queue runs occur only when the retry time for
the local address is reached.


32.1 Changing retry rules
-------------------------

If you change the retry rules in your configuration, you should consider
whether or not to delete the retry data that is stored in Exim's spool area in
files with names like db/retry. Deleting any of Exim's hints files is always
safe; that is why they are called "hints".

The hints retry data contains suggested retry times based on the previous
rules. In the case of a long-running problem with a remote host, it might
record the fact that the host has timed out. If your new rules increase the
timeout time for such a host, you should definitely remove the old retry data
and let Exim recreate it, based on the new rules. Otherwise Exim might bounce
messages that it should now be retaining.


32.2 Format of retry rules
--------------------------

Each retry rule occupies one line and consists of three or four parts,
separated by white space: a pattern, an error name, an optional list of sender
addresses, and a list of retry parameters. The pattern and sender lists must be
enclosed in double quotes if they contain white space. The rules are searched
in order until one is found where the pattern, error name, and sender list (if
present) match the failing host or address, the error that occurred, and the
message's sender, respectively.

The pattern is any single item that may appear in an address list (see section
10.5). It is in fact processed as a one-item address list, which means that it
is expanded before being tested against the address that has been delayed. A
negated address list item is permitted. Address list processing treats a plain
domain name as if it were preceded by "*@", which makes it possible for many
retry rules to start with just a domain. For example,

lookingglass.fict.example        *  F,24h,30m;

provides a rule for any address in the lookingglass.fict.example domain,
whereas

alice@lookingglass.fict.example  *  F,24h,30m;

applies only to temporary failures involving the local part alice. In practice,
almost all rules start with a domain name pattern without a local part.

Warning: If you use a regular expression in a retry rule pattern, it must match
a complete address, not just a domain, because that is how regular expressions
work in address lists.

^\Nxyz\d+\.abc\.example$\N        *  G,1h,10m,2     Wrong
^\N[^@]+@xyz\d+\.abc\.example$\N  *  G,1h,10m,2     Right


32.3 Choosing which retry rule to use for address errors
--------------------------------------------------------

When Exim is looking for a retry rule after a routing attempt has failed (for
example, after a DNS timeout), each line in the retry configuration is tested
against the complete address only if retry_use_local_part is set for the
router. Otherwise, only the domain is used, except when matching against a
regular expression, when the local part of the address is replaced with "*". A
domain on its own can match a domain pattern, or a pattern that starts with
"*@". By default, retry_use_local_part is true for routers where 
check_local_user is true, and false for other routers.

Similarly, when Exim is looking for a retry rule after a local delivery has
failed (for example, after a mailbox full error), each line in the retry
configuration is tested against the complete address only if 
retry_use_local_part is set for the transport (it defaults true for all local
transports).

However, when Exim is looking for a retry rule after a remote delivery attempt
suffers an address error (a 4xx SMTP response for a recipient address), the
whole address is always used as the key when searching the retry rules. The
rule that is found is used to create a retry time for the combination of the
failing address and the message's sender. It is the combination of sender and
recipient that is delayed in subsequent queue runs until its retry time is
reached. You can delay the recipient without regard to the sender by setting 
address_retry_include_sender false in the smtp transport but this can lead to
problems with servers that regularly issue 4xx responses to RCPT commands.


32.4 Choosing which retry rule to use for host and message errors
-----------------------------------------------------------------

For a temporary error that is not related to an individual address (for
example, a connection timeout), each line in the retry configuration is checked
twice. First, the name of the remote host is used as a domain name (preceded by
"*@" when matching a regular expression). If this does not match the line, the
domain from the email address is tried in a similar fashion. For example,
suppose the MX records for a.b.c.example are

a.b.c.example  MX  5  x.y.z.example
               MX  6  p.q.r.example
               MX  7  m.n.o.example

and the retry rules are

p.q.r.example    *      F,24h,30m;
a.b.c.example    *      F,4d,45m;

and a delivery to the host x.y.z.example suffers a connection failure. The
first rule matches neither the host nor the domain, so Exim looks at the second
rule. This does not match the host, but it does match the domain, so it is used
to calculate the retry time for the host x.y.z.example. Meanwhile, Exim tries
to deliver to p.q.r.example. If this also suffers a host error, the first retry
rule is used, because it matches the host.

In other words, temporary failures to deliver to host p.q.r.example use the
first rule to determine retry times, but for all the other hosts for the domain
a.b.c.example, the second rule is used. The second rule is also used if routing
to a.b.c.example suffers a temporary failure.

Note: The host name is used when matching the patterns, not its IP address.
However, if a message is routed directly to an IP address without the use of a
host name, for example, if a manualroute router contains a setting such as:

route_list = *.a.example  192.168.34.23

then the "host name" that is used when searching for a retry rule is the
textual form of the IP address.


32.5 Retry rules for specific errors
------------------------------------

The second field in a retry rule is the name of a particular error, or an
asterisk, which matches any error. The errors that can be tested for are:

auth_failed

    Authentication failed when trying to send to a host in the 
    hosts_require_auth list in an smtp transport.

data_4xx

    A 4xx error was received for an outgoing DATA command, either immediately
    after the command, or after sending the message's data.

mail_4xx

    A 4xx error was received for an outgoing MAIL command.

rcpt_4xx

    A 4xx error was received for an outgoing RCPT command.

For the three 4xx errors, either the first or both of the x's can be given as
specific digits, for example: "mail_45x" or "rcpt_436". For example, to
recognize 452 errors given to RCPT commands for addresses in a certain domain,
and have retries every ten minutes with a one-hour timeout, you could set up a
retry rule of this form:

the.domain.name  rcpt_452   F,1h,10m

These errors apply to both outgoing SMTP (the smtp transport) and outgoing LMTP
(either the lmtp transport, or the smtp transport in LMTP mode).

lost_connection

    A server unexpectedly closed the SMTP connection. There may, of course,
    legitimate reasons for this (host died, network died), but if it repeats a
    lot for the same host, it indicates something odd.

lookup

    A DNS lookup for a host failed. Note that a dnslookup router will need to
    have matched its fail_defer_domains option for this retry type to be
    usable. Also note that a manualroute router will probably need its 
    host_find_failed option set to defer.

refused_MX

    A connection to a host obtained from an MX record was refused.

refused_A

    A connection to a host not obtained from an MX record was refused.

refused

    A connection was refused.

timeout_connect_MX

    A connection attempt to a host obtained from an MX record timed out.

timeout_connect_A

    A connection attempt to a host not obtained from an MX record timed out.

timeout_connect

    A connection attempt timed out.

timeout_MX

    There was a timeout while connecting or during an SMTP session with a host
    obtained from an MX record.

timeout_A

    There was a timeout while connecting or during an SMTP session with a host
    not obtained from an MX record.

timeout

    There was a timeout while connecting or during an SMTP session.

tls_required

    The server was required to use TLS (it matched hosts_require_tls in the 
    smtp transport), but either did not offer TLS, or it responded with 4xx to
    STARTTLS, or there was a problem setting up the TLS connection.

quota

    A mailbox quota was exceeded in a local delivery by the appendfile
    transport.

quota_<time>

    A mailbox quota was exceeded in a local delivery by the appendfile
    transport, and the mailbox has not been accessed for <time>. For example, 
    quota_4d applies to a quota error when the mailbox has not been accessed
    for four days.

The idea of quota_<time> is to make it possible to have shorter timeouts when
the mailbox is full and is not being read by its owner. Ideally, it should be
based on the last time that the user accessed the mailbox. However, it is not
always possible to determine this. Exim uses the following heuristic rules:

  * If the mailbox is a single file, the time of last access (the "atime") is
    used. As no new messages are being delivered (because the mailbox is over
    quota), Exim does not access the file, so this is the time of last user
    access.

  * For a maildir delivery, the time of last modification of the new
    subdirectory is used. As the mailbox is over quota, no new files are
    created in the new subdirectory, because no new messages are being
    delivered. Any change to the new subdirectory is therefore assumed to be
    the result of an MUA moving a new message to the cur directory when it is
    first read. The time that is used is therefore the last time that the user
    read a new message.

  * For other kinds of multi-file mailbox, the time of last access cannot be
    obtained, so a retry rule that uses this type of error field is never
    matched.

The quota errors apply both to system-enforced quotas and to Exim's own quota
mechanism in the appendfile transport. The quota error also applies when a
local delivery is deferred because a partition is full (the ENOSPC error).


32.6 Retry rules for specified senders
--------------------------------------

You can specify retry rules that apply only when the failing message has a
specific sender. In particular, this can be used to define retry rules that
apply only to bounce messages. The third item in a retry rule can be of this
form:

senders=<address list>

The retry timings themselves are then the fourth item. For example:

*   rcpt_4xx   senders=:   F,1h,30m

matches recipient 4xx errors for bounce messages sent to any address at any
host. If the address list contains white space, it must be enclosed in quotes.
For example:

a.domain  rcpt_452  senders="xb.dom : yc.dom"  G,8h,10m,1.5

Warning: This facility can be unhelpful if it is used for host errors (which do
not depend on the recipient). The reason is that the sender is used only to
match the retry rule. Once the rule has been found for a host error, its
contents are used to set a retry time for the host, and this will apply to all
messages, not just those with specific senders.

When testing retry rules using -brt, you can supply a sender using the -f
command line option, like this:

exim -f "" -brt user@dom.ain

If you do not set -f with -brt, a retry rule that contains a senders list is
never matched.


32.7 Retry parameters
---------------------

The third (or fourth, if a senders list is present) field in a retry rule is a
sequence of retry parameter sets, separated by semicolons. Each set consists of

<letter>,<cutoff time>,<arguments>

The letter identifies the algorithm for computing a new retry time; the cutoff
time is the time beyond which this algorithm no longer applies, and the
arguments vary the algorithm's action. The cutoff time is measured from the
time that the first failure for the domain (combined with the local part if
relevant) was detected, not from the time the message was received.

The available algorithms are:

  * F: retry at fixed intervals. There is a single time parameter specifying
    the interval.

  * G: retry at geometrically increasing intervals. The first argument
    specifies a starting value for the interval, and the second a multiplier,
    which is used to increase the size of the interval at each retry.

  * H: retry at randomized intervals. The arguments are as for G. For each
    retry, the previous interval is multiplied by the factor in order to get a
    maximum for the next interval. The minimum interval is the first argument
    of the parameter, and an actual interval is chosen randomly between them.
    Such a rule has been found to be helpful in cluster configurations when all
    the members of the cluster restart at once, and may therefore synchronize
    their queue processing times.

When computing the next retry time, the algorithm definitions are scanned in
order until one whose cutoff time has not yet passed is reached. This is then
used to compute a new retry time that is later than the current time. In the
case of fixed interval retries, this simply means adding the interval to the
current time. For geometrically increasing intervals, retry intervals are
computed from the rule's parameters until one that is greater than the previous
interval is found. The main configuration variable retry_interval_max limits
the maximum interval between retries. It cannot be set greater than "24h",
which is its default value.

A single remote domain may have a number of hosts associated with it, and each
host may have more than one IP address. Retry algorithms are selected on the
basis of the domain name, but are applied to each IP address independently. If,
for example, a host has two IP addresses and one is unusable, Exim will
generate retry times for it and will not try to use it until its next retry
time comes. Thus the good IP address is likely to be tried first most of the
time.

Retry times are hints rather than promises. Exim does not make any attempt to
run deliveries exactly at the computed times. Instead, a queue runner process
starts delivery processes for delayed messages periodically, and these attempt
new deliveries only for those addresses that have passed their next retry time.
If a new message arrives for a deferred address, an immediate delivery attempt
occurs only if the address has passed its retry time. In the absence of new
messages, the minimum time between retries is the interval between queue runner
processes. There is not much point in setting retry times of five minutes if
your queue runners happen only once an hour, unless there are a significant
number of incoming messages (which might be the case on a system that is
sending everything to a smart host, for example).

The data in the retry hints database can be inspected by using the exim_dumpdb
or exim_fixdb utility programs (see chapter 54). The latter utility can also be
used to change the data. The exinext utility script can be used to find out
what the next retry times are for the hosts associated with a particular mail
domain, and also for local deliveries that have been deferred.


32.8 Retry rule examples
------------------------

Here are some example retry rules:

alice@wonderland.fict.example quota_5d  F,7d,3h
wonderland.fict.example       quota_5d
wonderland.fict.example       *         F,1h,15m; G,2d,1h,2;
lookingglass.fict.example     *         F,24h,30m;
*                 refused_A   F,2h,20m;
*                 *           F,2h,15m; G,16h,1h,1.5; F,5d,8h

The first rule sets up special handling for mail to 
alice@wonderland.fict.example when there is an over-quota error and the mailbox
has not been read for at least 5 days. Retries continue every three hours for 7
days. The second rule handles over-quota errors for all other local parts at 
wonderland.fict.example; the absence of a local part has the same effect as
supplying "*@". As no retry algorithms are supplied, messages that fail are
bounced immediately if the mailbox has not been read for at least 5 days.

The third rule handles all other errors at wonderland.fict.example; retries
happen every 15 minutes for an hour, then with geometrically increasing
intervals until two days have passed since a delivery first failed. After the
first hour there is a delay of one hour, then two hours, then four hours, and
so on (this is a rather extreme example).

The fourth rule controls retries for the domain lookingglass.fict.example. They
happen every 30 minutes for 24 hours only. The remaining two rules handle all
other domains, with special action for connection refusal from hosts that were
not obtained from an MX record.

The final rule in a retry configuration should always have asterisks in the
first two fields so as to provide a general catch-all for any addresses that do
not have their own special handling. This example tries every 15 minutes for 2
hours, then with intervals starting at one hour and increasing by a factor of
1.5 up to 16 hours, then every 8 hours up to 5 days.


32.9 Timeout of retry data
--------------------------

Exim timestamps the data that it writes to its retry hints database. When it
consults the data during a delivery it ignores any that is older than the value
set in retry_data_expire (default 7 days). If, for example, a host hasn't been
tried for 7 days, Exim will try to deliver to it immediately a message arrives,
and if that fails, it will calculate a retry time as if it were failing for the
first time.

This improves the behaviour for messages routed to rarely-used hosts such as MX
backups. If such a host was down at one time, and happens to be down again when
Exim tries a month later, using the old retry data would imply that it had been
down all the time, which is not a justified assumption.

If a host really is permanently dead, this behaviour causes a burst of retries
every now and again, but only if messages routed to it are rare. If there is a
message at least once every 7 days the retry data never expires.


32.10 Long-term failures
------------------------

Special processing happens when an email address has been failing for so long
that the cutoff time for the last algorithm is reached. For example, using the
default retry rule:

* * F,2h,15m; G,16h,1h,1.5; F,4d,6h

the cutoff time is four days. Reaching the retry cutoff is independent of how
long any specific message has been failing; it is the length of continuous
failure for the recipient address that counts.

When the cutoff time is reached for a local delivery, or for all the IP
addresses associated with a remote delivery, a subsequent delivery failure
causes Exim to give up on the address, and a bounce message is generated. In
order to cater for new messages that use the failing address, a next retry time
is still computed from the final algorithm, and is used as follows:

For local deliveries, one delivery attempt is always made for any subsequent
messages. If this delivery fails, the address fails immediately. The
post-cutoff retry time is not used.

If the delivery is remote, there are two possibilities, controlled by the 
delay_after_cutoff option of the smtp transport. The option is true by default.
Until the post-cutoff retry time for one of the IP addresses, as set by the 
retry_data_expire option, is reached, the failing email address is bounced
immediately, without a delivery attempt taking place. After that time, one new
delivery attempt is made to those IP addresses that are past their retry times,
and if that still fails, the address is bounced and new retry times are
computed.

In other words, when all the hosts for a given email address have been failing
for a long time, Exim bounces rather then defers until one of the hosts' retry
times is reached. Then it tries once, and bounces if that attempt fails. This
behaviour ensures that few resources are wasted in repeatedly trying to deliver
to a broken destination, but if the host does recover, Exim will eventually
notice.

If delay_after_cutoff is set false, Exim behaves differently. If all IP
addresses are past their final cutoff time, Exim tries to deliver to those IP
addresses that have not been tried since the message arrived. If there are no
suitable IP addresses, or if they all fail, the address is bounced. In other
words, it does not delay when a new message arrives, but tries the expired
addresses immediately, unless they have been tried since the message arrived.
If there is a continuous stream of messages for the failing domains, setting 
delay_after_cutoff false means that there will be many more attempts to deliver
to permanently failing IP addresses than when delay_after_cutoff is true.


32.11 Deliveries that work intermittently
-----------------------------------------

Some additional logic is needed to cope with cases where a host is
intermittently available, or when a message has some attribute that prevents
its delivery when others to the same address get through. In this situation,
because some messages are successfully delivered, the "retry clock" for the
host or address keeps getting reset by the successful deliveries, and so
failing messages remain in the queue for ever because the cutoff time is never
reached.

Two exceptional actions are applied to prevent this happening. The first
applies to errors that are related to a message rather than a remote host.
Section 49.1.1 has a discussion of the different kinds of error; examples of
message-related errors are 4xx responses to MAIL or DATA commands, and quota
failures. For this type of error, if a message's arrival time is earlier than
the "first failed" time for the error, the earlier time is used when scanning
the retry rules to decide when to try next and when to time out the address.

The exceptional second action applies in all cases. If a message has been on
the queue for longer than the cutoff time of any applicable retry rule for a
given address, a delivery is attempted for that address, even if it is not yet
time, and if this delivery fails, the address is timed out. A new retry time is
not computed in this case, so that other messages for the same address are
considered immediately.



===============================================================================
33. SMTP AUTHENTICATION

The "authenticators" section of Exim's runtime configuration is concerned with
SMTP authentication. This facility is an extension to the SMTP protocol,
described in RFC 2554, which allows a client SMTP host to authenticate itself
to a server. This is a common way for a server to recognize clients that are
permitted to use it as a relay. SMTP authentication is not of relevance to the
transfer of mail between servers that have no managerial connection with each
other.

The name of an authenticator is limited to be 64 ASCII characters long; prior
to Exim 4.95 names would be silently truncated at this length, but now it is
enforced.

Very briefly, the way SMTP authentication works is as follows:

 1. The server advertises a number of authentication mechanisms in response to
    the client's EHLO command.

 2. The client issues an AUTH command, naming a specific mechanism. The command
    may, optionally, contain some authentication data.

 3. The server may issue one or more challenges, to which the client must send
    appropriate responses. In simple authentication mechanisms, the challenges
    are just prompts for user names and passwords. The server does not have to
    issue any challenges - in some mechanisms the relevant data may all be
    transmitted with the AUTH command.

 4. The server either accepts or denies authentication.

 5. If authentication succeeds, the client may optionally make use of the AUTH
    option on the MAIL command to pass an authenticated sender in subsequent
    mail transactions. Authentication lasts for the remainder of the SMTP
    connection.

 6. If authentication fails, the client may give up, or it may try a different
    authentication mechanism, or it may try transferring mail over the
    unauthenticated connection.

If you are setting up a client, and want to know which authentication
mechanisms the server supports, you can use Telnet to connect to port 25 (the
SMTP port) on the server, and issue an EHLO command. The response to this
includes the list of supported mechanisms. For example:

$ telnet server.example 25
Trying 192.168.34.25...
Connected to server.example.
Escape character is '^]'.
220 server.example ESMTP Exim 4.20 ...
ehlo client.example
250-server.example Hello client.example [10.8.4.5]
250-SIZE 52428800
250-PIPELINING
250-AUTH PLAIN
250 HELP

The second-last line of this example output shows that the server supports
authentication using the PLAIN mechanism. In Exim, the different authentication
mechanisms are configured by specifying authenticator drivers. Like the routers
and transports, which authenticators are included in the binary is controlled
by build-time definitions. The following are currently available, included by
setting

AUTH_CRAM_MD5=yes
AUTH_CYRUS_SASL=yes
AUTH_DOVECOT=yes
AUTH_EXTERNAL=yes
AUTH_GSASL=yes
AUTH_HEIMDAL_GSSAPI=yes
AUTH_PLAINTEXT=yes
AUTH_SPA=yes
AUTH_TLS=yes

in Local/Makefile, respectively. The first of these supports the CRAM-MD5
authentication mechanism (RFC 2195), and the second provides an interface to
the Cyrus SASL authentication library. The third is an interface to Dovecot's
authentication system, delegating the work via a socket interface. The fourth
provides for negotiation of authentication done via non-SMTP means, as defined
by RFC 4422 Appendix A. The fifth provides an interface to the GNU SASL
authentication library, which provides mechanisms but typically not data
sources. The sixth provides direct access to Heimdal GSSAPI, geared for
Kerberos, but supporting setting a server keytab. The seventh can be configured
to support the PLAIN authentication mechanism (RFC 2595) or the LOGIN
mechanism, which is not formally documented, but used by several MUAs. The
eighth authenticator supports Microsoft's Secure Password Authentication
mechanism. The last is an Exim authenticator but not an SMTP one; instead it
can use information from a TLS negotiation.

The authenticators are configured using the same syntax as other drivers (see
section 6.23). If no authenticators are required, no authentication section
need be present in the configuration file. Each authenticator can in principle
have both server and client functions. When Exim is receiving SMTP mail, it is
acting as a server; when it is sending out messages over SMTP, it is acting as
a client. Authenticator configuration options are provided for use in both
these circumstances.

To make it clear which options apply to which situation, the prefixes server_
and client_ are used on option names that are specific to either the server or
the client function, respectively. Server and client functions are disabled if
none of their options are set. If an authenticator is to be used for both
server and client functions, a single definition, using both sets of options,
is required. For example:

cram:
  driver = cram_md5
  public_name = CRAM-MD5
  server_secret = ${if eq{$auth1}{ph10}{secret1}fail}
  client_name = ph10
  client_secret = secret2

The server_ option is used when Exim is acting as a server, and the client_
options when it is acting as a client.

Descriptions of the individual authenticators are given in subsequent chapters.
The remainder of this chapter covers the generic options for the
authenticators, followed by general discussion of the way authentication works
in Exim.

Beware: the meaning of $auth1, $auth2, ... varies on a per-driver and
per-mechanism basis. Please read carefully to determine which variables hold
account labels such as usercodes and which hold passwords or other
authenticating data.

Note that some mechanisms support two different identifiers for accounts: the 
authentication id and the authorization id. The contractions authn and authz
are commonly encountered. The American spelling is standard here. Conceptually,
authentication data such as passwords are tied to the identifier used to
authenticate; servers may have rules to permit one user to act as a second
user, so that after login the session is treated as though that second user had
logged in. That second user is the authorization id. A robust configuration
might confirm that the authz field is empty or matches the authn field. Often
this is just ignored. The authn can be considered as verified data, the authz
as an unverified request which the server might choose to honour.

A realm is a text string, typically a domain name, presented by a server to a
client to help it select an account and credentials to use. In some mechanisms,
the client and server provably agree on the realm, but clients typically can
not treat the realm as secure data to be blindly trusted.


33.1 Generic options for authenticators
---------------------------------------

+-----------------------------------------------------------------+
|client_condition|Use: authenticators|Type: string*|Default: unset|
+-----------------------------------------------------------------+

When Exim is authenticating as a client, it skips any authenticator whose 
client_condition expansion yields "0", "no", or "false". This can be used, for
example, to skip plain text authenticators when the connection is not encrypted
by a setting such as:

client_condition = ${if !eq{$tls_out_cipher}{}}

+--------------------------------------------------------------+
|client_set_id|Use: authenticators|Type: string*|Default: unset|
+--------------------------------------------------------------+

When client authentication succeeds, this condition is expanded; the result is
used in the log lines for outbound messages. Typically it will be the user name
used for authentication.

+------------------------------------------------------+
|driver|Use: authenticators|Type: string|Default: unset|
+------------------------------------------------------+

This option must always be set. It specifies which of the available
authenticators is to be used.

+-----------------------------------------------------------+
|public_name|Use: authenticators|Type: string|Default: unset|
+-----------------------------------------------------------+

This option specifies the name of the authentication mechanism that the driver
implements, and by which it is known to the outside world. These names should
contain only upper case letters, digits, underscores, and hyphens (RFC 2222),
but Exim in fact matches them caselessly. If public_name is not set, it
defaults to the driver's instance name.

+---------------------------------------------------------------------------+
|server_advertise_condition|Use: authenticators|Type: string*|Default: unset|
+---------------------------------------------------------------------------+

When a server is about to advertise an authentication mechanism, the condition
is expanded. If it yields the empty string, "0", "no", or "false", the
mechanism is not advertised. If the expansion fails, the mechanism is not
advertised. If the failure was not forced, and was not caused by a lookup
defer, the incident is logged. See section 33.3 below for further discussion.

+-----------------------------------------------------------------+
|server_condition|Use: authenticators|Type: string*|Default: unset|
+-----------------------------------------------------------------+

This option must be set for a plaintext server authenticator, where it is used
directly to control authentication. See section 34.3 for details.

For the gsasl authenticator, this option is required for various mechanisms;
see chapter 38 for details.

For the other authenticators, server_condition can be used as an additional
authentication or authorization mechanism that is applied after the other
authenticator conditions succeed. If it is set, it is expanded when the
authenticator would otherwise return a success code. If the expansion is forced
to fail, authentication fails. Any other expansion failure causes a temporary
error code to be returned. If the result of a successful expansion is an empty
string, "0", "no", or "false", authentication fails. If the result of the
expansion is "1", "yes", or "true", authentication succeeds. For any other
result, a temporary error code is returned, with the expanded string as the
error text.

+-------------------------------------------------------------------+
|server_debug_print|Use: authenticators|Type: string*|Default: unset|
+-------------------------------------------------------------------+

If this option is set and authentication debugging is enabled (see the -d
command line option), the string is expanded and included in the debugging
output when the authenticator is run as a server. This can help with checking
out the values of variables. If expansion of the string fails, the error
message is written to the debugging output, and Exim carries on processing.

+--------------------------------------------------------------+
|server_set_id|Use: authenticators|Type: string*|Default: unset|
+--------------------------------------------------------------+

When an Exim server successfully authenticates a client, this string is
expanded using data from the authentication, and preserved for any incoming
messages in the variable $authenticated_id. It is also included in the log
lines for incoming messages. For example, a user/password authenticator
configuration might preserve the user name that was used to authenticate, and
refer to it subsequently during delivery of the message. On a failing
authentication the expansion result is instead saved in the
$authenticated_fail_id variable. If expansion fails, the option is ignored.

+---------------------------------------------------------------------------+
|server_mail_auth_condition|Use: authenticators|Type: string*|Default: unset|
+---------------------------------------------------------------------------+

This option allows a server to discard authenticated sender addresses supplied
as part of MAIL commands in SMTP connections that are authenticated by the
driver on which server_mail_auth_condition is set. The option is not used as
part of the authentication process; instead its (unexpanded) value is
remembered for later use. How it is used is described in the following section.


33.2 The AUTH parameter on MAIL commands
----------------------------------------

When a client supplied an AUTH= item on a MAIL command, Exim applies the
following checks before accepting it as the authenticated sender of the
message:

  * If the connection is not using extended SMTP (that is, HELO was used rather
    than EHLO), the use of AUTH= is a syntax error.

  * If the value of the AUTH= parameter is "<>", it is ignored.

  * If acl_smtp_mailauth is defined, the ACL it specifies is run. While it is
    running, the value of $authenticated_sender is set to the value obtained
    from the AUTH= parameter. If the ACL does not yield "accept", the value of
    $authenticated_sender is deleted. The acl_smtp_mailauth ACL may not return
    "drop" or "discard". If it defers, a temporary error code (451) is given
    for the MAIL command.

  * If acl_smtp_mailauth is not defined, the value of the AUTH= parameter is
    accepted and placed in $authenticated_sender only if the client has
    authenticated.

  * If the AUTH= value was accepted by either of the two previous rules, and
    the client has authenticated, and the authenticator has a setting for the 
    server_mail_auth_condition, the condition is checked at this point. The
    valued that was saved from the authenticator is expanded. If the expansion
    fails, or yields an empty string, "0", "no", or "false", the value of
    $authenticated_sender is deleted. If the expansion yields any other value,
    the value of $authenticated_sender is retained and passed on with the
    message.

When $authenticated_sender is set for a message, it is passed on to other hosts
to which Exim authenticates as a client. Do not confuse this value with
$authenticated_id, which is a string obtained from the authentication process,
and which is not usually a complete email address.

Whenever an AUTH= value is ignored, the incident is logged. The ACL for MAIL,
if defined, is run after AUTH= is accepted or ignored. It can therefore make
use of $authenticated_sender. The converse is not true: the value of
$sender_address is not yet set up when the acl_smtp_mailauth ACL is run.


33.3 Authentication on an Exim server
-------------------------------------

When Exim receives an EHLO command, it advertises the public names of those
authenticators that are configured as servers, subject to the following
conditions:

  * The client host must match auth_advertise_hosts (default *).

  * If the server_advertise_condition option is set, its expansion must not
    yield the empty string, "0", "no", or "false".

The order in which the authenticators are defined controls the order in which
the mechanisms are advertised.

Some mail clients (for example, some versions of Netscape) require the user to
provide a name and password for authentication whenever AUTH is advertised,
even though authentication may not in fact be needed (for example, Exim may be
set up to allow unconditional relaying from the client by an IP address check).
You can make such clients more friendly by not advertising AUTH to them. For
example, if clients on the 10.9.8.0/24 network are permitted (by the ACL that
runs for RCPT) to relay without authentication, you should set

auth_advertise_hosts = ! 10.9.8.0/24

so that no authentication mechanisms are advertised to them.

The server_advertise_condition controls the advertisement of individual
authentication mechanisms. For example, it can be used to restrict the
advertisement of a particular mechanism to encrypted connections, by a setting
such as:

server_advertise_condition = ${if eq{$tls_in_cipher}{}{no}{yes}}

If the session is encrypted, $tls_in_cipher is not empty, and so the expansion
yields "yes", which allows the advertisement to happen.

When an Exim server receives an AUTH command from a client, it rejects it
immediately if AUTH was not advertised in response to an earlier EHLO command.
This is the case if

  * The client host does not match auth_advertise_hosts; or

  * No authenticators are configured with server options; or

  * Expansion of server_advertise_condition blocked the advertising of all the
    server authenticators.

Otherwise, Exim runs the ACL specified by acl_smtp_auth in order to decide
whether to accept the command. If acl_smtp_auth is not set, AUTH is accepted
from any client host.

If AUTH is not rejected by the ACL, Exim searches its configuration for a
server authentication mechanism that was advertised in response to EHLO and
that matches the one named in the AUTH command. If it finds one, it runs the
appropriate authentication protocol, and authentication either succeeds or
fails. If there is no matching advertised mechanism, the AUTH command is
rejected with a 504 error.

When a message is received from an authenticated host, the value of
$received_protocol is set to "esmtpa" or "esmtpsa" instead of "esmtp" or
"esmtps", and $sender_host_authenticated contains the name (not the public
name) of the authenticator driver that successfully authenticated the client
from which the message was received. This variable is empty if there was no
successful authentication.

Successful authentication sets up information used by the authresults expansion
item.

If an authenticator is run and does not succeed, an event (see 61) of type
"auth:fail" is raised. While the event is being processed the variables
$sender_host_authenticated (with the authenticator name) and
$authenticated_fail_id (as set by the authenticator server_set_id option) will
be valid. If the event is serviced and a string is returned then the string
will be logged instead of the default log line. See <<CHAPevents>> for details
on events.


33.4 Testing server authentication
----------------------------------

Exim's -bh option can be useful for testing server authentication
configurations. The data for the AUTH command has to be sent using base64
encoding. A quick way to produce such data for testing is the following Perl
script:

use MIME::Base64;
printf ("%s", encode_base64(eval "\"$ARGV[0]\""));

This interprets its argument as a Perl string, and then encodes it. The
interpretation as a Perl string allows binary zeros, which are required for
some kinds of authentication, to be included in the data. For example, a
command line to run this script on such data might be

encode '\0user\0password'

Note the use of single quotes to prevent the shell interpreting the
backslashes, so that they can be interpreted by Perl to specify characters
whose code value is zero.

Warning 1: If either of the user or password strings starts with an octal
digit, you must use three zeros instead of one after the leading backslash. If
you do not, the octal digit that starts your string will be incorrectly
interpreted as part of the code for the first character.

Warning 2: If there are characters in the strings that Perl interprets
specially, you must use a Perl escape to prevent them being misinterpreted. For
example, a command such as

encode '\0user@domain.com\0pas$$word'

gives an incorrect answer because of the unescaped "@" and "$" characters.

If you have the mimencode command installed, another way to produce
base64-encoded strings is to run the command

echo -e -n `\0user\0password' | mimencode

The -e option of echo enables the interpretation of backslash escapes in the
argument, and the -n option specifies no newline at the end of its output.
However, not all versions of echo recognize these options, so you should check
your version before relying on this suggestion.


33.5 Authentication by an Exim client
-------------------------------------

The smtp transport has two options called hosts_require_auth and hosts_try_auth
. When the smtp transport connects to a server that announces support for
authentication, and the host matches an entry in either of these options, Exim
(as a client) tries to authenticate as follows:

  * For each authenticator that is configured as a client, in the order in
    which they are defined in the configuration, it searches the authentication
    mechanisms announced by the server for one whose name matches the public
    name of the authenticator.

  * When it finds one that matches, it runs the authenticator's client code.
    The variables $host and $host_address are available for any string
    expansions that the client might do. They are set to the server's name and
    IP address. If any expansion is forced to fail, the authentication attempt
    is abandoned, and Exim moves on to the next authenticator. Otherwise an
    expansion failure causes delivery to be deferred.

  * If the result of the authentication attempt is a temporary error or a
    timeout, Exim abandons trying to send the message to the host for the
    moment. It will try again later. If there are any backup hosts available,
    they are tried in the usual way.

  * If the response to authentication is a permanent error (5xx code), an event
    (see 61) of type "auth:fail" is raised. While the event is being processed
    the variable $sender_host_authenticated (with the authenticator name) will
    be valid. If the event is serviced and a string is returned then the string
    will be logged. See <<CHAPevents>> for details on events.

  * If the response to authentication is a permanent error (5xx code), Exim
    carries on searching the list of authenticators and tries another one if
    possible. If all authentication attempts give permanent errors, or if there
    are no attempts because no mechanisms match (or option expansions force
    failure), what happens depends on whether the host matches 
    hosts_require_auth or hosts_try_auth. In the first case, a temporary error
    is generated, and delivery is deferred. The error can be detected in the
    retry rules, and thereby turned into a permanent error if you wish. In the
    second case, Exim tries to deliver the message unauthenticated.

Note that the hostlist test for whether to do authentication can be confused if
name-IP lookups change between the time the peer is decided upon and the time
that the transport runs. For example, with a manualroute router given a host
name, and with DNS "round-robin" used by that name: if the local resolver cache
times out between the router and the transport running, the transport may get
an IP for the name for its authentication check which does not match the
connection peer IP. No authentication will then be done, despite the names
being identical.

For such cases use a separate transport which always authenticates.

When Exim has authenticated itself to a remote server, it adds the AUTH
parameter to the MAIL commands it sends, if it has an authenticated sender for
the message. If the message came from a remote host, the authenticated sender
is the one that was receiving on an incoming MAIL command, provided that the
incoming connection was authenticated and the server_mail_auth condition
allowed the authenticated sender to be retained. If a local process calls Exim
to send a message, the sender address that is built from the login name and 
qualify_domain is treated as authenticated. However, if the 
authenticated_sender option is set on the smtp transport, it overrides the
authenticated sender that was received with the message.



===============================================================================
34. THE PLAINTEXT AUTHENTICATOR

The plaintext authenticator can be configured to support the PLAIN and LOGIN
authentication mechanisms, both of which transfer authentication data as plain
(unencrypted) text (though base64 encoded). The use of plain text is a security
risk; you are strongly advised to insist on the use of SMTP encryption (see
chapter 43) if you use the PLAIN or LOGIN mechanisms. If you do use unencrypted
plain text, you should not use the same passwords for SMTP connections as you
do for login accounts.


34.1 Avoiding cleartext use
---------------------------

The following generic option settings will disable plaintext authenticators
when TLS is not being used:

  server_advertise_condition = ${if def:tls_in_cipher }
  client_condition =           ${if def:tls_out_cipher}

Note: a plaintext SMTP AUTH done inside TLS is not vulnerable to casual
snooping, but is still vulnerable to a Man In The Middle attack unless
certificates (including their names) have been properly verified.


34.2 Plaintext server options
-----------------------------

When configured as a server, plaintext uses the following options:

+-----------------------------------------------------------------+
|server_condition|Use: authenticators|Type: string*|Default: unset|
+-----------------------------------------------------------------+

This is actually a global authentication option, but it must be set in order to
configure the plaintext driver as a server. Its use is described below.

+---------------------------------------------------------------+
|server_prompts|Use: plaintext|Type: string list*|Default: unset|
+---------------------------------------------------------------+

The contents of this option, after expansion, must be a colon-separated list of
prompt strings. If expansion fails, a temporary authentication rejection is
given.


34.3 Using plaintext in a server
--------------------------------

When running as a server, plaintext performs the authentication test by
expanding a string. The data sent by the client with the AUTH command, or in
response to subsequent prompts, is base64 encoded, and so may contain any byte
values when decoded. If any data is supplied with the command, it is treated as
a list of strings, separated by NULs (binary zeros), the first three of which
are placed in the expansion variables $auth1, $auth2, and $auth3 (neither LOGIN
nor PLAIN uses more than three strings).

For compatibility with previous releases of Exim, the values are also placed in
the expansion variables $1, $2, and $3. However, the use of these variables for
this purpose is now deprecated, as it can lead to confusion in string
expansions that also use them for other things.

If there are more strings in server_prompts than the number of strings supplied
with the AUTH command, the remaining prompts are used to obtain more data. Each
response from the client may be a list of NUL-separated strings.

Once a sufficient number of data strings have been received, server_condition
is expanded. If the expansion is forced to fail, authentication fails. Any
other expansion failure causes a temporary error code to be returned. If the
result of a successful expansion is an empty string, "0", "no", or "false",
authentication fails. If the result of the expansion is "1", "yes", or "true",
authentication succeeds and the generic server_set_id option is expanded and
saved in $authenticated_id. For any other result, a temporary error code is
returned, with the expanded string as the error text.

Warning: If you use a lookup in the expansion to find the user's password, be
sure to make the authentication fail if the user is unknown. There are good and
bad examples at the end of the next section.


34.4 The PLAIN authentication mechanism
---------------------------------------

The PLAIN authentication mechanism (RFC 2595) specifies that three strings be
sent as one item of data (that is, one combined string containing two NUL
separators). The data is sent either as part of the AUTH command, or
subsequently in response to an empty prompt from the server.

The second and third strings are a user name and a corresponding password.
Using a single fixed user name and password as an example, this could be
configured as follows:

fixed_plain:
  driver = plaintext
  public_name = PLAIN
  server_prompts = :
  server_condition = \
    ${if and {{eq{$auth2}{username}}{eq{$auth3}{mysecret}}}}
  server_set_id = $auth2

Note that the default result strings from if ("true" or an empty string) are
exactly what we want here, so they need not be specified. Obviously, if the
password contains expansion-significant characters such as dollar, backslash,
or closing brace, they have to be escaped.

The server_prompts setting specifies a single, empty prompt (empty items at the
end of a string list are ignored). If all the data comes as part of the AUTH
command, as is commonly the case, the prompt is not used. This authenticator is
advertised in the response to EHLO as

250-AUTH PLAIN

and a client host can authenticate itself by sending the command

AUTH PLAIN AHVzZXJuYW1lAG15c2VjcmV0

As this contains three strings (more than the number of prompts), no further
data is required from the client. Alternatively, the client may just send

AUTH PLAIN

to initiate authentication, in which case the server replies with an empty
prompt. The client must respond with the combined data string.

The data string is base64 encoded, as required by the RFC. This example, when
decoded, is <NUL>"username"<NUL>"mysecret", where <NUL> represents a zero byte.
This is split up into three strings, the first of which is empty. The 
server_condition option in the authenticator checks that the second two are
"username" and "mysecret" respectively.

Having just one fixed user name and password, as in this example, is not very
realistic, though for a small organization with only a handful of
authenticating clients it could make sense.

A more sophisticated instance of this authenticator could use the user name in
$auth2 to look up a password in a file or database, and maybe do an encrypted
comparison (see crypteq in chapter 11). Here is a example of this approach,
where the passwords are looked up in a DBM file. Warning: This is an incorrect
example:

server_condition = \
  ${if eq{$auth3}{${lookup{$auth2}dbm{/etc/authpwd}}}}

The expansion uses the user name ($auth2) as the key to look up a password,
which it then compares to the supplied password ($auth3). Why is this example
incorrect? It works fine for existing users, but consider what happens if a
non-existent user name is given. The lookup fails, but as no success/failure
strings are given for the lookup, it yields an empty string. Thus, to defeat
the authentication, all a client has to do is to supply a non-existent user
name and an empty password. The correct way of writing this test is:

server_condition = ${lookup{$auth2}dbm{/etc/authpwd}\
  {${if eq{$value}{$auth3}}} {false}}

In this case, if the lookup succeeds, the result is checked; if the lookup
fails, "false" is returned and authentication fails. If crypteq is being used
instead of eq, the first example is in fact safe, because crypteq always fails
if its second argument is empty. However, the second way of writing the test
makes the logic clearer.


34.5 The LOGIN authentication mechanism
---------------------------------------

The LOGIN authentication mechanism is not documented in any RFC, but is in use
in a number of programs. No data is sent with the AUTH command. Instead, a user
name and password are supplied separately, in response to prompts. The
plaintext authenticator can be configured to support this as in this example:

fixed_login:
  driver = plaintext
  public_name = LOGIN
  server_prompts = User Name : Password
  server_condition = \
    ${if and {{eq{$auth1}{username}}{eq{$auth2}{mysecret}}}}
  server_set_id = $auth1

Because of the way plaintext operates, this authenticator accepts data supplied
with the AUTH command (in contravention of the specification of LOGIN), but if
the client does not supply it (as is the case for LOGIN clients), the prompt
strings are used to obtain two data items.

Some clients are very particular about the precise text of the prompts. For
example, Outlook Express is reported to recognize only "Username:" and
"Password:". Here is an example of a LOGIN authenticator that uses those
strings. It uses the ldapauth expansion condition to check the user name and
password by binding to an LDAP server:

login:
  driver = plaintext
  public_name = LOGIN
  server_prompts = Username:: : Password::
  server_condition = ${if and{{ \
    !eq{}{$auth1} }{ \
    ldapauth{\
      user="uid=${quote_ldap_dn:$auth1},ou=people,o=example.org" \
      pass=${quote:$auth2} \
      ldap://ldap.example.org/} }} }
  server_set_id = uid=$auth1,ou=people,o=example.org

We have to check that the username is not empty before using it, because LDAP
does not permit empty DN components. We must also use the quote_ldap_dn
operator to correctly quote the DN for authentication. However, the basic quote
operator, rather than any of the LDAP quoting operators, is the correct one to
use for the password, because quoting is needed only to make the password
conform to the Exim syntax. At the LDAP level, the password is an uninterpreted
string.


34.6 Support for different kinds of authentication
--------------------------------------------------

A number of string expansion features are provided for the purpose of
interfacing to different ways of user authentication. These include checking
traditionally encrypted passwords from /etc/passwd (or equivalent), PAM,
Radius, ldapauth, pwcheck, and saslauthd. For details see section 11.7.


34.7 Using plaintext in a client
--------------------------------

The plaintext authenticator has two client options:

+------------------------------------------------------------------------+
|client_ignore_invalid_base64|Use: plaintext|Type: boolean|Default: false|
+------------------------------------------------------------------------+

If the client receives a server prompt that is not a valid base64 string,
authentication is abandoned by default. However, if this option is set true,
the error in the challenge is ignored and the client sends the response as
usual.

+-------------------------------------------------------+
|client_send|Use: plaintext|Type: string*|Default: unset|
+-------------------------------------------------------+

The string is a colon-separated list of authentication data strings. Each
string is independently expanded before being sent to the server. The first
string is sent with the AUTH command; any more strings are sent in response to
prompts from the server. Before each string is expanded, the value of the most
recent prompt is placed in the next $auth<n> variable, starting with $auth1 for
the first prompt. Up to three prompts are stored in this way. Thus, the prompt
that is received in response to sending the first string (with the AUTH
command) can be used in the expansion of the second string, and so on. If an
invalid base64 string is received when client_ignore_invalid_base64 is set, an
empty string is put in the $auth<n> variable.

Note: You cannot use expansion to create multiple strings, because splitting
takes priority and happens first.

Because the PLAIN authentication mechanism requires NUL (binary zero) bytes in
the data, further processing is applied to each string before it is sent. If
there are any single circumflex characters in the string, they are converted to
NULs. Should an actual circumflex be required as data, it must be doubled in
the string.

This is an example of a client configuration that implements the PLAIN
authentication mechanism with a fixed user name and password:

fixed_plain:
  driver = plaintext
  public_name = PLAIN
  client_send = ^username^mysecret

The lack of colons means that the entire text is sent with the AUTH command,
with the circumflex characters converted to NULs. Note that due to the
ambiguity of parsing three consectutive circumflex characters there is no way
to provide a password having a leading circumflex.

A similar example that uses the LOGIN mechanism is:

fixed_login:
  driver = plaintext
  public_name = LOGIN
  client_send = : username : mysecret

The initial colon means that the first string is empty, so no data is sent with
the AUTH command itself. The remaining strings are sent in response to prompts.



===============================================================================
35. THE CRAM_MD5 AUTHENTICATOR

The CRAM-MD5 authentication mechanism is described in RFC 2195. The server
sends a challenge string to the client, and the response consists of a user
name and the CRAM-MD5 digest of the challenge string combined with a secret
string (password) which is known to both server and client. Thus, the secret is
not sent over the network as plain text, which makes this authenticator more
secure than plaintext. However, the downside is that the secret has to be
available in plain text at either end.


35.1 Using cram_md5 as a server
-------------------------------

This authenticator has one server option, which must be set to configure the
authenticator as a server:

+--------------------------------------------------------+
|server_secret|Use: cram_md5|Type: string*|Default: unset|
+--------------------------------------------------------+

When the server receives the client's response, the user name is placed in the
expansion variable $auth1, and server_secret is expanded to obtain the password
for that user. The server then computes the CRAM-MD5 digest that the client
should have sent, and checks that it received the correct string. If the
expansion of server_secret is forced to fail, authentication fails. If the
expansion fails for some other reason, a temporary error code is returned to
the client.

For compatibility with previous releases of Exim, the user name is also placed
in $1. However, the use of this variables for this purpose is now deprecated,
as it can lead to confusion in string expansions that also use numeric
variables for other things.

For example, the following authenticator checks that the user name given by the
client is "ph10", and if so, uses "secret" as the password. For any other user
name, authentication fails.

fixed_cram:
  driver = cram_md5
  public_name = CRAM-MD5
  server_secret = ${if eq{$auth1}{ph10}{secret}fail}
  server_set_id = $auth1

If authentication succeeds, the setting of server_set_id preserves the user
name in $authenticated_id. A more typical configuration might look up the
secret string in a file, using the user name as the key. For example:

lookup_cram:
  driver = cram_md5
  public_name = CRAM-MD5
  server_secret = ${lookup{$auth1}lsearch{/etc/authpwd}\
                  {$value}fail}
  server_set_id = $auth1

Note that this expansion explicitly forces failure if the lookup fails because
$auth1 contains an unknown user name.

As another example, if you wish to re-use a Cyrus SASL sasldb2 file without
using the relevant libraries, you need to know the realm to specify in the
lookup and then ask for the "userPassword" attribute for that user in that
realm, with:

cyrusless_crammd5:
  driver = cram_md5
  public_name = CRAM-MD5
  server_secret = ${lookup{$auth1:mail.example.org:userPassword}\
                  dbmjz{/etc/sasldb2}{$value}fail}
  server_set_id = $auth1


35.2 Using cram_md5 as a client
-------------------------------

When used as a client, the cram_md5 authenticator has two options:

+----------------------------------------------------------------------+
|client_name|Use: cram_md5|Type: string*|Default: the primary host name|
+----------------------------------------------------------------------+

This string is expanded, and the result used as the user name data when
computing the response to the server's challenge.

+--------------------------------------------------------+
|client_secret|Use: cram_md5|Type: string*|Default: unset|
+--------------------------------------------------------+

This option must be set for the authenticator to work as a client. Its value is
expanded and the result used as the secret string when computing the response.

Different user names and secrets can be used for different servers by referring
to $host or $host_address in the options. Forced failure of either expansion
string is treated as an indication that this authenticator is not prepared to
handle this case. Exim moves on to the next configured client authenticator.
Any other expansion failure causes Exim to give up trying to send the message
to the current server.

A simple example configuration of a cram_md5 authenticator, using fixed
strings, is:

fixed_cram:
  driver = cram_md5
  public_name = CRAM-MD5
  client_name = ph10
  client_secret = secret



===============================================================================
36. THE CYRUS_SASL AUTHENTICATOR

The code for this authenticator was provided by Matthew Byng-Maddick while at A
L Digital Ltd.

The cyrus_sasl authenticator provides server support for the Cyrus SASL library
implementation of the RFC 2222 ("Simple Authentication and Security Layer").
This library supports a number of authentication mechanisms, including PLAIN
and LOGIN, but also several others that Exim does not support directly. In
particular, there is support for Kerberos authentication.

The cyrus_sasl authenticator provides a gatewaying mechanism directly to the
Cyrus interface, so if your Cyrus library can do, for example, CRAM-MD5, then
so can the cyrus_sasl authenticator. By default it uses the public name of the
driver to determine which mechanism to support.

Where access to some kind of secret file is required, for example, in GSSAPI or
CRAM-MD5, it is worth noting that the authenticator runs as the Exim user, and
that the Cyrus SASL library has no way of escalating privileges by default. You
may also find you need to set environment variables, depending on the driver
you are using.

The application name provided by Exim is "exim", so various SASL options may be
set in exim.conf in your SASL directory. If you are using GSSAPI for Kerberos,
note that because of limitations in the GSSAPI interface, changing the server
keytab might need to be communicated down to the Kerberos layer independently.
The mechanism for doing so is dependent upon the Kerberos implementation.

For example, for older releases of Heimdal, the environment variable
KRB5_KTNAME may be set to point to an alternative keytab file. Exim will pass
this variable through from its own inherited environment when started as root
or the Exim user. The keytab file needs to be readable by the Exim user. With
newer releases of Heimdal, a setuid Exim may cause Heimdal to discard the
environment variable. In practice, for those releases, the Cyrus authenticator
is not a suitable interface for GSSAPI (Kerberos) support. Instead, consider
the heimdal_gssapi authenticator, described in chapter 39


36.1 Using cyrus_sasl as a server
---------------------------------

The cyrus_sasl authenticator has four private options. It puts the username (on
a successful authentication) into $auth1. For compatibility with previous
releases of Exim, the username is also placed in $1. However, the use of this
variable for this purpose is now deprecated, as it can lead to confusion in
string expansions that also use numeric variables for other things.

+----------------------------------------------------------------+
|server_hostname|Use: cyrus_sasl|Type: string*|Default: see below|
+----------------------------------------------------------------+

This option selects the hostname that is used when communicating with the
library. The default value is "$primary_hostname". It is up to the underlying
SASL plug-in what it does with this data.

+-----------------------------------------------------------+
|server_mech|Use: cyrus_sasl|Type: string|Default: see below|
+-----------------------------------------------------------+

This option selects the authentication mechanism this driver should use. The
default is the value of the generic public_name option. This option allows you
to use a different underlying mechanism from the advertised name. For example:

sasl:
  driver = cyrus_sasl
  public_name = X-ANYTHING
  server_mech = CRAM-MD5
  server_set_id = $auth1

+---------------------------------------------------------+
|server_realm|Use: cyrus_sasl|Type: string*|Default: unset|
+---------------------------------------------------------+

This specifies the SASL realm that the server claims to be in.

+-----------------------------------------------------------+
|server_service|Use: cyrus_sasl|Type: string|Default: "smtp"|
+-----------------------------------------------------------+

This is the SASL service that the server claims to implement.

For straightforward cases, you do not need to set any of the authenticator's
private options. All you need to do is to specify an appropriate mechanism as
the public name. Thus, if you have a SASL library that supports CRAM-MD5 and
PLAIN, you could have two authenticators as follows:

sasl_cram_md5:
  driver = cyrus_sasl
  public_name = CRAM-MD5
  server_set_id = $auth1

sasl_plain:
  driver = cyrus_sasl
  public_name = PLAIN
  server_set_id = $auth2

Cyrus SASL does implement the LOGIN authentication method, even though it is
not a standard method. It is disabled by default in the source distribution,
but it is present in many binary distributions.



===============================================================================
37. THE DOVECOT AUTHENTICATOR

This authenticator is an interface to the authentication facility of the
Dovecot 2 POP/IMAP server, which can support a number of authentication
methods. Note that Dovecot must be configured to use auth-client not
auth-userdb. If you are using Dovecot to authenticate POP/IMAP clients, it
might be helpful to use the same mechanisms for SMTP authentication. This is a
server authenticator only. There is only one non-generic option:

+------------------------------------------------------+
|server_socket|Use: dovecot|Type: string|Default: unset|
+------------------------------------------------------+

This option must specify the UNIX socket that is the interface to Dovecot
authentication. The public_name option must specify an authentication mechanism
that Dovecot is configured to support. You can have several authenticators for
different mechanisms. For example:

dovecot_plain:
  driver = dovecot
  public_name = PLAIN
  server_advertise_condition = ${if def:tls_in_cipher}
  server_socket = /var/run/dovecot/auth-client
  server_set_id = $auth1

dovecot_ntlm:
  driver = dovecot
  public_name = NTLM
  server_socket = /var/run/dovecot/auth-client
  server_set_id = $auth1

Note: plaintext authentication methods such as PLAIN and LOGIN should not be
advertised on cleartext SMTP connections. See the discussion in section 34.1.

If the SMTP connection is encrypted, or if $sender_host_address is equal to
$received_ip_address (that is, the connection is local), the "secured" option
is passed in the Dovecot authentication command. If, for a TLS connection, a
client certificate has been verified, the "valid-client-cert" option is passed.
When authentication succeeds, the identity of the user who authenticated is
placed in $auth1.

The Dovecot configuration to match the above will look something like:

conf.d/10-master.conf :-

service auth {
...
#SASL
  unix_listener auth-client {
    mode = 0660
    user = mail
  }
...
}

conf.d/10-auth.conf :-

auth_mechanisms = plain login ntlm



===============================================================================
38. THE GSASL AUTHENTICATOR

The gsasl authenticator provides integration for the GNU SASL library and the
mechanisms it provides. This is new as of the 4.80 release and there are a few
areas where the library does not let Exim smoothly scale to handle future
authentication mechanisms, so no guarantee can be made that any particular new
authentication mechanism will be supported without code changes in Exim.

The library is expected to add support in an upcoming realease for the
SCRAM-SHA-256 method. The macro _HAVE_AUTH_GSASL_SCRAM_SHA_256 will be defined
when this happens.

To see the list of mechanisms supported by the library run Exim with "auth"
debug enabled and look for a line containing "GNU SASL supports". Note however
that some may not have been tested from Exim.

+----------------------------------------------------+
|client_authz|Use: gsasl|Type: string*|Default: unset|
+----------------------------------------------------+

This option can be used to supply an authorization id which is different to the
authentication_id provided by client_username option. If unset or (after
expansion) empty it is not used, which is the common case.

+-------------------------------------------------------------+
|client_channelbinding|Use: gsasl|Type: boolean|Default: false|
+-------------------------------------------------------------+

See server_channelbinding below.

+-------------------------------------------------------+
|client_password|Use: gsasl|Type: string*|Default: unset|
+-------------------------------------------------------+

This option is exapanded before use, and should result in the password to be
used, in clear.

+-------------------------------------------------------+
|client_username|Use: gsasl|Type: string*|Default: unset|
+-------------------------------------------------------+

This option is exapanded before use, and should result in the account name to
be used.

+--------------------------------------------------------+
|client_spassword|Use: gsasl|Type: string*|Default: unset|
+--------------------------------------------------------+

This option is only supported for library versions 1.9.1 and greater. The macro
_HAVE_AUTH_GSASL_SCRAM_S_KEY will be defined when this is so.

If a SCRAM mechanism is being used and this option is set and correctly sized
it is used in preference to client_password. The value after expansion should
be a 40 (for SHA-1) or 64 (for SHA-256) character string with the
PBKDF2-prepared password, hex-encoded.

Note that this value will depend on the salt and iteration-count supplied by
the server. The option is expanded before use. During the expansion $auth1 is
set with the client username, $auth2 with the iteration count, and $auth3 with
the salt.

The intent of this option is to support clients that can cache thes salted
password to save on recalculation costs. The cache lookup should return an
unusable value (eg. an empty string) if the salt or iteration count has changed

If the authentication succeeds then the above variables are set, plus the
calculated salted password value value in $auth4, during the expansion of the 
client_set_id option. A side-effect of this expansion can be used to prime the
cache.

+-------------------------------------------------------------+
|server_channelbinding|Use: gsasl|Type: boolean|Default: false|
+-------------------------------------------------------------+

Some authentication mechanisms are able to use external context at both ends of
the session to bind the authentication to that context, and fail the
authentication process if that context differs. Specifically, some TLS
ciphersuites can provide identifying information about the cryptographic
context.

This should have meant that certificate identity and verification becomes a
non-issue, as a man-in-the-middle attack will cause the correct client and
server to see different identifiers and authentication will fail.

This is only usable by mechanisms which support "channel binding"; at time of
writing, that's the SCRAM family. When using this feature the "-PLUS" variants
of the method names need to be used.

This defaults off to ensure smooth upgrade across Exim releases, in case this
option causes some clients to start failing. Some future release of Exim might
have switched the default to be true.

This option was deprecated in previous releases due to doubts over the "Triple
Handshake" vulnerability. Exim takes suitable precausions (requiring Extended
Master Secret if TLS Session Resumption was used) for safety.

+-----------------------------------------------------------+
|server_hostname|Use: gsasl|Type: string*|Default: see below|
+-----------------------------------------------------------+

This option selects the hostname that is used when communicating with the
library. The default value is "$primary_hostname". Some mechanisms will use
this data.

+------------------------------------------------------+
|server_mech|Use: gsasl|Type: string|Default: see below|
+------------------------------------------------------+

This option selects the authentication mechanism this driver should use. The
default is the value of the generic public_name option. This option allows you
to use a different underlying mechanism from the advertised name. For example:

sasl:
  driver = gsasl
  public_name = X-ANYTHING
  server_mech = CRAM-MD5
  server_set_id = $auth1

+-------------------------------------------------------+
|server_password|Use: gsasl|Type: string*|Default: unset|
+-------------------------------------------------------+

Various mechanisms need access to the cleartext password on the server, so that
proof-of-possession can be demonstrated on the wire, without sending the
password itself.

The data available for lookup varies per mechanism. In all cases, $auth1 is set
to the authentication id. The $auth2 variable will always be the authorization
id (authz) if available, else the empty string. The $auth3 variable will always
be the realm if available, else the empty string.

A forced failure will cause authentication to defer.

If using this option, it may make sense to set the server_condition option to
be simply "true".

+----------------------------------------------------+
|server_realm|Use: gsasl|Type: string*|Default: unset|
+----------------------------------------------------+

This specifies the SASL realm that the server claims to be in. Some mechanisms
will use this data.

+--------------------------------------------------------+
|server_scram_iter|Use: gsasl|Type: string*|Default: 4096|
+--------------------------------------------------------+

This option provides data for the SCRAM family of mechanisms. The $auth1,
$auth2 and $auth3 variables are available when this option is expanded.

The result of expansion should be a decimal number, and represents both a
lower-bound on the security, and a compute cost factor imposed on the client
(if it does not cache results, or the server changes either the iteration count
or the salt). A minimum value of 4096 is required by the standards for all
current SCRAM mechanism variants.

+---------------------------------------------------------+
|server_scram_salt|Use: gsasl|Type: string*|Default: unset|
+---------------------------------------------------------+

This option provides data for the SCRAM family of mechanisms. The $auth1,
$auth2 and $auth3 variables are available when this option is expanded. The
value should be a base64-encoded string, of random data typically 4-to-16 bytes
long. If unset or empty after expansion the library will provides a value for
the protocol conversation.

+--------------------------------------------------+
|server_key|Use: gsasl|Type: string*|Default: unset|
+--------------------------------------------------+

+---------------------------------------------------+
|server_skey|Use: gsasl|Type: string*|Default: unset|
+---------------------------------------------------+

These options can be used for the SCRAM family of mechanisms to provide stored
information related to a password, the storage of which is preferable to
plaintext.

server_key is the value defined in the SCRAM standards as ServerKey; 
server_skey is StoredKey.

They are only available for version 1.9.0 (or later) of the gsasl library. When
this is so, the macros _OPT_AUTHENTICATOR_GSASL_SERVER_KEY and
_HAVE_AUTH_GSASL_SCRAM_S_KEY will be defined.

The $authN variables are available when these options are expanded.

If set, the results of expansion should for each should be a 28 (for SHA-1) or
44 (for SHA-256) character string of base64-coded data, and will be used in
preference to the server_password option. If unset or not of the right length, 
server_password will be used.

The libgsasl library release includes a utility gsasl which can be used to
generate these values.

+------------------------------------------------------+
|server_service|Use: gsasl|Type: string|Default: "smtp"|
+------------------------------------------------------+

This is the SASL service that the server claims to implement. Some mechanisms
will use this data.


38.1 gsasl auth variables
-------------------------

These may be set when evaluating specific options, as detailed above. They will
also be set when evaluating server_condition.

Unless otherwise stated below, the gsasl integration will use the following
meanings for these variables:

  * $auth1: the authentication id

  * $auth2: the authorization id

  * $auth3: the realm

On a per-mechanism basis:

  * EXTERNAL: only $auth1 is set, to the possibly empty authorization id; the 
    server_condition option must be present.

  * ANONYMOUS: only $auth1 is set, to the possibly empty anonymous token; the 
    server_condition option must be present.

  * GSSAPI: $auth1 will be set to the GSSAPI Display Name; $auth2 will be set
    to the authorization id, the server_condition option must be present.

An anonymous token is something passed along as an unauthenticated identifier;
this is analogous to FTP anonymous authentication passing an email address, or
software-identifier@, as the "password".

An example showing the password having the realm specified in the callback and
demonstrating a Cyrus SASL to GSASL migration approach is:

gsasl_cyrusless_crammd5:
  driver = gsasl
  public_name = CRAM-MD5
  server_realm = imap.example.org
  server_password = ${lookup{$auth1:$auth3:userPassword}\
                    dbmjz{/etc/sasldb2}{$value}fail}
  server_set_id = ${quote:$auth1}
  server_condition = yes



===============================================================================
39. THE HEIMDAL_GSSAPI AUTHENTICATOR

The heimdal_gssapi authenticator provides server integration for the Heimdal
GSSAPI/Kerberos library, permitting Exim to set a keytab pathname reliably.

+--------------------------------------------------------------------+
|server_hostname|Use: heimdal_gssapi|Type: string*|Default: see below|
+--------------------------------------------------------------------+

This option selects the hostname that is used, with server_service, for
constructing the GSS server name, as a GSS_C_NT_HOSTBASED_SERVICE identifier.
The default value is "$primary_hostname".

+--------------------------------------------------------------+
|server_keytab|Use: heimdal_gssapi|Type: string*|Default: unset|
+--------------------------------------------------------------+

If set, then Heimdal will not use the system default keytab (typically /etc/
krb5.keytab) but instead the pathname given in this option. The value should be
a pathname, with no "file:" prefix.

+--------------------------------------------------------------+
|server_service|Use: heimdal_gssapi|Type: string*|Default: smtp|
+--------------------------------------------------------------+

This option specifies the service identifier used, in conjunction with 
server_hostname, for building the identifier for finding credentials from the
keytab.


39.1 heimdal_gssapi auth variables
----------------------------------

Beware that these variables will typically include a realm, thus will appear to
be roughly like an email address already. The authzid in $auth2 is not
verified, so a malicious client can set it to anything.

The $auth1 field should be safely trustable as a value from the Key
Distribution Center. Note that these are not quite email addresses. Each
identifier is for a role, and so the left-hand-side may include a role suffix.
For instance, "joe/admin@EXAMPLE.ORG".

  * $auth1: the authentication id, set to the GSS Display Name.

  * $auth2: the authorization id, sent within SASL encapsulation after
    authentication. If that was empty, this will also be set to the GSS Display
    Name.



===============================================================================
40. THE SPA AUTHENTICATOR

The spa authenticator provides client support for Microsoft's Secure Password
Authentication mechanism, which is also sometimes known as NTLM (NT LanMan).
The code for client side of this authenticator was contributed by Marc
Prud'hommeaux, and much of it is taken from the Samba project (https://
www.samba.org/). The code for the server side was subsequently contributed by
Tom Kistner. The mechanism works as follows:

  * After the AUTH command has been accepted, the client sends an SPA
    authentication request based on the user name and optional domain.

  * The server sends back a challenge.

  * The client builds a challenge response which makes use of the user's
    password and sends it to the server, which then accepts or rejects it.

Encryption is used to protect the password in transit.


40.1 Using spa as a server
--------------------------

The spa authenticator has just one server option:

+-----------------------------------------------------+
|server_password|Use: spa|Type: string*|Default: unset|
+-----------------------------------------------------+

This option is expanded, and the result must be the cleartext password for the
authenticating user, whose name is at this point in $auth1. For compatibility
with previous releases of Exim, the user name is also placed in $1. However,
the use of this variable for this purpose is now deprecated, as it can lead to
confusion in string expansions that also use numeric variables for other
things. For example:

spa:
  driver = spa
  public_name = NTLM
  server_password = \
    ${lookup{$auth1}lsearch{/etc/exim/spa_clearpass}{$value}fail}

If the expansion is forced to fail, authentication fails. Any other expansion
failure causes a temporary error code to be returned.


40.2 Using spa as a client
--------------------------

The spa authenticator has the following client options:

+---------------------------------------------------+
|client_domain|Use: spa|Type: string*|Default: unset|
+---------------------------------------------------+

This option specifies an optional domain for the authentication.

+-----------------------------------------------------+
|client_password|Use: spa|Type: string*|Default: unset|
+-----------------------------------------------------+

This option specifies the user's password, and must be set.

+-----------------------------------------------------+
|client_username|Use: spa|Type: string*|Default: unset|
+-----------------------------------------------------+

This option specifies the user name, and must be set. Here is an example of a
configuration of this authenticator for use with the mail servers at msn.com:

msn:
  driver = spa
  public_name = MSN
  client_username = msn/msn_username
  client_password = msn_plaintext_password
  client_domain = DOMAIN_OR_UNSET



===============================================================================
41. THE EXTERNAL AUTHENTICATOR

The external authenticator provides support for authentication based on
non-SMTP information. The specification is in RFC 4422 Appendix A (https://
tools.ietf.org/html/rfc4422). It is only a transport and negotiation mechanism;
the process of authentication is entirely controlled by the server
configuration.

The client presents an identity in-clear. It is probably wise for a server to
only advertise, and for clients to only attempt, this authentication method on
a secure (eg. under TLS) connection.

One possible use, compatible with the K-9 Mail Android client (https://
k9mail.github.io/), is for using X509 client certificates.

It thus overlaps in function with the TLS authenticator (see 42) but is a full
SMTP SASL authenticator rather than being implicit for TLS-connection carried
client certificates only.

The examples and discussion in this chapter assume that client-certificate
authentication is being done.

The client must present a certificate, for which it must have been requested
via the tls_verify_hosts or tls_try_verify_hosts main options (see 43). For
authentication to be effective the certificate should be verifiable against a
trust-anchor certificate known to the server.


41.1 External options
---------------------

The external authenticator has two server options:

+--------------------------------------------------------+
|server_param2|Use: external|Type: string*|Default: unset|
+--------------------------------------------------------+

+--------------------------------------------------------+
|server_param3|Use: external|Type: string*|Default: unset|
+--------------------------------------------------------+

These options are expanded before the server_condition option and the result
are placed in $auth2 and $auth3 resectively. If the expansion is forced to
fail, authentication fails. Any other expansion failure causes a temporary
error code to be returned.

They can be used to clarify the coding of a complex server_condition.


41.2 Using external in a server
-------------------------------

When running as a server, external performs the authentication test by
expanding a string. The data sent by the client with the AUTH command, or in
response to subsequent prompts, is base64 encoded, and so may contain any byte
values when decoded. The decoded value is treated as an identity for
authentication and placed in the expansion variable $auth1.

For compatibility with previous releases of Exim, the value is also placed in
the expansion variable $1. However, the use of this variable for this purpose
is now deprecated, as it can lead to confusion in string expansions that also
use them for other things.

Once an identity has been received, server_condition is expanded. If the
expansion is forced to fail, authentication fails. Any other expansion failure
causes a temporary error code to be returned. If the result of a successful
expansion is an empty string, "0", "no", or "false", authentication fails. If
the result of the expansion is "1", "yes", or "true", authentication succeeds
and the generic server_set_id option is expanded and saved in $authenticated_id
. For any other result, a temporary error code is returned, with the expanded
string as the error text.

Example:

ext_ccert_san_mail:
  driver =            external
  public_name =       EXTERNAL

  server_advertise_condition = $tls_in_certificate_verified
  server_param2 =     ${certextract {subj_altname,mail,>:} \
                                    {$tls_in_peercert}}
  server_condition =  ${if forany {$auth2} \
                            {eq {$item}{$auth1}}}
  server_set_id =     $auth1

This accepts a client certificate that is verifiable against any of your
configured trust-anchors (which usually means the full set of public CAs) and
which has a mail-SAN matching the claimed identity sent by the client.

Note: up to TLS1.2, the client cert is on the wire in-clear, including the SAN.
The account name is therefore guessable by an opponent. TLS 1.3 protects both
server and client certificates, and is not vulnerable in this way.


41.3 Using external in a client
-------------------------------

The external authenticator has one client option:

+------------------------------------------------------+
|client_send|Use: external|Type: string*|Default: unset|
+------------------------------------------------------+

This option is expanded and sent with the AUTH command as the identity being
asserted.

Example:

ext_ccert:
  driver =      external
  public_name = EXTERNAL

  client_condition = ${if !eq{$tls_out_cipher}{}}
  client_send = myaccount@smarthost.example.net



===============================================================================
42. THE TLS AUTHENTICATOR

The tls authenticator provides server support for authentication based on
client certificates.

It is not an SMTP authentication mechanism and is not advertised by the server
as part of the SMTP EHLO response. It is an Exim authenticator in the sense
that it affects the protocol element of the log line, can be tested for by the 
authenticated ACL condition, and can set the $authenticated_id variable.

The client must present a verifiable certificate, for which it must have been
requested via the tls_verify_hosts or tls_try_verify_hosts main options (see 43
).

If an authenticator of this type is configured it is run immediately after a
TLS connection being negotiated (due to either STARTTLS or TLS-on-connect) and
can authenticate the connection. If it does, SMTP authentication is not
subsequently offered.

A maximum of one authenticator of this type may be present.

The tls authenticator has three server options:

+---------------------------------------------------+
|server_param1|Use: tls|Type: string*|Default: unset|
+---------------------------------------------------+

This option is expanded after the TLS negotiation and the result is placed in
$auth1. If the expansion is forced to fail, authentication fails. Any other
expansion failure causes a temporary error code to be returned.

+---------------------------------------------------+
|server_param2|Use: tls|Type: string*|Default: unset|
+---------------------------------------------------+

+---------------------------------------------------+
|server_param3|Use: tls|Type: string*|Default: unset|
+---------------------------------------------------+

As above, for $auth2 and $auth3.

server_param1 may also be spelled server_param.

Example:

tls:
  driver = tls
  server_param1 =     ${certextract {subj_altname,mail,>:} \
                                    {$tls_in_peercert}}
  server_condition =  ${if and { {eq{$tls_in_certificate_verified}{1}} \
                                 {forany {$auth1} \
                            {!= {0} \
                                {${lookup ldap{ldap:///\
                         mailname=${quote_ldap_dn:${lc:$item}},\
                         ou=users,LDAP_DC?mailid} {$value}{0} \
                       }    }  } }}}
  server_set_id =     ${if = {1}{${listcount:$auth1}} {$auth1}{}}

This accepts a client certificate that is verifiable against any of your
configured trust-anchors (which usually means the full set of public CAs) and
which has a SAN with a good account name.

Note that, up to TLS1.2, the client cert is on the wire in-clear, including the
SAN, The account name is therefore guessable by an opponent. TLS 1.3 protects
both server and client certificates, and is not vulnerable in this way.
Likewise, a traditional plaintext SMTP AUTH done inside TLS is not.

Note that because authentication is traditionally an SMTP operation, the 
authenticated ACL condition cannot be used in a connect- or helo-ACL.



===============================================================================
43. ENCRYPTED SMTP CONNECTIONS USING TLS/SSL

Support for TLS (Transport Layer Security), formerly known as SSL (Secure
Sockets Layer), is implemented by making use of the OpenSSL library or the
GnuTLS library (Exim requires GnuTLS release 1.0 or later). There is no
cryptographic code in the Exim distribution itself for implementing TLS. In
order to use this feature you must install OpenSSL or GnuTLS, and then build a
version of Exim that includes TLS support (see section 4.7). You also need to
understand the basic concepts of encryption at a managerial level, and in
particular, the way that public keys, private keys, and certificates are used.

RFC 3207 defines how SMTP connections can make use of encryption. Once a
connection is established, the client issues a STARTTLS command. If the server
accepts this, the client and the server negotiate an encryption mechanism. If
the negotiation succeeds, the data that subsequently passes between them is
encrypted.

Exim's ACLs can detect whether the current SMTP session is encrypted or not,
and if so, what cipher suite is in use, whether the client supplied a
certificate, and whether or not that certificate was verified. This makes it
possible for an Exim server to deny or accept certain commands based on the
encryption state.

Warning: Certain types of firewall and certain anti-virus products can disrupt
TLS connections. You need to turn off SMTP scanning for these products in order
to get TLS to work.


43.1 Support for the "submissions" (aka "ssmtp" and "smtps") protocol
---------------------------------------------------------------------

The history of port numbers for TLS in SMTP is a little messy and has been
contentious. As of RFC 8314, the common practice of using the historically
allocated port 465 for "email submission but with TLS immediately upon connect
instead of using STARTTLS" is officially blessed by the IETF, and recommended
by them in preference to STARTTLS.

The name originally assigned to the port was "ssmtp" or "smtps", but as clarity
emerged over the dual roles of SMTP, for MX delivery and Email Submission,
nomenclature has shifted. The modern name is now "submissions".

This approach was, for a while, officially abandoned when encrypted SMTP was
standardized, but many clients kept using it, even as the TCP port number was
reassigned for other use. Thus you may encounter guidance claiming that you
shouldn't enable use of this port. In practice, a number of mail-clients have
only ever supported submissions, not submission with STARTTLS upgrade. Ideally,
offer both submission (587) and submissions (465) service.

Exim supports TLS-on-connect by means of the tls_on_connect_ports global
option. Its value must be a list of port numbers; the most common use is
expected to be:

tls_on_connect_ports = 465

The port numbers specified by this option apply to all SMTP connections, both
via the daemon and via inetd. You still need to specify all the ports that the
daemon uses (by setting daemon_smtp_ports or local_interfaces or the -oX
command line option) because tls_on_connect_ports does not add an extra port -
rather, it specifies different behaviour on a port that is defined elsewhere.

There is also a -tls-on-connect command line option. This overrides 
tls_on_connect_ports; it forces the TLS-only behaviour for all ports.


43.2 OpenSSL vs GnuTLS
----------------------

TLS is supported in Exim using either the OpenSSL or GnuTLS library. To build
Exim to use OpenSSL you need to set

USE_OPENSSL=yes

in Local/Makefile.

To build Exim to use GnuTLS, you need to set

USE_GNUTLS=yes

in Local/Makefile.

You must also set TLS_LIBS and TLS_INCLUDE appropriately, so that the include
files and libraries for GnuTLS can be found.

There are some differences in usage when using GnuTLS instead of OpenSSL:

  * The tls_verify_certificates option cannot be the path of a directory for
    GnuTLS versions before 3.3.6 (for later versions, or OpenSSL, it can be
    either).

  * The default value for tls_dhparam differs for historical reasons.

  * Distinguished Name (DN) strings reported by the OpenSSL library use a slash
    for separating fields; GnuTLS uses commas, in accordance with RFC 2253.
    This affects the value of the $tls_in_peerdn and $tls_out_peerdn variables.

  * OpenSSL identifies cipher suites using hyphens as separators, for example:
    DES-CBC3-SHA. GnuTLS historically used underscores, for example:
    RSA_ARCFOUR_SHA. What is more, OpenSSL complains if underscores are present
    in a cipher list. To make life simpler, Exim changes underscores to hyphens
    for OpenSSL and passes the string unchanged to GnuTLS (expecting the
    library to handle its own older variants) when processing lists of cipher
    suites in the tls_require_ciphers options (the global option and the smtp
    transport option).

  * The tls_require_ciphers options operate differently, as described in the
    sections 43.4 and 43.5.

  * The tls_dh_min_bits SMTP transport option is only honoured by GnuTLS. When
    using OpenSSL, this option is ignored. (If an API is found to let OpenSSL
    be configured in this way, let the Exim Maintainers know and we'll likely
    use it).

  * With GnuTLS, if an explicit list is used for the tls_privatekey main
    option, it must be ordered to match the tls_certificate list.

  * Some other recently added features may only be available in one or the
    other. This should be documented with the feature. If the documentation
    does not explicitly state that the feature is infeasible in the other TLS
    implementation, then patches are welcome.

  * The output from "exim -bV" will show which (if any) support was included in
    the build. Also, the macro "_HAVE_OPENSSL" or "_HAVE_GNUTLS" will be
    defined.


43.3 GnuTLS parameter computation
---------------------------------

This section only applies if tls_dhparam is set to "historic" or to an explicit
path; if the latter, then the text about generation still applies, but not the
chosen filename. By default, as of Exim 4.80 a hard-coded D-H prime is used.
See the documentation of tls_dhparam for more information.

GnuTLS uses D-H parameters that may take a substantial amount of time to
compute. It is unreasonable to re-compute them for every TLS session.
Therefore, Exim keeps this data in a file in its spool directory, called
gnutls-params-NNNN for some value of NNNN, corresponding to the number of bits
requested. The file is owned by the Exim user and is readable only by its
owner. Every Exim process that start up GnuTLS reads the D-H parameters from
this file. If the file does not exist, the first Exim process that needs it
computes the data and writes it to a temporary file which is renamed once it is
complete. It does not matter if several Exim processes do this simultaneously
(apart from wasting a few resources). Once a file is in place, new Exim
processes immediately start using it.

For maximum security, the parameters that are stored in this file should be
recalculated periodically, the frequency depending on your paranoia level. If
you are avoiding using the fixed D-H primes published in RFCs, then you are
concerned about some advanced attacks and will wish to do this; if you do not
regenerate then you might as well stick to the standard primes.

Arranging this is easy in principle; just delete the file when you want new
values to be computed. However, there may be a problem. The calculation of new
parameters needs random numbers, and these are obtained from /dev/random. If
the system is not very active, /dev/random may delay returning data until
enough randomness (entropy) is available. This may cause Exim to hang for a
substantial amount of time, causing timeouts on incoming connections.

The solution is to generate the parameters externally to Exim. They are stored
in gnutls-params-N in PEM format, which means that they can be generated
externally using the certtool command that is part of GnuTLS.

To replace the parameters with new ones, instead of deleting the file and
letting Exim re-create it, you can generate new parameters using certtool and,
when this has been done, replace Exim's cache file by renaming. The relevant
commands are something like this:

# ls
[ look for file; assume gnutls-params-2236 is the most recent ]
# rm -f new-params
# touch new-params
# chown exim:exim new-params
# chmod 0600 new-params
# certtool --generate-dh-params --bits 2236 >>new-params
# openssl dhparam -noout -text -in new-params | head
[ check the first line, make sure it's not more than 2236;
  if it is, then go back to the start ("rm") and repeat
  until the size generated is at most the size requested ]
# chmod 0400 new-params
# mv new-params gnutls-params-2236

If Exim never has to generate the parameters itself, the possibility of
stalling is removed.

The filename changed in Exim 4.80, to gain the -bits suffix. The value which
Exim will choose depends upon the version of GnuTLS in use. For older GnuTLS,
the value remains hard-coded in Exim as 1024. As of GnuTLS 2.12.x, there is a
way for Exim to ask for the "normal" number of bits for D-H public-key usage,
and Exim does so. This attempt to remove Exim from TLS policy decisions failed,
as GnuTLS 2.12 returns a value higher than the current hard-coded limit of the
NSS library. Thus Exim gains the tls_dh_max_bits global option, which applies
to all D-H usage, client or server. If the value returned by GnuTLS is greater
than tls_dh_max_bits then the value will be clamped down to tls_dh_max_bits.
The default value has been set at the current NSS limit, which is still much
higher than Exim historically used.

The filename and bits used will change as the GnuTLS maintainers change the
value for their parameter "GNUTLS_SEC_PARAM_NORMAL", as clamped by 
tls_dh_max_bits. At the time of writing (mid 2012), GnuTLS 2.12 recommends 2432
bits, while NSS is limited to 2236 bits.

In fact, the requested value will be *lower* than tls_dh_max_bits, to increase
the chance of the generated prime actually being within acceptable bounds, as
GnuTLS has been observed to overshoot. Note the check step in the procedure
above. There is no sane procedure available to Exim to double-check the size of
the generated prime, so it might still be too large.


43.4 Requiring specific ciphers in OpenSSL
------------------------------------------

There is a function in the OpenSSL library that can be passed a list of cipher
suites before the cipher negotiation takes place. This specifies which ciphers
are acceptable for TLS versions prior to 1.3. The list is colon separated and
may contain names like DES-CBC3-SHA. Exim passes the expanded value of 
tls_require_ciphers directly to this function call. Many systems will install
the OpenSSL manual-pages, so you may have ciphers(1) available to you. The
following quotation from the OpenSSL documentation specifies what forms of item
are allowed in the cipher string:

  * It can consist of a single cipher suite such as RC4-SHA.

  * It can represent a list of cipher suites containing a certain algorithm, or
    cipher suites of a certain type. For example SHA1 represents all ciphers
    suites using the digest algorithm SHA1 and SSLv3 represents all SSL v3
    algorithms.

  * Lists of cipher suites can be combined in a single cipher string using the
    + character. This is used as a logical and operation. For example SHA1+DES
    represents all cipher suites containing the SHA1 and the DES algorithms.

Each cipher string can be optionally preceded by one of the characters "!", "-"
or "+".

  * If "!" is used, the ciphers are permanently deleted from the list. The
    ciphers deleted can never reappear in the list even if they are explicitly
    stated.

  * If "-" is used, the ciphers are deleted from the list, but some or all of
    the ciphers can be added again by later options.

  * If "+" is used, the ciphers are moved to the end of the list. This option
    does not add any new ciphers; it just moves matching existing ones.

If none of these characters is present, the string is interpreted as a list of
ciphers to be appended to the current preference list. If the list includes any
ciphers already present they will be ignored: that is, they will not be moved
to the end of the list.

The OpenSSL ciphers(1) command may be used to test the results of a given
string:

# note single-quotes to get ! past any shell history expansion
$ openssl ciphers 'HIGH:!MD5:!SHA1'

This example will let the library defaults be permitted on the MX port, where
there's probably no identity verification anyway, but ups the ante on the
submission ports where the administrator might have some influence on the
choice of clients used:

# OpenSSL variant; see man ciphers(1)
tls_require_ciphers = ${if =={$received_port}{25}\
                           {DEFAULT}\
                           {HIGH:!MD5:!SHA1}}

This example will prefer ECDSA-authenticated ciphers over RSA ones:

tls_require_ciphers = ECDSA:RSA:!COMPLEMENTOFDEFAULT

For TLS version 1.3 the control available is less fine-grained and Exim does
not provide access to it at present. The value of the tls_require_ciphers
option is ignored when TLS version 1.3 is negotiated.

As of writing the library default cipher suite list for TLSv1.3 is

TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256


43.5 Requiring specific ciphers or other parameters in GnuTLS
-------------------------------------------------------------

The GnuTLS library allows the caller to provide a "priority string", documented
as part of the gnutls_priority_init function. This is very similar to the
ciphersuite specification in OpenSSL.

The tls_require_ciphers option is treated as the GnuTLS priority string and
controls both protocols and ciphers.

The tls_require_ciphers option is available both as an global option,
controlling how Exim behaves as a server, and also as an option of the smtp
transport, controlling how Exim behaves as a client. In both cases the value is
string expanded. The resulting string is not an Exim list and the string is
given to the GnuTLS library, so that Exim does not need to be aware of future
feature enhancements of GnuTLS.

Documentation of the strings accepted may be found in the GnuTLS manual, under
"Priority strings". This is online as https://www.gnutls.org/manual/html_node/
Priority-Strings.html, but beware that this relates to GnuTLS 3, which may be
newer than the version installed on your system. If you are using GnuTLS 3,
then the example code https://www.gnutls.org/manual/gnutls.html#
Listing-the-ciphersuites-in-a-priority-string on that site can be used to test
a given string.

For example:

# Disable older versions of protocols
tls_require_ciphers = NORMAL:%LATEST_RECORD_VERSION:-VERS-SSL3.0

Prior to Exim 4.80, an older API of GnuTLS was used, and Exim supported three
additional options, "gnutls_require_kx", "gnutls_require_mac" and "
gnutls_require_protocols". tls_require_ciphers was an Exim list.

This example will let the library defaults be permitted on the MX port, where
there's probably no identity verification anyway, and lowers security further
by increasing compatibility; but this ups the ante on the submission ports
where the administrator might have some influence on the choice of clients
used:

# GnuTLS variant
tls_require_ciphers = ${if =={$received_port}{25}\
                           {NORMAL:%COMPAT}\
                           {SECURE128}}


43.6 Configuring an Exim server to use TLS
------------------------------------------

When Exim has been built with TLS support, it advertises the availability of
the STARTTLS command to client hosts that match tls_advertise_hosts, but not to
any others. The default value of this option is *, which means that STARTTLS is
always advertised. Set it to blank to never advertise; this is reasonable for
systems that want to use TLS only as a client.

If STARTTLS is to be used you need to set some other options in order to make
TLS available.

If a client issues a STARTTLS command and there is some configuration problem
in the server, the command is rejected with a 454 error. If the client persists
in trying to issue SMTP commands, all except QUIT are rejected with the error

554 Security failure

If a STARTTLS command is issued within an existing TLS session, it is rejected
with a 554 error code.

To enable TLS operations on a server, the tls_advertise_hosts option must be
set to match some hosts. The default is * which matches all hosts.

If this is all you do, TLS encryption will be enabled but not authentication -
meaning that the peer has no assurance it is actually you he is talking to. You
gain protection from a passive sniffer listening on the wire but not from
someone able to intercept the communication.

Further protection requires some further configuration at the server end.

To make TLS work you need to set, in the server,

tls_certificate = /some/file/name
tls_privatekey = /some/file/name

These options are, in fact, expanded strings, so you can make them depend on
the identity of the client that is connected if you wish. The first file
contains the server's X509 certificate, and the second contains the private key
that goes with it. These files need to be PEM format and readable by the Exim
user, and must always be given as full path names. The key must not be
password-protected. They can be the same file if both the certificate and the
key are contained within it. If tls_privatekey is not set, or if its expansion
is forced to fail or results in an empty string, this is assumed to be the
case. The certificate file may also contain intermediate certificates that need
to be sent to the client to enable it to authenticate the server's certificate.

For dual-stack (eg. RSA and ECDSA) configurations, these options can be
colon-separated lists of file paths. Ciphers using given authentication
algorithms require the presence of a suitable certificate to supply the
public-key. The server selects among the certificates to present to the client
depending on the selected cipher, hence the priority ordering for ciphers will
affect which certificate is used.

If you do not understand about certificates and keys, please try to find a
source of this background information, which is not Exim-specific. (There are a
few comments below in section 43.10.)

Note: These options do not apply when Exim is operating as a client - they
apply only in the case of a server. If you need to use a certificate in an Exim
client, you must set the options of the same names in an smtp transport.

With just these options, an Exim server will be able to use TLS. It does not
require the client to have a certificate (but see below for how to insist on
this). There is one other option that may be needed in other situations. If

tls_dhparam = /some/file/name

is set, the SSL library is initialized for the use of Diffie-Hellman ciphers
with the parameters contained in the file. Set this to "none" to disable use of
DH entirely, by making no prime available:

tls_dhparam = none

This may also be set to a string identifying a standard prime to be used for
DH; if it is set to "default" or, for OpenSSL, is unset, then the prime used is
"ike23". There are a few standard primes available, see the documentation for 
tls_dhparam for the complete list.

See the command

openssl dhparam

for a way of generating file data.

The strings supplied for these three options are expanded every time a client
host connects. It is therefore possible to use different certificates and keys
for different hosts, if you so wish, by making use of the client's IP address
in $sender_host_address to control the expansion. If a string expansion is
forced to fail, Exim behaves as if the option is not set.

The variable $tls_in_cipher is set to the cipher suite that was negotiated for
an incoming TLS connection. It is included in the Received: header of an
incoming message (by default - you can, of course, change this), and it is also
included in the log line that records a message's arrival, keyed by "X=",
unless the tls_cipher log selector is turned off. The encrypted condition can
be used to test for specific cipher suites in ACLs.

Once TLS has been established, the ACLs that run for subsequent SMTP commands
can check the name of the cipher suite and vary their actions accordingly. The
cipher suite names vary, depending on which TLS library is being used. For
example, OpenSSL uses the name DES-CBC3-SHA for the cipher suite which in other
contexts is known as TLS_RSA_WITH_3DES_EDE_CBC_SHA. Check the OpenSSL or GnuTLS
documentation for more details.

For outgoing SMTP deliveries, $tls_out_cipher is used and logged (again
depending on the tls_cipher log selector).

43.6.1 Requesting and verifying client certificates

If you want an Exim server to request a certificate when negotiating a TLS
session with a client, you must set either tls_verify_hosts or 
tls_try_verify_hosts. You can, of course, set either of them to * to apply to
all TLS connections. For any host that matches one of these options, Exim
requests a certificate as part of the setup of the TLS session. The contents of
the certificate are verified by comparing it with a list of expected
trust-anchors or certificates. These may be the system default set (depending
on library version), an explicit file or, depending on library version, a
directory, identified by tls_verify_certificates.

A file can contain multiple certificates, concatenated end to end. If a
directory is used (OpenSSL only), each certificate must be in a separate file,
with a name (or a symbolic link) of the form <hash>.0, where <hash> is a hash
value constructed from the certificate. You can compute the relevant hash by
running the command

openssl x509 -hash -noout -in /cert/file

where /cert/file contains a single certificate.

There is no checking of names of the client against the certificate Subject
Name or Subject Alternate Names.

The difference between tls_verify_hosts and tls_try_verify_hosts is what
happens if the client does not supply a certificate, or if the certificate does
not match any of the certificates in the collection named by 
tls_verify_certificates. If the client matches tls_verify_hosts, the attempt to
set up a TLS session is aborted, and the incoming connection is dropped. If the
client matches tls_try_verify_hosts, the (encrypted) SMTP session continues.
ACLs that run for subsequent SMTP commands can detect the fact that no
certificate was verified, and vary their actions accordingly. For example, you
can insist on a certificate before accepting a message for relaying, but not
when the message is destined for local delivery.

When a client supplies a certificate (whether it verifies or not), the value of
the Distinguished Name of the certificate is made available in the variable
$tls_in_peerdn during subsequent processing of the message.

Because it is often a long text string, it is not included in the log line or 
Received: header by default. You can arrange for it to be logged, keyed by "DN=
", by setting the tls_peerdn log selector, and you can use received_header_text
to change the Received: header. When no certificate is supplied, $tls_in_peerdn
is empty.

43.6.2 Caching of static server configuration items

If any of the main configuration options tls_certificate, tls_privatekey, 
tls_crl and tls_ocsp_file have values with no expandable elements, then the
associated information is loaded at daemon startup. It is made available to
child processes forked for handling received SMTP connections.

This caching is currently only supported under Linux and FreeBSD.

If caching is not possible, for example if an item has to be dependent on the
peer host so contains a $sender_host_name expansion, the load of the associated
information is done at the startup of the TLS connection.

The cache is invalidated and reloaded after any changes to the directories
containing files specified by these options.

The information specified by the main option tls_verify_certificates is
similarly cached so long as it specifies files explicitly or (under GnuTLS) is
the string "system,cache". The latter case is not automatically invalidated; it
is the operator's responsibility to arrange for a daemon restart any time the
system certificate authority bundle is updated. A HUP signal is sufficient for
this. The value "system" results in no caching under GnuTLS.

The macro _HAVE_TLS_CA_CACHE will be defined if the suffix for "system" is
acceptable in configurations for the Exim executable.

Caching of the system Certificate Authorities bundle can save significant time
and processing on every TLS connection accepted by Exim.


43.7 Configuring an Exim client to use TLS
------------------------------------------

The tls_cipher and tls_peerdn log selectors apply to outgoing SMTP deliveries
as well as to incoming, the latter one causing logging of the server
certificate's DN. The remaining client configuration for TLS is all within the 
smtp transport.

It is not necessary to set any options to have TLS work in the smtp transport.
If Exim is built with TLS support, and TLS is advertised by a server, the smtp
transport always tries to start a TLS session. However, this can be prevented
by setting hosts_avoid_tls (an option of the transport) to a list of server
hosts for which TLS should not be used.

If you do not want Exim to attempt to send messages unencrypted when an attempt
to set up an encrypted connection fails in any way, you can set 
hosts_require_tls to a list of hosts for which encryption is mandatory. For
those hosts, delivery is always deferred if an encrypted connection cannot be
set up. If there are any other hosts for the address, they are tried in the
usual way.

When the server host is not in hosts_require_tls, Exim may try to deliver the
message unencrypted. It always does this if the response to STARTTLS is a 5xx
code. For a temporary error code, or for a failure to negotiate a TLS session
after a success response code, what happens is controlled by the 
tls_tempfail_tryclear option of the smtp transport. If it is false, delivery to
this host is deferred, and other hosts (if available) are tried. If it is true,
Exim attempts to deliver unencrypted after a 4xx response to STARTTLS, and if
STARTTLS is accepted, but the subsequent TLS negotiation fails, Exim closes the
current connection (because it is in an unknown state), opens a new one to the
same host, and then tries the delivery unencrypted.

The tls_certificate and tls_privatekey options of the smtp transport provide
the client with a certificate, which is passed to the server if it requests it.
This is an optional thing for TLS connections, although either end may insist
on it. If the server is Exim, it will request a certificate only if 
tls_verify_hosts or tls_try_verify_hosts matches the client.

Note: Do not use a certificate which has the OCSP-must-staple extension, for
client use (they are usable for server use). As the TLS protocol has no means
for the client to staple before TLS 1.3 it will result in failed connections.

If the tls_verify_certificates option is set on the smtp transport, it
specifies a collection of expected server certificates. These may be the system
default set (depending on library version), a file, or (depending on library
version) a directory. The client verifies the server's certificate against this
collection, taking into account any revoked certificates that are in the list
defined by tls_crl. Failure to verify fails the TLS connection unless either of
the tls_verify_hosts or tls_try_verify_hosts options are set.

The tls_verify_hosts and tls_try_verify_hosts options restrict certificate
verification to the listed servers. Verification either must or need not
succeed respectively.

The tls_verify_cert_hostnames option lists hosts for which additional name
checks are made on the server certificate. The match against this list is, as
per other Exim usage, the IP for the host. That is most closely associated with
the name on the DNS A (or AAAA) record for the host. However, the name that
needs to be in the certificate is the one at the head of any CNAME chain
leading to the A record. The option defaults to always checking.

The smtp transport has two OCSP-related options: hosts_require_ocsp; a
host-list for which a Certificate Status is requested and required for the
connection to proceed. The default value is empty. hosts_request_ocsp; a
host-list for which (additionally) a Certificate Status is requested (but not
necessarily verified). The default value is "*" meaning that requests are made
unless configured otherwise.

The host(s) should also be in hosts_require_tls, and tls_verify_certificates
configured for the transport, for OCSP to be relevant.

If tls_require_ciphers is set on the smtp transport, it must contain a list of
permitted cipher suites. If either of these checks fails, delivery to the
current host is abandoned, and the smtp transport tries to deliver to
alternative hosts, if any.

Note: These options must be set in the smtp transport for Exim to use TLS when
it is operating as a client. Exim does not assume that a server certificate
(set by the global options of the same name) should also be used when operating
as a client.

All the TLS options in the smtp transport are expanded before use, with $host
and $host_address containing the name and address of the server to which the
client is connected. Forced failure of an expansion causes Exim to behave as if
the relevant option were unset.

Before an SMTP connection is established, the $tls_out_bits, $tls_out_cipher,
$tls_out_peerdn and $tls_out_sni variables are emptied. (Until the first
connection, they contain the values that were set when the message was
received.) If STARTTLS is subsequently successfully obeyed, these variables are
set to the relevant values for the outgoing connection.

43.7.1 Caching of static client configuration items

If any of the transport configuration options tls_certificate, tls_privatekey
and tls_crl have values with no expandable elements, then the associated
information is loaded per smtp transport at daemon startup, at the start of a
queue run, or on a command-line specified message delivery. It is made
available to child processes forked for handling making SMTP connections.

This caching is currently only supported under Linux.

If caching is not possible, the load of the associated information is done at
the startup of the TLS connection.

The cache is invalidated in the daemon and reloaded after any changes to the
directories containing files specified by these options.

The information specified by the main option tls_verify_certificates is
similarly cached so long as it specifies files explicitly or (under GnuTLS) is
the string "system,cache". The latter case is not automatically invaludated; it
is the operator's responsibility to arrange for a daemon restart any time the
system certificate authority bundle is updated. A HUP signal is sufficient for
this. The value "system" results in no caching under GnuTLS.

The macro _HAVE_TLS_CA_CACHE will be defined if the suffix for "system" is
acceptable in configurations for the Exim executable.

Caching of the system Certificate Authorities bundle can save significant time
and processing on every TLS connection initiated by Exim.


43.8 Use of TLS Server Name Indication
--------------------------------------

With TLS1.0 or above, there is an extension mechanism by which extra
information can be included at various points in the protocol. One of these
extensions, documented in RFC 6066 (and before that RFC 4366) is "Server Name
Indication", commonly "SNI". This extension is sent by the client in the
initial handshake, so that the server can examine the servername within and
possibly choose to use different certificates and keys (and more) for this
session.

This is analogous to HTTP's "Host:" header, and is the main mechanism by which
HTTPS-enabled web-sites can be virtual-hosted, many sites to one IP address.

With SMTP to MX, there are the same problems here as in choosing the identity
against which to validate a certificate: you can't rely on insecure DNS to
provide the identity which you then cryptographically verify. So this will be
of limited use in that environment.

With SMTP to Submission, there is a well-defined hostname which clients are
connecting to and can validate certificates against. Thus clients can choose to
include this information in the TLS negotiation. If this becomes wide-spread,
then hosters can choose to present different certificates to different clients.
Or even negotiate different cipher suites.

The tls_sni option on an SMTP transport is an expanded string; the result, if
not empty, will be sent on a TLS session as part of the handshake. There's
nothing more to it. Choosing a sensible value not derived insecurely is the
only point of caution. The $tls_out_sni variable will be set to this string for
the lifetime of the client connection (including during authentication).

If DANE validated the connection attempt then the value of the tls_sni option
is forced to the name of the destination host, after any MX- or
CNAME-following.

Except during SMTP client sessions, if $tls_in_sni is set then it is a string
received from a client. It can be logged with the log_selector item "+tls_sni".

If the string "tls_in_sni" appears in the main section's tls_certificate option
(prior to expansion) then the following options will be re-expanded during TLS
session handshake, to permit alternative values to be chosen:

  * tls_certificate

  * tls_crl

  * tls_privatekey

  * tls_verify_certificates

  * tls_ocsp_file

Great care should be taken to deal with matters of case, various injection
attacks in the string ("../" or SQL), and ensuring that a valid filename can
always be referenced; it is important to remember that $tls_in_sni is arbitrary
unverified data provided prior to authentication. Further, the initial
certificate is loaded before SNI has arrived, so an expansion for 
tls_certificate must have a default which is used when $tls_in_sni is empty.

The Exim developers are proceeding cautiously and so far no other TLS options
are re-expanded.

When Exim is built against OpenSSL, OpenSSL must have been built with support
for TLS Extensions. This holds true for OpenSSL 1.0.0+ and 0.9.8+ with
enable-tlsext in EXTRACONFIGURE. If you invoke openssl s_client -h and see
"-servername" in the output, then OpenSSL has support.

When Exim is built against GnuTLS, SNI support is available as of GnuTLS
0.5.10. (Its presence predates the current API which Exim uses, so if Exim
built, then you have SNI support).

43.8.1 ALPN

There is a TLS feature related to SNI called Application Layer Protocol Name
(ALPN). This is intended to declare, or select, what protocol layer will be
using a TLS connection. The client for the connection proposes a set of
protocol names, and the server responds with a selected one. It is not, as of
2021, commonly used for SMTP connections. However, to guard against misdirected
or malicious use of web clients (which often do use ALPN) against MTA ports,
Exim by default check that there is no incompatible ALPN specified by a client
for a TLS connection. If there is, the connection is rejected.

As a client Exim does not supply ALPN by default. The behaviour of both client
and server can be configured using the options tls_alpn and hosts_require_alpn.
There are no variables providing observability. Some feature-specific logging
may appear on denied connections, but this depends on the behaviour of the peer
(not all peers can send a feature-specific TLS Alert).

This feature is available when Exim is built with OpenSSL 1.1.0 or later or
GnuTLS 3.2.0 or later; the macro _HAVE_TLS_ALPN will be defined when this is
so.


43.9 Multiple messages on the same encrypted TCP/IP connection
--------------------------------------------------------------

Exim sends multiple messages down the same TCP/IP connection by starting up an
entirely new delivery process for each message, passing the socket from one
process to the next. This implementation does not fit well with the use of TLS,
because there is quite a lot of state information associated with a TLS
connection, not just a socket identification. Passing all the state information
to a new process is not feasible. Consequently, for sending using TLS Exim
starts an additional proxy process for handling the encryption, piping the
unencrypted data stream from and to the delivery processes.

An older mode of operation can be enabled on a per-host basis by the 
hosts_noproxy_tls option on the smtp transport. If the host matches this list
the proxy process described above is not used; instead Exim shuts down an
existing TLS session being run by the delivery process before passing the
socket to a new process. The new process may then try to start a new TLS
session, and if successful, may try to re-authenticate if AUTH is in use,
before sending the next message.

The RFC is not clear as to whether or not an SMTP session continues in clear
after TLS has been shut down, or whether TLS may be restarted again later, as
just described. However, if the server is Exim, this shutdown and
reinitialization works. It is not known which (if any) other servers operate
successfully if the client closes a TLS session and continues with unencrypted
SMTP, but there are certainly some that do not work. For such servers, Exim
should not pass the socket to another process, because the failure of the
subsequent attempt to use it would cause Exim to record a temporary host error,
and delay other deliveries to that host.

To test for this case, Exim sends an EHLO command to the server after closing
down the TLS session. If this fails in any way, the connection is closed
instead of being passed to a new delivery process, but no retry information is
recorded.

There is also a manual override; you can set hosts_nopass_tls on the smtp
transport to match those hosts for which Exim should not pass connections to
new processes if TLS has been used.


43.10 Certificates and all that
-------------------------------

In order to understand fully how TLS works, you need to know about
certificates, certificate signing, and certificate authorities. This is a large
topic and an introductory guide is unsuitable for the Exim reference manual, so
instead we provide pointers to existing documentation.

The Apache web-server was for a long time the canonical guide, so their
documentation is a good place to start; their SSL module's Introduction
document is currently at

https://httpd.apache.org/docs/current/ssl/ssl_intro.html

and their FAQ is at

https://httpd.apache.org/docs/current/ssl/ssl_faq.html

Eric Rescorla's book, SSL and TLS, published by Addison-Wesley (ISBN
0-201-61598-3) in 2001, contains both introductory and more in-depth
descriptions. More recently Ivan Risti?'s book Bulletproof SSL and TLS,
published by Feisty Duck (ISBN 978-1907117046) in 2013 is good. Ivan is the
author of the popular TLS testing tools at https://www.ssllabs.com/.

43.10.1 Certificate chains

A file named by tls_certificate may contain more than one certificate. This is
useful in the case where the certificate that is being sent is validated by an
intermediate certificate which the other end does not have. Multiple
certificates must be in the correct order in the file. First the host's
certificate itself, then the first intermediate certificate to validate the
issuer of the host certificate, then the next intermediate certificate to
validate the issuer of the first intermediate certificate, and so on, until
finally (optionally) the root certificate. The root certificate must already be
trusted by the recipient for validation to succeed, of course, but if it's not
preinstalled, sending the root certificate along with the rest makes it
available for the user to install if the receiving end is a client MUA that can
interact with a user.

Note that certificates using MD5 are unlikely to work on today's Internet; even
if your libraries allow loading them for use in Exim when acting as a server,
increasingly clients will not accept such certificates. The error diagnostics
in such a case can be frustratingly vague.

43.10.2 Self-signed certificates

You can create a self-signed certificate using the req command provided with
OpenSSL, like this:

openssl req -x509 -newkey rsa:1024 -keyout file1 -out file2 \
            -days 9999 -nodes

file1 and file2 can be the same file; the key and the certificate are delimited
and so can be identified independently. The -days option specifies a period for
which the certificate is valid. The -nodes option is important: if you do not
set it, the key is encrypted with a passphrase that you are prompted for, and
any use that is made of the key causes more prompting for the passphrase. This
is not helpful if you are going to use this certificate and key in an MTA,
where prompting is not possible.

NB: we are now past the point where 9999 days takes us past the 32-bit Unix
epoch. If your system uses unsigned time_t (most do) and is 32-bit, then the
above command might produce a date in the past. Think carefully about the
lifetime of the systems you're deploying, and either reduce the duration of the
certificate or reconsider your platform deployment. (At time of writing,
reducing the duration is the most likely choice, but the inexorable progression
of time takes us steadily towards an era where this will not be a sensible
resolution).

A self-signed certificate made in this way is sufficient for testing, and may
be adequate for all your requirements if you are mainly interested in
encrypting transfers, and not in secure identification.

However, many clients require that the certificate presented by the server be a
user (also called "leaf" or "site") certificate, and not a self-signed
certificate. In this situation, the self-signed certificate described above
must be installed on the client host as a trusted root certification authority
(CA), and the certificate used by Exim must be a user certificate signed with
that self-signed certificate.

For information on creating self-signed CA certificates and using them to sign
user certificates, see the General implementation overview chapter of the
Open-source PKI book, available online at https://sourceforge.net/projects/
ospkibook/.

43.10.3 Revoked certificates

There are three ways for a certificate to be made unusable before its expiry.

  * Certificate issuing authorities issue Certificate Revocation Lists (CRLs)
    when certificates are revoked. If you have such a list, you can pass it to
    an Exim server using the global option called tls_crl and to an Exim client
    using an identically named option for the smtp transport. In each case, the
    value of the option is expanded and must then be the name of a file that
    contains a CRL in PEM format. The downside is that clients have to
    periodically re-download a potentially huge file from every certificate
    authority they know of.

  * The way with most moving parts at query time is Online Certificate Status
    Protocol (OCSP), where the client verifies the certificate against an OCSP
    server run by the CA. This lets the CA track all usage of the certs. It
    requires running software with access to the private key of the CA, to sign
    the responses to the OCSP queries. OCSP is based on HTTP and can be proxied
    accordingly.

    The only widespread OCSP server implementation (known to this writer) comes
    as part of OpenSSL and aborts on an invalid request, such as connecting to
    the port and then disconnecting. This requires re-entering the passphrase
    each time some random client does this.

  * The third way is OCSP Stapling; in this, the server using a certificate
    issued by the CA periodically requests an OCSP proof of validity from the
    OCSP server, then serves it up inline as part of the TLS negotiation. This
    approach adds no extra round trips, does not let the CA track users, scales
    well with number of certs issued by the CA and is resilient to temporary
    OCSP server failures, as long as the server starts retrying to fetch an
    OCSP proof some time before its current proof expires. The downside is that
    it requires server support.

    Unless Exim is built with the support disabled, or with GnuTLS earlier than
    version 3.3.16 / 3.4.8 support for OCSP stapling is included.

    There is a global option called tls_ocsp_file. The file specified therein
    is expected to be in DER format, and contain an OCSP proof. Exim will serve
    it as part of the TLS handshake. This option will be re-expanded for SNI,
    if the tls_certificate option contains "tls_in_sni", as per other TLS
    options.

    Exim does not at this time implement any support for fetching a new OCSP
    proof. The burden is on the administrator to handle this, outside of Exim.
    The file specified should be replaced atomically, so that the contents are
    always valid. Exim will expand the tls_ocsp_file option on each connection,
    so a new file will be handled transparently on the next connection.

    When built with OpenSSL Exim will check for a valid next update timestamp
    in the OCSP proof; if not present, or if the proof has expired, it will be
    ignored.

    For the client to be able to verify the stapled OCSP the server must also
    supply, in its stapled information, any intermediate certificates for the
    chain leading to the OCSP proof from the signer of the server certificate.
    There may be zero or one such. These intermediate certificates should be
    added to the server OCSP stapling file named by tls_ocsp_file.

    Note that the proof only covers the terminal server certificate, not any of
    the chain from CA to it.

    There is no current way to staple a proof for a client certificate.

      A helper script "ocsp_fetch.pl" for fetching a proof from a CA
      OCSP server is supplied.  The server URL may be included in the
      server certificate, if the CA is helpful.

      One failure mode seen was the OCSP Signer cert expiring before the end
      of validity of the OCSP proof. The checking done by Exim/OpenSSL
      noted this as invalid overall, but the re-fetch script did not.


43.11 TLS Resumption
--------------------

TLS Session Resumption for TLS 1.2 and TLS 1.3 connections can be used (defined
in RFC 5077 for 1.2). The support for this requires GnuTLS 3.6.3 or OpenSSL
1.1.1 (or later).

Session resumption (this is the "stateless" variant) involves the server
sending a "session ticket" to the client on one connection, which can be stored
by the client and used for a later session. The ticket contains sufficient
state for the server to reconstruct the TLS session, avoiding some expensive
crypto calculation and (on TLS1.2) one full packet roundtrip time.

  * Operational cost/benefit:

    The extra data being transmitted costs a minor amount, and the client has
    extra costs in storing and retrieving the data.

    In the Exim/Gnutls implementation the extra cost on an initial connection
    which is TLS1.2 over a loopback path is about 6ms on 2017-laptop class
    hardware. The saved cost on a subsequent connection is about 4ms; three or
    more connections become a net win. On longer network paths, two or more
    connections will have an average lower startup time thanks to the one saved
    packet roundtrip. TLS1.3 will save the crypto cpu costs but not any packet
    roundtrips.

    Since a new hints DB is used on the TLS client, the hints DB maintenance
    should be updated to additionally handle "tls".

  * Security aspects:

    The session ticket is encrypted, but is obviously an additional security
    vulnarability surface. An attacker able to decrypt it would have access all
    connections using the resumed session. The session ticket encryption key is
    not committed to storage by the server and is rotated regularly (OpenSSL:
    1hr, and one previous key is used for overlap; GnuTLS 6hr but does not
    specify any overlap). Tickets have limited lifetime (2hr, and new ones
    issued after 1hr under OpenSSL. GnuTLS 2hr, appears to not do overlap).

    There is a question-mark over the security of the Diffie-Helman parameters
    used for session negotiation.

  * Observability:

    The log_selector "tls_resumption" appends an asterisk to the tls_cipher "X=
    " element.

    The variables $tls_in_resumption and $tls_out_resumption have bits 0-4
    indicating respectively support built, client requested ticket, client
    offered session, server issued ticket, resume used. A suitable decode list
    is provided in the builtin macro _RESUME_DECODE for in listextract
    expansions.

  * Control:

    The tls_resumption_hosts main option specifies a hostlist for which exim,
    operating as a server, will offer resumption to clients. Current best
    practice is to not offer the feature to MUA connection. Commonly this can
    be done like this:

    tls_resumption_hosts = ${if inlist {$received_port}{587:465} {:}{*}}

    If the peer host matches the list after expansion then resumption is
    offered and/or accepted.

    The tls_resumption_hosts smtp transport option performs the equivalent
    function for operation as a client. If the peer host matches the list after
    expansion then resumption is attempted (if a stored session is available)
    or the information stored (if supplied by the peer).

  * Issues:

    In a resumed session:

      + The variables $tls_{in,out}_cipher will have values different to the
        original (under GnuTLS).

      + The variables $tls_{in,out}_ocsp will be "not requested" or "no
        response", and the hosts_require_ocsp smtp trasnport option will fail.


43.12 DANE
----------

DNS-based Authentication of Named Entities, as applied to SMTP over TLS,
provides assurance to a client that it is actually talking to the server it
wants to rather than some attacker operating a Man In The Middle (MITM)
operation. The latter can terminate the TLS connection you make, and make
another one to the server (so both you and the server still think you have an
encrypted connection) and, if one of the "well known" set of Certificate
Authorities has been suborned - something which *has* been seen already (2014),
a verifiable certificate (if you're using normal root CAs, eg. the Mozilla set,
as your trust anchors).

What DANE does is replace the CAs with the DNS as the trust anchor. The
assurance is limited to a) the possibility that the DNS has been suborned, b)
mistakes made by the admins of the target server. The attack surface presented
by (a) is thought to be smaller than that of the set of root CAs.

It also allows the server to declare (implicitly) that connections to it should
use TLS. An MITM could simply fail to pass on a server's STARTTLS.

DANE scales better than having to maintain (and communicate via side-channel)
copies of server certificates for every possible target server. It also scales
(slightly) better than having to maintain on an SMTP client a copy of the
standard CAs bundle. It also means not having to pay a CA for certificates.

DANE requires a server operator to do three things:

 1. Run DNSSEC. This provides assurance to clients that DNS lookups they do for
    the server have not been tampered with. The domain MX record applying to
    this server, its A record, its TLSA record and any associated CNAME records
    must all be covered by DNSSEC.

 2. Add TLSA DNS records. These say what the server certificate for a TLS
    connection should be.

 3. Offer a server certificate, or certificate chain, in TLS connections which
    is anchored by one of the TLSA records.

There are no changes to Exim specific to server-side operation of DANE. Support
for client-side operation of DANE can be included at compile time by defining
SUPPORT_DANE=yes in Local/Makefile. If it has been included, the macro
"_HAVE_DANE" will be defined.

43.12.1 DNS records

A TLSA record consist of 4 fields, the "Certificate Usage", the "Selector", the
"Matching type", and the "Certificate Association Data". For a detailed
description of the TLSA record see RFC 7671.

The TLSA record for the server may have "Certificate Usage" (1st) field of
DANE-TA(2) or DANE-EE(3). These are the "Trust Anchor" and "End Entity"
variants. The latter specifies the End Entity directly, i.e. the certificate
involved is that of the server (and if only DANE-EE is used then it should be
the sole one transmitted during the TLS handshake); this is appropriate for a
single system, using a self-signed certificate. DANE-TA usage is effectively
declaring a specific CA to be used; this might be a private CA or a public,
well-known one. A private CA at simplest is just a self-signed certificate
(with certain attributes) which is used to sign server certificates, but
running one securely does require careful arrangement. With DANE-TA, as
implemented in Exim and commonly in other MTAs, the server TLS handshake must
transmit the entire certificate chain from CA to server-certificate. DANE-TA is
commonly used for several services and/or servers, each having a TLSA
query-domain CNAME record, all of which point to a single TLSA record. DANE-TA
and DANE-EE can both be used together.

Our recommendation is to use DANE with a certificate from a public CA, because
this enables a variety of strategies for remote clients to verify your
certificate. You can then publish information both via DANE and another
technology, "MTA-STS", described below.

When you use DANE-TA to publish trust anchor information, you ask entities
outside your administrative control to trust the Certificate Authority for
connections to you. If using a private CA then you should expect others to
still apply the technical criteria they'd use for a public CA to your
certificates. In particular, you should probably try to follow current best
practices for CA operation around hash algorithms and key sizes. Do not expect
other organizations to lower their security expectations just because a
particular profile might be reasonable for your own internal use.

When this text was last updated, this in practice means to avoid use of SHA-1
and MD5; if using RSA to use key sizes of at least 2048 bits (and no larger
than 4096, for interoperability); to use keyUsage fields correctly; to use
random serial numbers. The list of requirements is subject to change as best
practices evolve. If you're not already using a private CA, or it doesn't meet
these requirements, then we encourage you to avoid all these issues and use a
public CA such as Let's Encrypt instead.

The TLSA record should have a "Selector" (2nd) field of SPKI(1) and a "Matching
Type" (3rd) field of SHA2-512(2).

For the "Certificate Authority Data" (4th) field, commands like

  openssl x509 -pubkey -noout <certificate.pem \
  | openssl rsa -outform der -pubin 2>/dev/null \
  | openssl sha512 \
  | awk '{print $2}'

are workable to create a hash of the certificate's public key.

An example TLSA record for DANE-EE(3), SPKI(1), and SHA-512 (2) looks like

  _25._tcp.mail.example.com. TLSA 3 1 2 8BA8A336E...

At the time of writing, https://www.huque.com/bin/gen_tlsa is useful for
quickly generating TLSA records.

For use with the DANE-TA model, server certificates must have a correct name
(SubjectName or SubjectAltName).

The Certificate issued by the CA published in the DANE-TA model should be
issued using a strong hash algorithm. Exim, and importantly various other MTAs
sending to you, will not re-enable hash algorithms which have been disabled by
default in TLS libraries. This means no MD5 and no SHA-1. SHA2-256 is the
minimum for reliable interoperability (and probably the maximum too, in 2018).

43.12.2 Interaction with OCSP

The use of OCSP-stapling should be considered, allowing for fast revocation of
certificates (which would otherwise be limited by the DNS TTL on the TLSA
records). However, this is likely to only be usable with DANE-TA. NOTE: the
default of requesting OCSP for all hosts is modified iff DANE is in use, to:

  hosts_request_ocsp = ${if or { {= {0}{$tls_out_tlsa_usage}} \
                                 {= {4}{$tls_out_tlsa_usage}} } \
                         {*}{}}

The (new) variable $tls_out_tlsa_usage is a bitfield with numbered bits set for
TLSA record usage codes. The zero above means DANE was not in use, the four
means that only DANE-TA usage TLSA records were found. If the definition of 
hosts_request_ocsp includes the string "tls_out_tlsa_usage", they are
re-expanded in time to control the OCSP request.

This modification of hosts_request_ocsp is only done if it has the default
value of "*". Admins who change it, and those who use hosts_require_ocsp,
should consider the interaction with DANE in their OCSP settings.

43.12.3 Client configuration

For client-side DANE there are three new smtp transport options, hosts_try_dane
, hosts_require_dane and dane_require_tls_ciphers. The "require" variant will
result in failure if the target host is not DNSSEC-secured. To get
DNSSEC-secured hostname resolution, use the dnssec_request_domains router or
transport option.

DANE will only be usable if the target host has DNSSEC-secured MX, A and TLSA
records.

A TLSA lookup will be done if either of the above options match and the
host-lookup succeeded using DNSSEC. If a TLSA lookup is done and succeeds, a
DANE-verified TLS connection will be required for the host. If it does not, the
host will not be used; there is no fallback to non-DANE or non-TLS.

If DANE is requested and usable, then the TLS cipher list configuration prefers
to use the option dane_require_tls_ciphers and falls back to 
tls_require_ciphers only if that is unset. This lets you configure "decent
crypto" for DANE and "better than nothing crypto" as the default. Note though
that while GnuTLS lets the string control which versions of TLS/SSL will be
negotiated, OpenSSL does not and you're limited to ciphersuite constraints.

If DANE is requested and useable (see above) the following transport options
are ignored:

  hosts_require_tls
  tls_verify_hosts
  tls_try_verify_hosts
  tls_verify_certificates
  tls_crl
  tls_verify_cert_hostnames
  tls_sni

If DANE is not usable, whether requested or not, and CA-anchored verification
evaluation is wanted, the above variables should be set appropriately.

The router and transport option dnssec_request_domains must not be set to
"never", and dnssec_require_domains is ignored.

43.12.4 Observability

If verification was successful using DANE then the "CV" item in the delivery
log line will show as "CV=dane".

There is a new variable $tls_out_dane which will have "yes" if verification
succeeded using DANE and "no" otherwise (only useful in combination with
events; see 61), and a new variable $tls_out_tlsa_usage (detailed above).

An event (see 61) of type "dane:fail" will be raised on failures to achieve
DANE-verified connection, if one was either requested and offered, or required.
This is intended to support TLS-reporting as defined in https://tools.ietf.org/
html/draft-ietf-uta-smtp-tlsrpt-17. The $event_data will be one of the Result
Types defined in Section 4.3 of that document.

43.12.5 General

Under GnuTLS, DANE is only supported from version 3.0.0 onwards.

DANE is specified in published RFCs and decouples certificate authority trust
selection from a "race to the bottom" of "you must trust everything for mail to
get through". There is an alternative technology called MTA-STS, which instead
publishes MX trust anchor information on an HTTPS website. At the time this
text was last updated, MTA-STS was still a draft, not yet an RFC. Exim has no
support for MTA-STS as a client, but Exim mail server operators can choose to
publish information describing their TLS configuration using MTA-STS to let
those clients who do use that protocol derive trust information.

The MTA-STS design requires a certificate from a public Certificate Authority
which is recognized by clients sending to you. That selection of which CAs are
trusted by others is outside your control.

The most interoperable course of action is probably to use Let's Encrypt, with
automated certificate renewal; to publish the anchor information in
DNSSEC-secured DNS via TLSA records for DANE clients (such as Exim and Postfix)
and to publish anchor information for MTA-STS as well. This is what is done for
the exim.org domain itself (with caveats around occasionally broken MTA-STS
because of incompatible specification changes prior to reaching RFC status).



===============================================================================
44. ACCESS CONTROL LISTS

Access Control Lists (ACLs) are defined in a separate section of the runtime
configuration file, headed by "begin acl". Each ACL definition starts with a
name, terminated by a colon. Here is a complete ACL section that contains just
one very small ACL:

begin acl
small_acl:
  accept   hosts = one.host.only

You can have as many lists as you like in the ACL section, and the order in
which they appear does not matter. The lists are self-terminating.

The majority of ACLs are used to control Exim's behaviour when it receives
certain SMTP commands. This applies both to incoming TCP/IP connections, and
when a local process submits a message using SMTP by specifying the -bs option.
The most common use is for controlling which recipients are accepted in
incoming messages. In addition, you can define an ACL that is used to check
local non-SMTP messages. The default configuration file contains an example of
a realistic ACL for checking RCPT commands. This is discussed in chapter 7.


44.1 Testing ACLs
-----------------

The -bh command line option provides a way of testing your ACL configuration
locally by running a fake SMTP session with which you interact.


44.2 Specifying when ACLs are used
----------------------------------

In order to cause an ACL to be used, you have to name it in one of the relevant
options in the main part of the configuration. These options are: 

    acl_not_smtp       ACL for non-SMTP messages
    acl_not_smtp_mime  ACL for non-SMTP MIME parts
    acl_not_smtp_start ACL at start of non-SMTP message
    acl_smtp_auth      ACL for AUTH
    acl_smtp_connect   ACL for start of SMTP connection
    acl_smtp_data      ACL after DATA is complete
    acl_smtp_data_prdr ACL for each recipient, after DATA is complete
    acl_smtp_dkim      ACL for each DKIM signer
    acl_smtp_etrn      ACL for ETRN
    acl_smtp_expn      ACL for EXPN
    acl_smtp_helo      ACL for HELO or EHLO
    acl_smtp_mail      ACL for MAIL
    acl_smtp_mailauth  ACL for the AUTH parameter of MAIL
    acl_smtp_mime      ACL for content-scanning MIME parts
    acl_smtp_notquit   ACL for non-QUIT terminations
    acl_smtp_predata   ACL at start of DATA command
    acl_smtp_quit      ACL for QUIT
    acl_smtp_rcpt      ACL for RCPT
    acl_smtp_starttls  ACL for STARTTLS
    acl_smtp_vrfy      ACL for VRFY

For example, if you set

acl_smtp_rcpt = small_acl

the little ACL defined above is used whenever Exim receives a RCPT command in
an SMTP dialogue. The majority of policy tests on incoming messages can be done
when RCPT commands arrive. A rejection of RCPT should cause the sending MTA to
give up on the recipient address contained in the RCPT command, whereas
rejection at other times may cause the client MTA to keep on trying to deliver
the message. It is therefore recommended that you do as much testing as
possible at RCPT time.

44.2.1 The non-SMTP ACLs

The non-SMTP ACLs apply to all non-interactive incoming messages, that is, they
apply to batched SMTP as well as to non-SMTP messages. (Batched SMTP is not
really SMTP.) Many of the ACL conditions (for example, host tests, and tests on
the state of the SMTP connection such as encryption and authentication) are not
relevant and are forbidden in these ACLs. However, the sender and recipients
are known, so the senders and sender_domains conditions and the $sender_address
and $recipients variables can be used. Variables such as $authenticated_sender
are also available. You can specify added header lines in any of these ACLs.

The acl_not_smtp_start ACL is run right at the start of receiving a non-SMTP
message, before any of the message has been read. (This is the analogue of the 
acl_smtp_predata ACL for SMTP input.) In the case of batched SMTP input, it
runs after the DATA command has been reached. The result of this ACL is
ignored; it cannot be used to reject a message. If you really need to, you
could set a value in an ACL variable here and reject based on that in the 
acl_not_smtp ACL. However, this ACL can be used to set controls, and in
particular, it can be used to set

control = suppress_local_fixups

This cannot be used in the other non-SMTP ACLs because by the time they are
run, it is too late.

The acl_not_smtp_mime ACL is available only when Exim is compiled with the
content-scanning extension. For details, see chapter 45.

The acl_not_smtp ACL is run just before the local_scan() function. Any kind of
rejection is treated as permanent, because there is no way of sending a
temporary error for these kinds of message.

44.2.2 The SMTP connect ACL

The ACL test specified by acl_smtp_connect happens at the start of an SMTP
session, after the test specified by host_reject_connection (which is now an
anomaly) and any TCP Wrappers testing (if configured). If the connection is
accepted by an accept verb that has a message modifier, the contents of the
message override the banner message that is otherwise specified by the 
smtp_banner option.

For tls-on-connect connections, the ACL is run before the TLS connection is
accepted; if the ACL does not accept then the TCP connection is dropped without
any TLS startup attempt and without any SMTP response being transmitted.

44.2.3 The EHLO/HELO ACL

The ACL test specified by acl_smtp_helo happens when the client issues an EHLO
or HELO command, after the tests specified by helo_accept_junk_hosts, 
helo_allow_chars, helo_verify_hosts, and helo_try_verify_hosts. Note that a
client may issue more than one EHLO or HELO command in an SMTP session, and
indeed is required to issue a new EHLO or HELO after successfully setting up
encryption following a STARTTLS command.

Note also that a deny neither forces the client to go away nor means that mail
will be refused on the connection. Consider checking for $sender_helo_name
being defined in a MAIL or RCPT ACL to do that.

If the command is accepted by an accept verb that has a message modifier, the
message may not contain more than one line (it will be truncated at the first
newline and a panic logged if it does). Such a message cannot affect the EHLO
options that are listed on the second and subsequent lines of an EHLO response.

44.2.4 The DATA ACLs

Two ACLs are associated with the DATA command, because it is two-stage command,
with two responses being sent to the client. When the DATA command is received,
the ACL defined by acl_smtp_predata is obeyed. This gives you control after all
the RCPT commands, but before the message itself is received. It offers the
opportunity to give a negative response to the DATA command before the data is
transmitted. Header lines added by MAIL or RCPT ACLs are not visible at this
time, but any that are defined here are visible when the acl_smtp_data ACL is
run.

You cannot test the contents of the message, for example, to verify addresses
in the headers, at RCPT time or when the DATA command is received. Such tests
have to appear in the ACL that is run after the message itself has been
received, before the final response to the DATA command is sent. This is the
ACL specified by acl_smtp_data, which is the second ACL that is associated with
the DATA command.

If CHUNKING was advertised and a BDAT command sequence is received, the 
acl_smtp_predata ACL is not run. The acl_smtp_data is run after the last BDAT
command and all of the data specified is received.

For both of these ACLs, it is not possible to reject individual recipients. An
error response rejects the entire message. Unfortunately, it is known that some
MTAs do not treat hard (5xx) responses to the DATA command (either before or
after the data) correctly - they keep the message on their queues and try again
later, but that is their problem, though it does waste some of your resources.

The acl_smtp_data ACL is run after the acl_smtp_data_prdr, the acl_smtp_dkim
and the acl_smtp_mime ACLs.

44.2.5 The SMTP DKIM ACL

The acl_smtp_dkim ACL is available only when Exim is compiled with DKIM support
enabled (which is the default).

The ACL test specified by acl_smtp_dkim happens after a message has been
received, and is executed for each DKIM signature found in a message. If not
otherwise specified, the default action is to accept.

This ACL is evaluated before acl_smtp_mime and acl_smtp_data.

For details on the operation of DKIM, see section 58.1.

44.2.6 The SMTP MIME ACL

The acl_smtp_mime option is available only when Exim is compiled with the
content-scanning extension. For details, see chapter 45.

This ACL is evaluated after acl_smtp_dkim but before acl_smtp_data.

44.2.7 The SMTP PRDR ACL

The acl_smtp_data_prdr ACL is available only when Exim is compiled with PRDR
support enabled (which is the default). It becomes active only when the PRDR
feature is negotiated between client and server for a message, and more than
one recipient has been accepted.

The ACL test specified by acl_smtp_data_prdr happens after a message has been
received, and is executed once for each recipient of the message with
$local_part and $domain valid. The test may accept, defer or deny for
individual recipients. The acl_smtp_data will still be called after this ACL
and can reject the message overall, even if this ACL has accepted it for some
or all recipients.

PRDR may be used to support per-user content filtering. Without it one must
defer any recipient after the first that has a different content-filter
configuration. With PRDR, the RCPT-time check for this can be disabled when the
variable $prdr_requested is "yes". Any required difference in behaviour of the
main DATA-time ACL should however depend on the PRDR-time ACL having run, as
Exim will avoid doing so in some situations (e.g. single-recipient mails).

See also the prdr_enable global option and the hosts_try_prdr smtp transport
option.

This ACL is evaluated after acl_smtp_dkim but before acl_smtp_data. If the ACL
is not defined, processing completes as if the feature was not requested by the
client.

44.2.8 The QUIT ACL

The ACL for the SMTP QUIT command is anomalous, in that the outcome of the ACL
does not affect the response code to QUIT, which is always 221. Thus, the ACL
does not in fact control any access. For this reason, it may only accept or
warn as its final result.

This ACL can be used for tasks such as custom logging at the end of an SMTP
session. For example, you can use ACL variables in other ACLs to count
messages, recipients, etc., and log the totals at QUIT time using one or more 
logwrite modifiers on a warn verb.

Warning: Only the $acl_cx variables can be used for this, because the $acl_mx
variables are reset at the end of each incoming message.

You do not need to have a final accept, but if you do, you can use a message
modifier to specify custom text that is sent as part of the 221 response to
QUIT.

This ACL is run only for a "normal" QUIT. For certain kinds of disastrous
failure (for example, failure to open a log file, or when Exim is bombing out
because it has detected an unrecoverable error), all SMTP commands from the
client are given temporary error responses until QUIT is received or the
connection is closed. In these special cases, the QUIT ACL does not run.

44.2.9 The not-QUIT ACL

The not-QUIT ACL, specified by acl_smtp_notquit, is run in most cases when an
SMTP session ends without sending QUIT. However, when Exim itself is in bad
trouble, such as being unable to write to its log files, this ACL is not run,
because it might try to do things (such as write to log files) that make the
situation even worse.

Like the QUIT ACL, this ACL is provided to make it possible to do customized
logging or to gather statistics, and its outcome is ignored. The delay modifier
is forbidden in this ACL, and the only permitted verbs are accept and warn.

When the not-QUIT ACL is running, the variable $smtp_notquit_reason is set to a
string that indicates the reason for the termination of the SMTP connection.
The possible values are:

    "acl-drop"              Another ACL issued a drop command
    "bad-commands"          Too many unknown or non-mail commands
    "command-timeout"       Timeout while reading SMTP commands
    "connection-lost"       The SMTP connection has been lost
    "data-timeout"          Timeout while reading message data
    "local-scan-error"      The local_scan() function crashed
    "local-scan-timeout"    The local_scan() function timed out
    "signal-exit"           SIGTERM or SIGINT
    "synchronization-error" SMTP synchronization error
    "tls-failed"            TLS failed to start

In most cases when an SMTP connection is closed without having received QUIT,
Exim sends an SMTP response message before actually closing the connection.
With the exception of the "acl-drop" case, the default message can be
overridden by the message modifier in the not-QUIT ACL. In the case of a drop
verb in another ACL, it is the message from the other ACL that is used.


44.3 Finding an ACL to use
--------------------------

The value of an acl_smtp_xxx option is expanded before use, so you can use
different ACLs in different circumstances. For example,

acl_smtp_rcpt = ${if ={25}{$interface_port} \
                     {acl_check_rcpt} {acl_check_rcpt_submit} }

In the default configuration file there are some example settings for providing
an RFC 4409 message "submission" service on port 587 and an RFC 8314
"submissions" service on port 465. You can use a string expansion like this to
choose an ACL for MUAs on these ports which is more appropriate for this
purpose than the default ACL on port 25.

The expanded string does not have to be the name of an ACL in the configuration
file; there are other possibilities. Having expanded the string, Exim searches
for an ACL as follows:

  * If the string begins with a slash, Exim uses it as a filename, and reads
    its contents as an ACL. The lines are processed in the same way as lines in
    the Exim configuration file. In particular, continuation lines are
    supported, blank lines are ignored, as are lines whose first non-whitespace
    character is "#". If the file does not exist or cannot be read, an error
    occurs (typically causing a temporary failure of whatever caused the ACL to
    be run). For example:

    acl_smtp_data = /etc/acls/\
      ${lookup{$sender_host_address}lsearch\
      {/etc/acllist}{$value}{default}}

    This looks up an ACL file to use on the basis of the host's IP address,
    falling back to a default if the lookup fails. If an ACL is successfully
    read from a file, it is retained in memory for the duration of the Exim
    process, so that it can be re-used without having to re-read the file.

  * If the string does not start with a slash, and does not contain any spaces,
    Exim searches the ACL section of the configuration for an ACL whose name
    matches the string.

  * If no named ACL is found, or if the string contains spaces, Exim parses the
    string as an inline ACL. This can save typing in cases where you just want
    to have something like

    acl_smtp_vrfy = accept

    in order to allow free use of the VRFY command. Such a string may contain
    newlines; it is processed in the same way as an ACL that is read from a
    file.


44.4 ACL return codes
---------------------

Except for the QUIT ACL, which does not affect the SMTP return code (see
section 44.2.8 above), the result of running an ACL is either "accept" or
"deny", or, if some test cannot be completed (for example, if a database is
down), "defer". These results cause 2xx, 5xx, and 4xx return codes,
respectively, to be used in the SMTP dialogue. A fourth return, "error", occurs
when there is an error such as invalid syntax in the ACL. This also causes a 4
xx return code.

For the non-SMTP ACL, "defer" and "error" are treated in the same way as
"deny", because there is no mechanism for passing temporary errors to the
submitters of non-SMTP messages.

ACLs that are relevant to message reception may also return "discard". This has
the effect of "accept", but causes either the entire message or an individual
recipient address to be discarded. In other words, it is a blackholing
facility. Use it with care.

If the ACL for MAIL returns "discard", all recipients are discarded, and no ACL
is run for subsequent RCPT commands. The effect of "discard" in a RCPT ACL is
to discard just the one recipient address. If there are no recipients left when
the message's data is received, the DATA ACL is not run. A "discard" return
from the DATA or the non-SMTP ACL discards all the remaining recipients. The
"discard" return is not permitted for the acl_smtp_predata ACL.

If the ACL for VRFY returns "accept", a recipient verify (without callout) is
done on the address and the result determines the SMTP response.

The local_scan() function is always run, even if there are no remaining
recipients; it may create new recipients.


44.5 Unset ACL options
----------------------

The default actions when any of the acl_xxx options are unset are not all the
same. Note: These defaults apply only when the relevant ACL is not defined at
all. For any defined ACL, the default action when control reaches the end of
the ACL statements is "deny".

For acl_smtp_quit and acl_not_smtp_start there is no default because these two
are ACLs that are used only for their side effects. They cannot be used to
accept or reject anything.

For acl_not_smtp, acl_smtp_auth, acl_smtp_connect, acl_smtp_data, acl_smtp_helo
, acl_smtp_mail, acl_smtp_mailauth, acl_smtp_mime, acl_smtp_predata, and 
acl_smtp_starttls, the action when the ACL is not defined is "accept".

For the others (acl_smtp_etrn, acl_smtp_expn, acl_smtp_rcpt, and acl_smtp_vrfy
), the action when the ACL is not defined is "deny". This means that 
acl_smtp_rcpt must be defined in order to receive any messages over an SMTP
connection. For an example, see the ACL in the default configuration file.


44.6 Data for message ACLs
--------------------------

When a MAIL or RCPT ACL, or either of the DATA ACLs, is running, the variables
that contain information about the host and the message's sender (for example,
$sender_host_address and $sender_address) are set, and can be used in ACL
statements. In the case of RCPT (but not MAIL or DATA), $domain and $local_part
are set from the argument address. The entire SMTP command is available in
$smtp_command.

When an ACL for the AUTH parameter of MAIL is running, the variables that
contain information about the host are set, but $sender_address is not yet set.
Section 33.2 contains a discussion of this parameter and how it is used.

The $message_size variable is set to the value of the SIZE parameter on the
MAIL command at MAIL, RCPT and pre-data time, or to -1 if that parameter is not
given. The value is updated to the true message size by the time the final DATA
ACL is run (after the message data has been received).

The $rcpt_count variable increases by one for each RCPT command received. The
$recipients_count variable increases by one each time a RCPT command is
accepted, so while an ACL for RCPT is being processed, it contains the number
of previously accepted recipients. At DATA time (for both the DATA ACLs),
$rcpt_count contains the total number of RCPT commands, and $recipients_count
contains the total number of accepted recipients.


44.7 Data for non-message ACLs
------------------------------

When an ACL is being run for AUTH, EHLO, ETRN, EXPN, HELO, STARTTLS, or VRFY,
the remainder of the SMTP command line is placed in $smtp_command_argument, and
the entire SMTP command is available in $smtp_command. These variables can be
tested using a condition condition. For example, here is an ACL for use with
AUTH, which insists that either the session is encrypted, or the CRAM-MD5
authentication method is used. In other words, it does not permit
authentication methods that use cleartext passwords on unencrypted connections.

acl_check_auth:
  accept encrypted = *
  accept condition = ${if eq{${uc:$smtp_command_argument}}\
                     {CRAM-MD5}}
  deny   message   = TLS encryption or CRAM-MD5 required

(Another way of applying this restriction is to arrange for the authenticators
that use cleartext passwords not to be advertised when the connection is not
encrypted. You can use the generic server_advertise_condition authenticator
option to do this.)


44.8 Format of an ACL
---------------------

An individual ACL consists of a number of statements. Each statement starts
with a verb, optionally followed by a number of conditions and "modifiers".
Modifiers can change the way the verb operates, define error and log messages,
set variables, insert delays, and vary the processing of accepted messages.

If all the conditions are met, the verb is obeyed. The same condition may be
used (with different arguments) more than once in the same statement. This
provides a means of specifying an "and" conjunction between conditions. For
example:

deny  dnslists = list1.example
      dnslists = list2.example

If there are no conditions, the verb is always obeyed. Exim stops evaluating
the conditions and modifiers when it reaches a condition that fails. What
happens then depends on the verb (and in one case, on a special modifier). Not
all the conditions make sense at every testing point. For example, you cannot
test a sender address in the ACL that is run for a VRFY command.


44.9 ACL verbs
--------------

The ACL verbs are as follows:

  * accept: If all the conditions are met, the ACL returns "accept". If any of
    the conditions are not met, what happens depends on whether endpass appears
    among the conditions (for syntax see below). If the failing condition is
    before endpass, control is passed to the next ACL statement; if it is after
    endpass, the ACL returns "deny". Consider this statement, used to check a
    RCPT command:

    accept domains = +local_domains
           endpass
           verify = recipient

    If the recipient domain does not match the domains condition, control
    passes to the next statement. If it does match, the recipient is verified,
    and the command is accepted if verification succeeds. However, if
    verification fails, the ACL yields "deny", because the failing condition is
    after endpass.

    The endpass feature has turned out to be confusing to many people, so its
    use is not recommended nowadays. It is always possible to rewrite an ACL so
    that endpass is not needed, and it is no longer used in the default
    configuration.

    If a message modifier appears on an accept statement, its action depends on
    whether or not endpass is present. In the absence of endpass (when an 
    accept verb either accepts or passes control to the next statement), 
    message can be used to vary the message that is sent when an SMTP command
    is accepted. For example, in a RCPT ACL you could have:

    accept  <some conditions>
            message = OK, I will allow you through today

    You can specify an SMTP response code, optionally followed by an "extended
    response code" at the start of the message, but the first digit must be the
    same as would be sent by default, which is 2 for an accept verb.

    If endpass is present in an accept statement, message specifies an error
    message that is used when access is denied. This behaviour is retained for
    backward compatibility, but current "best practice" is to avoid the use of 
    endpass.

  * defer: If all the conditions are true, the ACL returns "defer" which, in an
    SMTP session, causes a 4xx response to be given. For a non-SMTP ACL, defer
    is the same as deny, because there is no way of sending a temporary error.
    For a RCPT command, defer is much the same as using a redirect router and
    ":defer:" while verifying, but the defer verb can be used in any ACL, and
    even for a recipient it might be a simpler approach.

  * deny: If all the conditions are met, the ACL returns "deny". If any of the
    conditions are not met, control is passed to the next ACL statement. For
    example,

    deny dnslists = blackholes.mail-abuse.org

    rejects commands from hosts that are on a DNS black list.

  * discard: This verb behaves like accept, except that it returns "discard"
    from the ACL instead of "accept". It is permitted only on ACLs that are
    concerned with receiving messages. When all the conditions are true, the
    sending entity receives a "success" response. However, discard causes
    recipients to be discarded. If it is used in an ACL for RCPT, just the one
    recipient is discarded; if used for MAIL, DATA or in the non-SMTP ACL, all
    the message's recipients are discarded. Recipients that are discarded
    before DATA do not appear in the log line when the received_recipients log
    selector is set.

    If the log_message modifier is set when discard operates, its contents are
    added to the line that is automatically written to the log. The message
    modifier operates exactly as it does for accept.

  * drop: This verb behaves like deny, except that an SMTP connection is
    forcibly closed after the 5xx error message has been sent. For example:

    drop   condition = ${if > {$rcpt_count}{20}}
           message   = I don't take more than 20 RCPTs

    There is no difference between deny and drop for the connect-time ACL. The
    connection is always dropped after sending a 550 response.

  * require: If all the conditions are met, control is passed to the next ACL
    statement. If any of the conditions are not met, the ACL returns "deny".
    For example, when checking a RCPT command,

    require message = Sender did not verify
            verify  = sender

    passes control to subsequent statements only if the message's sender can be
    verified. Otherwise, it rejects the command. Note the positioning of the 
    message modifier, before the verify condition. The reason for this is
    discussed in section 44.11.

  * warn: If all the conditions are true, a line specified by the log_message
    modifier is written to Exim's main log. Control always passes to the next
    ACL statement. If any condition is false, the log line is not written. If
    an identical log line is requested several times in the same message, only
    one copy is actually written to the log. If you want to force duplicates to
    be written, use the logwrite modifier instead.

    If log_message is not present, a warn verb just checks its conditions and
    obeys any "immediate" modifiers (such as control, set, logwrite, add_header
    , and remove_header) that appear before the first failing condition. There
    is more about adding header lines in section 44.15.

    If any condition on a warn statement cannot be completed (that is, there is
    some sort of defer), the log line specified by log_message is not written.
    This does not include the case of a forced failure from a lookup, which is
    considered to be a successful completion. After a defer, no further
    conditions or modifiers in the warn statement are processed. The incident
    is logged, and the ACL continues to be processed, from the next statement
    onwards.

    When one of the warn conditions is an address verification that fails, the
    text of the verification failure message is in $acl_verify_message. If you
    want this logged, you must set it up explicitly. For example:

    warn   !verify = sender
           log_message = sender verify failed: $acl_verify_message

At the end of each ACL there is an implicit unconditional deny.

As you can see from the examples above, the conditions and modifiers are
written one to a line, with the first one on the same line as the verb, and
subsequent ones on following lines. If you have a very long condition, you can
continue it onto several physical lines by the usual backslash continuation
mechanism. It is conventional to align the conditions vertically.


44.10 ACL variables
-------------------

There are some special variables that can be set during ACL processing. They
can be used to pass information between different ACLs, different invocations
of the same ACL in the same SMTP connection, and between ACLs and the routers,
transports, and filters that are used to deliver a message. The names of these
variables must begin with $acl_c or $acl_m, followed either by a digit or an
underscore, but the remainder of the name can be any sequence of alphanumeric
characters and underscores that you choose. There is no limit on the number of
ACL variables. The two sets act as follows:

  * The values of those variables whose names begin with $acl_c persist
    throughout an SMTP connection. They are never reset. Thus, a value that is
    set while receiving one message is still available when receiving the next
    message on the same SMTP connection.

  * The values of those variables whose names begin with $acl_m persist only
    while a message is being received. They are reset afterwards. They are also
    reset by MAIL, RSET, EHLO, HELO, and after starting up a TLS session.

When a message is accepted, the current values of all the ACL variables are
preserved with the message and are subsequently made available at delivery
time. The ACL variables are set by a modifier called set. For example:

accept hosts = whatever
       set acl_m4 = some value
accept authenticated = *
       set acl_c_auth = yes

Note: A leading dollar sign is not used when naming a variable that is to be
set. If you want to set a variable without taking any action, you can use a 
warn verb without any other modifiers or conditions.

What happens if a syntactically valid but undefined ACL variable is referenced
depends on the setting of the strict_acl_vars option. If it is false (the
default), an empty string is substituted; if it is true, an error is generated.

Versions of Exim before 4.64 have a limited set of numbered variables, but
their names are compatible, so there is no problem with upgrading.


44.11 Condition and modifier processing
---------------------------------------

An exclamation mark preceding a condition negates its result. For example:

deny   domains = *.dom.example
      !verify  = recipient

causes the ACL to return "deny" if the recipient domain ends in dom.example and
the recipient address cannot be verified. Sometimes negation can be used on the
right-hand side of a condition. For example, these two statements are
equivalent:

deny  hosts = !192.168.3.4
deny !hosts =  192.168.3.4

However, for many conditions (verify being a good example), only left-hand side
negation of the whole condition is possible.

The arguments of conditions and modifiers are expanded. A forced failure of an
expansion causes a condition to be ignored, that is, it behaves as if the
condition is true. Consider these two statements:

accept  senders = ${lookup{$host_name}lsearch\
                  {/some/file}{$value}fail}
accept  senders = ${lookup{$host_name}lsearch\
                  {/some/file}{$value}{}}

Each attempts to look up a list of acceptable senders. If the lookup succeeds,
the returned list is searched, but if the lookup fails the behaviour is
different in the two cases. The fail in the first statement causes the
condition to be ignored, leaving no further conditions. The accept verb
therefore succeeds. The second statement, however, generates an empty list when
the lookup fails. No sender can match an empty list, so the condition fails,
and therefore the accept also fails.

ACL modifiers appear mixed in with conditions in ACL statements. Some of them
specify actions that are taken as the conditions for a statement are checked;
others specify text for messages that are used when access is denied or a
warning is generated. The control modifier affects the way an incoming message
is handled.

The positioning of the modifiers in an ACL statement is important, because the
processing of a verb ceases as soon as its outcome is known. Only those
modifiers that have already been encountered will take effect. For example,
consider this use of the message modifier:

require message = Can't verify sender
        verify  = sender
        message = Can't verify recipient
        verify  = recipient
        message = This message cannot be used

If sender verification fails, Exim knows that the result of the statement is
"deny", so it goes no further. The first message modifier has been seen, so its
text is used as the error message. If sender verification succeeds, but
recipient verification fails, the second message is used. If recipient
verification succeeds, the third message becomes "current", but is never used
because there are no more conditions to cause failure.

For the deny verb, on the other hand, it is always the last message modifier
that is used, because all the conditions must be true for rejection to happen.
Specifying more than one message modifier does not make sense, and the message
can even be specified after all the conditions. For example:

deny   hosts = ...
      !senders = *@my.domain.example
       message = Invalid sender from client host

The "deny" result does not happen until the end of the statement is reached, by
which time Exim has set up the message.


44.12 ACL modifiers
-------------------

The ACL modifiers are as follows:

add_header = <text>

    This modifier specifies one or more header lines that are to be added to an
    incoming message, assuming, of course, that the message is ultimately
    accepted. For details, see section 44.15.

continue = <text>

    This modifier does nothing of itself, and processing of the ACL always
    continues with the next condition or modifier. The value of continue is in
    the side effects of expanding its argument. Typically this could be used to
    update a database. It is really just a syntactic tidiness, to avoid having
    to write rather ugly lines like this:

    condition = ${if eq{0}{<some expansion>}{true}{true}}

    Instead, all you need is

    continue = <some expansion>

control = <text>

    This modifier affects the subsequent processing of the SMTP connection or
    of an incoming message that is accepted. The effect of the first type of
    control lasts for the duration of the connection, whereas the effect of the
    second type lasts only until the current message has been received. The
    message-specific controls always apply to the whole message, not to
    individual recipients, even if the control modifier appears in a RCPT ACL.

    As there are now quite a few controls that can be applied, they are
    described separately in section 44.13. The control modifier can be used in
    several different ways. For example:

      o It can be at the end of an accept statement:

            accept  ...some conditions
                    control = queue

        In this case, the control is applied when this statement yields
        "accept", in other words, when the conditions are all true.

      o It can be in the middle of an accept statement:

            accept  ...some conditions...
                    control = queue
                    ...some more conditions...

        If the first set of conditions are true, the control is applied, even
        if the statement does not accept because one of the second set of
        conditions is false. In this case, some subsequent statement must yield
        "accept" for the control to be relevant.

      o It can be used with warn to apply the control, leaving the decision
        about accepting or denying to a subsequent verb. For example:

            warn    ...some conditions...
                    control = freeze
            accept  ...

        This example of warn does not contain message, log_message, or logwrite
        , so it does not add anything to the message and does not write a log
        entry.

      o If you want to apply a control unconditionally, you can use it with a 
        require verb. For example:

            require  control = no_multiline_responses

delay = <time>

    This modifier may appear in any ACL except notquit. It causes Exim to wait
    for the time interval before proceeding. However, when testing Exim using
    the -bh option, the delay is not actually imposed (an appropriate message
    is output instead). The time is given in the usual Exim notation, and the
    delay happens as soon as the modifier is processed. In an SMTP session,
    pending output is flushed before the delay is imposed.

    Like control, delay can be used with accept or deny, for example:

    deny    ...some conditions...
            delay = 30s

    The delay happens if all the conditions are true, before the statement
    returns "deny". Compare this with:

    deny    delay = 30s
            ...some conditions...

    which waits for 30s before processing the conditions. The delay modifier
    can also be used with warn and together with control:

    warn    ...some conditions...
            delay = 2m
            control = freeze
    accept  ...

    If delay is encountered when the SMTP PIPELINING extension is in use,
    responses to several commands are no longer buffered and sent in one packet
    (as they would normally be) because all output is flushed before imposing
    the delay. This optimization is disabled so that a number of small delays
    do not appear to the client as one large aggregated delay that might
    provoke an unwanted timeout. You can, however, disable output flushing for 
    delay by using a control modifier to set no_delay_flush.

endpass

    This modifier, which has no argument, is recognized only in accept and 
    discard statements. It marks the boundary between the conditions whose
    failure causes control to pass to the next statement, and the conditions
    whose failure causes the ACL to return "deny". This concept has proved to
    be confusing to some people, so the use of endpass is no longer recommended
    as "best practice". See the description of accept above for more details.

log_message = <text>

    This modifier sets up a message that is used as part of the log message if
    the ACL denies access or a warn statement's conditions are true. For
    example:

    require log_message = wrong cipher suite $tls_in_cipher
            encrypted   = DES-CBC3-SHA

    log_message is also used when recipients are discarded by discard. For
    example:

    discard <some conditions>
            log_message = Discarded $local_part@$domain because...

    When access is denied, log_message adds to any underlying error message
    that may exist because of a condition failure. For example, while verifying
    a recipient address, a :fail: redirection might have already set up a
    message.

    The message may be defined before the conditions to which it applies,
    because the string expansion does not happen until Exim decides that access
    is to be denied. This means that any variables that are set by the
    condition are available for inclusion in the message. For example, the
    $dnslist_<xxx> variables are set after a DNS black list lookup succeeds. If
    the expansion of log_message fails, or if the result is an empty string,
    the modifier is ignored.

    If you want to use a warn statement to log the result of an address
    verification, you can use $acl_verify_message to include the verification
    error message.

    If log_message is used with a warn statement, "Warning:" is added to the
    start of the logged message. If the same warning log message is requested
    more than once while receiving a single email message, only one copy is
    actually logged. If you want to log multiple copies, use logwrite instead
    of log_message. In the absence of log_message and logwrite, nothing is
    logged for a successful warn statement.

    If log_message is not present and there is no underlying error message (for
    example, from the failure of address verification), but message is present,
    the message text is used for logging rejections. However, if any text for
    logging contains newlines, only the first line is logged. In the absence of
    both log_message and message, a default built-in message is used for
    logging rejections.

log_reject_target = <log name list>

    This modifier makes it possible to specify which logs are used for messages
    about ACL rejections. Its argument is a colon-separated list of words that
    can be "main", "reject", or "panic". The default is "main:reject". The list
    may be empty, in which case a rejection is not logged at all. For example,
    this ACL fragment writes no logging information when access is denied:

    deny <some conditions>
         log_reject_target =

    This modifier can be used in SMTP and non-SMTP ACLs. It applies to both
    permanent and temporary rejections. Its effect lasts for the rest of the
    current ACL.

logwrite = <text>

    This modifier writes a message to a log file as soon as it is encountered
    when processing an ACL. (Compare log_message, which, except in the case of 
    warn and discard, is used only if the ACL statement denies access.) The 
    logwrite modifier can be used to log special incidents in ACLs. For
    example:

    accept <some special conditions>
           control  = freeze
           logwrite = froze message because ...

    By default, the message is written to the main log. However, it may begin
    with a colon, followed by a comma-separated list of log names, and then
    another colon, to specify exactly which logs are to be written. For
    example:

    logwrite = :main,reject: text for main and reject logs
    logwrite = :panic: text for panic log only

message = <text>

    This modifier sets up a text string that is expanded and used as a response
    message when an ACL statement terminates the ACL with an "accept", "deny",
    or "defer" response. (In the case of the accept and discard verbs, there is
    some complication if endpass is involved; see the description of accept for
    details.)

    The expansion of the message happens at the time Exim decides that the ACL
    is to end, not at the time it processes message. If the expansion fails, or
    generates an empty string, the modifier is ignored. Here is an example
    where message must be specified first, because the ACL ends with a
    rejection if the hosts condition fails:

    require  message = Host not recognized
             hosts = 10.0.0.0/8

    (Once a condition has failed, no further conditions or modifiers are
    processed.)

    For ACLs that are triggered by SMTP commands, the message is returned as
    part of the SMTP response. The use of message with accept (or discard) is
    meaningful only for SMTP, as no message is returned when a non-SMTP message
    is accepted. In the case of the connect ACL, accepting with a message
    modifier overrides the value of smtp_banner. For the EHLO/HELO ACL, a
    customized accept message may not contain more than one line (otherwise it
    will be truncated at the first newline and a panic logged), and it cannot
    affect the EHLO options.

    When SMTP is involved, the message may begin with an overriding response
    code, consisting of three digits optionally followed by an "extended
    response code" of the form n.n.n, each code being followed by a space. For
    example:

    deny  message = 599 1.2.3 Host not welcome
          hosts = 192.168.34.0/24

    The first digit of the supplied response code must be the same as would be
    sent by default. A panic occurs if it is not. Exim uses a 550 code when it
    denies access, but for the predata ACL, note that the default success code
    is 354, not 2xx.

    Notwithstanding the previous paragraph, for the QUIT ACL, unlike the
    others, the message modifier cannot override the 221 response code.

    The text in a message modifier is literal; any quotes are taken as
    literals, but because the string is expanded, backslash escapes are
    processed anyway. If the message contains newlines, this gives rise to a
    multi-line SMTP response. A long message line will also be split into
    multi-line SMTP responses, on word boundaries if possible.

    While the text is being expanded, the $acl_verify_message variable contains
    any message previously set. Afterwards, $acl_verify_message is cleared.

    If message is used on a statement that verifies an address, the message
    specified overrides any message that is generated by the verification
    process. However, the original message is available in the variable
    $acl_verify_message, so you can incorporate it into your message if you
    wish. In particular, if you want the text from :fail: items in redirect
    routers to be passed back as part of the SMTP response, you should either
    not use a message modifier, or make use of $acl_verify_message.

    For compatibility with previous releases of Exim, a message modifier that
    is used with a warn verb behaves in a similar way to the add_header
    modifier, but this usage is now deprecated. However, message acts only when
    all the conditions are true, wherever it appears in an ACL command, whereas
    add_header acts as soon as it is encountered. If message is used with warn
    in an ACL that is not concerned with receiving a message, it has no effect.

queue = <text>

    This modifier specifies the use of a named queue for spool files for the
    message. It can only be used before the message is received (i.e. not in
    the DATA ACL). This could be used, for example, for known high-volume burst
    sources of traffic, or for quarantine of messages. Separate queue-runner
    processes will be needed for named queues. If the text after expansion is
    empty, the default queue is used.

remove_header = <text>

    This modifier specifies one or more header names in a colon-separated list
    that are to be removed from an incoming message, assuming, of course, that
    the message is ultimately accepted. For details, see section 44.16.

set <acl_name> = <value>

    This modifier puts a value into one of the ACL variables (see section 44.10
    ).

udpsend = <parameters>

    This modifier sends a UDP packet, for purposes such as statistics
    collection or behaviour monitoring. The parameters are expanded, and the
    result of the expansion must be a colon-separated list consisting of a
    destination server, port number, and the packet contents. The server can be
    specified as a host name or IPv4 or IPv6 address. The separator can be
    changed with the usual angle bracket syntax. For example, you might want to
    collect information on which hosts connect when:

    udpsend = <; 2001:dB8::dead:beef ; 1234 ;\
                 $tod_zulu $sender_host_address


44.13 Use of the control modifier
---------------------------------

The control modifier supports the following settings:

control = allow_auth_unadvertised

    This modifier allows a client host to use the SMTP AUTH command even when
    it has not been advertised in response to EHLO. Furthermore, because there
    are apparently some really broken clients that do this, Exim will accept
    AUTH after HELO (rather than EHLO) when this control is set. It should be
    used only if you really need it, and you should limit its use to those
    broken clients that do not work without it. For example:

    warn hosts   = 192.168.34.25
         control = allow_auth_unadvertised

    Normally, when an Exim server receives an AUTH command, it checks the name
    of the authentication mechanism that is given in the command to ensure that
    it matches an advertised mechanism. When this control is set, the check
    that a mechanism has been advertised is bypassed. Any configured mechanism
    can be used by the client. This control is permitted only in the connection
    and HELO ACLs.

control = caseful_local_part, control = caselower_local_part

    These two controls are permitted only in the ACL specified by acl_smtp_rcpt
    (that is, during RCPT processing). By default, the contents of $local_part
    are lower cased before ACL processing. If "caseful_local_part" is
    specified, any uppercase letters in the original local part are restored in
    $local_part for the rest of the ACL, or until a control that sets
    "caselower_local_part" is encountered.

    These controls affect only the current recipient. Moreover, they apply only
    to local part handling that takes place directly in the ACL (for example,
    as a key in lookups). If a test to verify the recipient is obeyed, the
    case-related handling of the local part during the verification is
    controlled by the router configuration (see the caseful_local_part generic
    router option).

    This facility could be used, for example, to add a spam score to local
    parts containing upper case letters. For example, using $acl_m4 to
    accumulate the spam score:

    warn  control = caseful_local_part
          set acl_m4 = ${eval:\
                         $acl_m4 + \
                         ${if match{$local_part}{[A-Z]}{1}{0}}\
                        }
          control = caselower_local_part

    Notice that we put back the lower cased version afterwards, assuming that
    is what is wanted for subsequent tests.

control = cutthrough_delivery/<options>

    This option requests delivery be attempted while the item is being
    received.

    The option is usable in the RCPT ACL. If enabled for a message received via
    smtp and routed to an smtp transport, and only one transport, interface,
    destination host and port combination is used for all recipients of the
    message, then the delivery connection is made while the receiving
    connection is open and data is copied from one to the other.

    An attempt to set this option for any recipient but the first for a mail
    will be quietly ignored. If a recipient-verify callout (with use_sender)
    connection is subsequently requested in the same ACL it is held open and
    used for any subsequent recipients and the data, otherwise one is made
    after the initial RCPT ACL completes.

    Note that routers are used in verify mode, and cannot depend on content of
    received headers. Note also that headers cannot be modified by any of the
    post-data ACLs (DATA, MIME and DKIM). Headers may be modified by routers
    (subject to the above) and transports. The Received-By: header is generated
    as soon as the body reception starts, rather than the traditional time
    after the full message is received; this will affect the timestamp.

    All the usual ACLs are called; if one results in the message being
    rejected, all effort spent in delivery (including the costs on the ultimate
    destination) will be wasted. Note that in the case of data-time ACLs this
    includes the entire message body.

    Cutthrough delivery is not supported via transport-filters or when DKIM
    signing of outgoing messages is done, because it sends data to the ultimate
    destination before the entire message has been received from the source. It
    is not supported for messages received with the SMTP PRDR or CHUNKING
    options in use.

    Should the ultimate destination system positively accept or reject the
    mail, a corresponding indication is given to the source system and nothing
    is queued. If the item is successfully delivered in cutthrough mode the
    delivery log lines are tagged with ">>" rather than "=>" and appear before
    the acceptance "<=" line.

    If there is a temporary error the item is queued for later delivery in the
    usual fashion. This behaviour can be adjusted by appending the option defer
    =<value> to the control; the default value is "spool" and the alternate
    value "pass" copies an SMTP defer response from the target back to the
    initiator and does not queue the message. Note that this is independent of
    any recipient verify conditions in the ACL.

    Delivery in this mode avoids the generation of a bounce mail to a (possibly
    faked) sender when the destination system is doing content-scan based
    rejection.

control = debug/<options>

    This control turns on debug logging, almost as though Exim had been invoked
    with "-d", with the output going to a new logfile in the usual logs
    directory, by default called debuglog.

    Logging set up by the control will be maintained across spool residency.

    Options are a slash-separated list. If an option takes an argument, the
    option name and argument are separated by an equals character. Several
    options are supported:

    tag=<suffix>         The filename can be adjusted with thise option.
         The argument, which may access any variables already defined,
                         is appended to the default name.

    opts=<debug options> The argument specififes what is to be logged,
                         using the same values as the -d command-line option.

    stop                 Logging started with this control may be
                         stopped by using this option.

    kill                 Logging started with this control may be
                         stopped by using this option.
                         Additionally the debug file will be removed,
                         providing one means for speculative debug tracing.

    pretrigger=<size>    This option specifies a memory buffuer to be used
                         for pre-trigger debug capture.
                         Debug lines are recorded in the buffer until
                         and if) a trigger occurs; at which time they are
                         dumped to the debug file.  Newer lines displace the
                         oldest if the buffer is full.  After a trigger,
                         immediate writes to file are done as normal.

    trigger=<reason>     This option selects cause for the pretrigger buffer
                         see above) to be copied to file.  A reason of now
                         take effect immediately; one of paniclog triggers
                         on a write to the panic log.

    Some examples (which depend on variables that don't exist in all contexts):

          control = debug
          control = debug/tag=.$sender_host_address
          control = debug/opts=+expand+acl
          control = debug/tag=.$message_exim_id/opts=+expand
          control = debug/kill
          control = debug/opts=+all/pretrigger=1024/trigger=paniclog
          control = debug/trigger=now

control = dkim_disable_verify

    This control turns off DKIM verification processing entirely. For details
    on the operation and configuration of DKIM, see section 58.1.

control = dmarc_disable_verify

    This control turns off DMARC verification processing entirely. For details
    on the operation and configuration of DMARC, see section 58.3.

control = dscp/<value>

    This option causes the DSCP value associated with the socket for the
    inbound connection to be adjusted to a given value, given as one of a
    number of fixed strings or to numeric value. The -bI:dscp option may be
    used to ask Exim which names it knows of. Common values include
    "throughput", "mincost", and on newer systems "ef", "af41", etc. Numeric
    values may be in the range 0 to 0x3F.

    The outbound packets from Exim will be marked with this value in the header
    (for IPv4, the TOS field; for IPv6, the TCLASS field); there is no
    guarantee that these values will have any effect, not be stripped by
    networking equipment, or do much of anything without cooperation with your
    Network Engineer and those of all network operators between the source and
    destination.

control = enforce_sync, control = no_enforce_sync

    These controls make it possible to be selective about when SMTP
    synchronization is enforced. The global option smtp_enforce_sync specifies
    the initial state of the switch (it is true by default). See the
    description of this option in chapter 14 for details of SMTP
    synchronization checking.

    The effect of these two controls lasts for the remainder of the SMTP
    connection. They can appear in any ACL except the one for the non-SMTP
    messages. The most straightforward place to put them is in the ACL defined
    by acl_smtp_connect, which is run at the start of an incoming SMTP
    connection, before the first synchronization check. The expected use is to
    turn off the synchronization checks for badly-behaved hosts that you
    nevertheless need to work with.

control = fakedefer/<message>

    This control works in exactly the same way as fakereject (described below)
    except that it causes an SMTP 450 response after the message data instead
    of a 550 response. You must take care when using fakedefer because it
    causes the messages to be duplicated when the sender retries. Therefore,
    you should not use fakedefer if the message is to be delivered normally.

control = fakereject/<message>

    This control is permitted only for the MAIL, RCPT, and DATA ACLs, in other
    words, only when an SMTP message is being received. If Exim accepts the
    message, instead the final 250 response, a 550 rejection message is sent.
    However, Exim proceeds to deliver the message as normal. The control
    applies only to the current message, not to any subsequent ones that may be
    received in the same SMTP connection.

    The text for the 550 response is taken from the control modifier. If no
    message is supplied, the following is used:

    550-Your message has been rejected but is being
    550-kept for evaluation.
    550-If it was a legitimate message, it may still be
    550 delivered to the target recipient(s).

    This facility should be used with extreme caution.

control = freeze

    This control is permitted only for the MAIL, RCPT, DATA, and non-SMTP ACLs,
    in other words, only when a message is being received. If the message is
    accepted, it is placed on Exim's queue and frozen. The control applies only
    to the current message, not to any subsequent ones that may be received in
    the same SMTP connection.

    This modifier can optionally be followed by "/no_tell". If the global
    option freeze_tell is set, it is ignored for the current message (that is,
    nobody is told about the freezing), provided all the control=freeze
    modifiers that are obeyed for the current message have the "/no_tell"
    option.

control = no_delay_flush

    Exim normally flushes SMTP output before implementing a delay in an ACL, to
    avoid unexpected timeouts in clients when the SMTP PIPELINING extension is
    in use. This control, as long as it is encountered before the delay
    modifier, disables such output flushing.

control = no_callout_flush

    Exim normally flushes SMTP output before performing a callout in an ACL, to
    avoid unexpected timeouts in clients when the SMTP PIPELINING extension is
    in use. This control, as long as it is encountered before the verify
    condition that causes the callout, disables such output flushing.

control = no_mbox_unspool

    This control is available when Exim is compiled with the content scanning
    extension. Content scanning may require a copy of the current message, or
    parts of it, to be written in "mbox format" to a spool file, for passing to
    a virus or spam scanner. Normally, such copies are deleted when they are no
    longer needed. If this control is set, the copies are not deleted. The
    control applies only to the current message, not to any subsequent ones
    that may be received in the same SMTP connection. It is provided for
    debugging purposes and is unlikely to be useful in production.

control = no_multiline_responses

    This control is permitted for any ACL except the one for non-SMTP messages.
    It seems that there are broken clients in use that cannot handle multiline
    SMTP responses, despite the fact that RFC 821 defined them over 20 years
    ago.

    If this control is set, multiline SMTP responses from ACL rejections are
    suppressed. One way of doing this would have been to put out these
    responses as one long line. However, RFC 2821 specifies a maximum of 512
    bytes per response ("use multiline responses for more" it says - ha!), and
    some of the responses might get close to that. So this facility, which is
    after all only a sop to broken clients, is implemented by doing two very
    easy things:

      o Extra information that is normally output as part of a rejection caused
        by sender verification failure is omitted. Only the final line
        (typically "sender verification failed") is sent.

      o If a message modifier supplies a multiline response, only the first
        line is output.

    The setting of the switch can, of course, be made conditional on the
    calling host. Its effect lasts until the end of the SMTP connection.

control = no_pipelining

    This control turns off the advertising of the PIPELINING extension to SMTP
    in the current session. To be useful, it must be obeyed before Exim sends
    its response to an EHLO command. Therefore, it should normally appear in an
    ACL controlled by acl_smtp_connect or acl_smtp_helo. See also 
    pipelining_advertise_hosts.

control = queue/<options>*, control = queue_only

    This control is permitted only for the MAIL, RCPT, DATA, and non-SMTP ACLs,
    in other words, only when a message is being received. If the message is
    accepted, it is placed on Exim's queue and left there for delivery by a
    subsequent queue runner. If used with no options set, no immediate delivery
    process is started. In other words, it has the effect as the queue_only
    global option or -odq command-line option.

    If the first_pass_route option is given then the behaviour is like the
    command-line -oqds option; a delivery process is started which stops short
    of making any SMTP delivery. The benefit is that the hints database will be
    updated for the message being waiting for a specific host, and a later
    queue run will be able to send all such messages on a single connection.

    The control only applies to the current message, not to any subsequent ones
    that may be received in the same SMTP connection.

control = submission/<options>

    This control is permitted only for the MAIL, RCPT, and start of data ACLs
    (the latter is the one defined by acl_smtp_predata). Setting it tells Exim
    that the current message is a submission from a local MUA. In this case,
    Exim operates in "submission mode", and applies certain fixups to the
    message if necessary. For example, it adds a Date: header line if one is
    not present. This control is not permitted in the acl_smtp_data ACL,
    because that is too late (the message has already been created).

    Chapter 48 describes the processing that Exim applies to messages. Section
    48.1 covers the processing that happens in submission mode; the available
    options for this control are described there. The control applies only to
    the current message, not to any subsequent ones that may be received in the
    same SMTP connection.

control = suppress_local_fixups

    This control applies to locally submitted (non TCP/IP) messages, and is the
    complement of "control = submission". It disables the fixups that are
    normally applied to locally-submitted messages. Specifically:

      o Any Sender: header line is left alone (in this respect, it is a dynamic
        version of local_sender_retain).

      o No Message-ID:, From:, or Date: header lines are added.

      o There is no check that From: corresponds to the actual sender.

    This control may be useful when a remotely-originated message is accepted,
    passed to some scanning program, and then re-submitted for delivery. It can
    be used only in the acl_smtp_mail, acl_smtp_rcpt, acl_smtp_predata, and 
    acl_not_smtp_start ACLs, because it has to be set before the message's data
    is read.

    Note: This control applies only to the current message, not to any others
    that are being submitted at the same time using -bs or -bS.

control = utf8_downconvert

    This control enables conversion of UTF-8 in message envelope addresses to
    a-label form. For details see section 60.1.


44.14 Summary of message fixup control
--------------------------------------

All four possibilities for message fixups can be specified:

  * Locally submitted, fixups applied: the default.

  * Locally submitted, no fixups applied: use "control =
    suppress_local_fixups".

  * Remotely submitted, no fixups applied: the default.

  * Remotely submitted, fixups applied: use "control = submission".


44.15 Adding header lines in ACLs
---------------------------------

The add_header modifier can be used to add one or more extra header lines to an
incoming message, as in this example:

warn dnslists = sbl.spamhaus.org : \
                dialup.mail-abuse.org
     add_header = X-blacklisted-at: $dnslist_domain

The add_header modifier is permitted in the MAIL, RCPT, PREDATA, DATA, MIME,
DKIM, and non-SMTP ACLs (in other words, those that are concerned with
receiving a message). The message must ultimately be accepted for add_header to
have any significant effect. You can use add_header with any ACL verb,
including deny (though this is potentially useful only in a RCPT ACL).

Headers will not be added to the message if the modifier is used in DATA, MIME
or DKIM ACLs for a message delivered by cutthrough routing.

Leading and trailing newlines are removed from the data for the add_header
modifier; if it then contains one or more newlines that are not followed by a
space or a tab, it is assumed to contain multiple header lines. Each one is
checked for valid syntax; "X-ACL-Warn:" is added to the front of any line that
is not a valid header line.

Added header lines are accumulated during the MAIL, RCPT, and predata ACLs.
They are added to the message before processing the DATA and MIME ACLs.
However, if an identical header line is requested more than once, only one copy
is actually added to the message. Further header lines may be accumulated
during the DATA and MIME ACLs, after which they are added to the message, again
with duplicates suppressed. Thus, it is possible to add two identical header
lines to an SMTP message, but only if one is added before DATA and one after.
In the case of non-SMTP messages, new headers are accumulated during the
non-SMTP ACLs, and are added to the message after all the ACLs have run. If a
message is rejected after DATA or by the non-SMTP ACL, all added header lines
are included in the entry that is written to the reject log.

Header lines are not visible in string expansions of message headers until they
are added to the message. It follows that header lines defined in the MAIL,
RCPT, and predata ACLs are not visible until the DATA ACL and MIME ACLs are
run. Similarly, header lines that are added by the DATA or MIME ACLs are not
visible in those ACLs. Because of this restriction, you cannot use header lines
as a way of passing data between (for example) the MAIL and RCPT ACLs. If you
want to do this, you can use ACL variables, as described in section 44.10.

The list of headers yet to be added is given by the $headers_added variable.

The add_header modifier acts immediately as it is encountered during the
processing of an ACL. Notice the difference between these two cases:

accept add_header = ADDED: some text
       <some condition>

accept <some condition>
       add_header = ADDED: some text

In the first case, the header line is always added, whether or not the
condition is true. In the second case, the header line is added only if the
condition is true. Multiple occurrences of add_header may occur in the same ACL
statement. All those that are encountered before a condition fails are
honoured.

For compatibility with previous versions of Exim, a message modifier for a warn
verb acts in the same way as add_header, except that it takes effect only if
all the conditions are true, even if it appears before some of them.
Furthermore, only the last occurrence of message is honoured. This usage of 
message is now deprecated. If both add_header and message are present on a warn
verb, both are processed according to their specifications.

By default, new header lines are added to a message at the end of the existing
header lines. However, you can specify that any particular header line should
be added right at the start (before all the Received: lines), immediately after
the first block of Received: lines, or immediately before any line that is not
a Received: or Resent-something: header.

This is done by specifying ":at_start:", ":after_received:", or
":at_start_rfc:" (or, for completeness, ":at_end:") before the text of the
header line, respectively. (Header text cannot start with a colon, as there has
to be a header name first.) For example:

warn add_header = \
       :after_received:X-My-Header: something or other...

If more than one header line is supplied in a single add_header modifier, each
one is treated independently and can therefore be placed differently. If you
add more than one line at the start, or after the Received: block, they end up
in reverse order.

Warning: This facility currently applies only to header lines that are added in
an ACL. It does NOT work for header lines that are added in a system filter or
in a router or transport.


44.16 Removing header lines in ACLs
-----------------------------------

The remove_header modifier can be used to remove one or more header lines from
an incoming message, as in this example:

warn   message        = Remove internal headers
       remove_header  = x-route-mail1 : x-route-mail2

The remove_header modifier is permitted in the MAIL, RCPT, PREDATA, DATA, MIME,
DKIM, and non-SMTP ACLs (in other words, those that are concerned with
receiving a message). The message must ultimately be accepted for remove_header
to have any significant effect. You can use remove_header with any ACL verb,
including deny, though this is really not useful for any verb that doesn't
result in a delivered message.

Headers will not be removed from the message if the modifier is used in DATA,
MIME or DKIM ACLs for a message delivered by cutthrough routing.

More than one header can be removed at the same time by using a colon separated
list of header specifiers.

If a specifier does not start with a circumflex (^) then it is treated as a
header name. The header name matching is case insensitive. If it does, then it
is treated as a (front-anchored) regular expression applied to the whole
header.

Note: The colon terminating a header name will need to be doubled if used in an
RE, and there can legitimately be whitepace before it.

Example:

remove_header = \N^(?i)Authentication-Results\s*::\s*example.org;\N

List expansion is not performed, so you cannot use hostlists to create a list
of headers, however both connection and message variable expansion are
performed ($acl_c_* and $acl_m_*), illustrated in this example:

warn   hosts           = +internal_hosts
       set acl_c_ihdrs = x-route-mail1 : x-route-mail2
warn   message         = Remove internal headers
       remove_header   = $acl_c_ihdrs

Header specifiers for removal are accumulated during the MAIL, RCPT, and
predata ACLs. Matching header lines are removed from the message before
processing the DATA and MIME ACLs. If multiple header lines match, all are
removed. There is no harm in attempting to remove the same header twice nor in
removing a non-existent header. Further header specifiers for removal may be
accumulated during the DATA and MIME ACLs, after which matching headers are
removed if present. In the case of non-SMTP messages, remove specifiers are
accumulated during the non-SMTP ACLs, and are acted on after all the ACLs have
run. If a message is rejected after DATA or by the non-SMTP ACL, there really
is no effect because there is no logging of what headers would have been
removed.

Header lines are not visible in string expansions until the DATA phase when it
is received. Any header lines removed in the MAIL, RCPT, and predata ACLs are
not visible in the DATA ACL and MIME ACLs. Similarly, header lines that are
removed by the DATA or MIME ACLs are still visible in those ACLs. Because of
this restriction, you cannot use header lines as a way of controlling data
passed between (for example) the MAIL and RCPT ACLs. If you want to do this,
you should instead use ACL variables, as described in section 44.10.

The remove_header modifier acts immediately as it is encountered during the
processing of an ACL. Notice the difference between these two cases:

accept remove_header = X-Internal
       <some condition>

accept <some condition>
       remove_header = X-Internal

In the first case, the header line is always removed, whether or not the
condition is true. In the second case, the header line is removed only if the
condition is true. Multiple occurrences of remove_header may occur in the same
ACL statement. All those that are encountered before a condition fails are
honoured.

Warning: This facility currently applies only to header lines that are present
during ACL processing. It does NOT remove header lines that are added in a
system filter or in a router or transport.


44.17 ACL conditions
--------------------

Some of the conditions listed in this section are available only when Exim is
compiled with the content-scanning extension. They are included here briefly
for completeness. More detailed descriptions can be found in the discussion on
content scanning in chapter 45.

Not all conditions are relevant in all circumstances. For example, testing
senders and recipients does not make sense in an ACL that is being run as the
result of the arrival of an ETRN command, and checks on message headers can be
done only in the ACLs specified by acl_smtp_data and acl_not_smtp. You can use
the same condition (with different parameters) more than once in the same ACL
statement. This provides a way of specifying an "and" conjunction. The
conditions are as follows:

acl = <name of acl or ACL string or file name >

    The possible values of the argument are the same as for the acl_smtp_xxx
    options. The named or inline ACL is run. If it returns "accept" the
    condition is true; if it returns "deny" the condition is false. If it
    returns "defer", the current ACL returns "defer" unless the condition is on
    a warn verb. In that case, a "defer" return makes the condition false. This
    means that further processing of the warn verb ceases, but processing of
    the ACL continues.

    If the argument is a named ACL, up to nine space-separated optional values
    can be appended; they appear within the called ACL in $acl_arg1 to
    $acl_arg9, and $acl_narg is set to the count of values. Previous values of
    these variables are restored after the call returns. The name and values
    are expanded separately. Note that spaces in complex expansions which are
    used as arguments will act as argument separators.

    If the nested acl returns "drop" and the outer condition denies access, the
    connection is dropped. If it returns "discard", the verb must be accept or 
    discard, and the action is taken immediately - no further conditions are
    tested.

    ACLs may be nested up to 20 deep; the limit exists purely to catch runaway
    loops. This condition allows you to use different ACLs in different
    circumstances. For example, different ACLs can be used to handle RCPT
    commands for different local users or different local domains.

authenticated = <string list>

    If the SMTP connection is not authenticated, the condition is false.
    Otherwise, the name of the authenticator is tested against the list. To
    test for authentication by any authenticator, you can set

    authenticated = *

condition = <string>

    This feature allows you to make up custom conditions. If the result of
    expanding the string is an empty string, the number zero, or one of the
    strings "no" or "false", the condition is false. If the result is any
    non-zero number, or one of the strings "yes" or "true", the condition is
    true. For any other value, some error is assumed to have occurred, and the
    ACL returns "defer". However, if the expansion is forced to fail, the
    condition is ignored. The effect is to treat it as true, whether it is
    positive or negative.

decode = <location>

    This condition is available only when Exim is compiled with the
    content-scanning extension, and it is allowed only in the ACL defined by 
    acl_smtp_mime. It causes the current MIME part to be decoded into a file.
    If all goes well, the condition is true. It is false only if there are
    problems such as a syntax error or a memory shortage. For more details, see
    chapter 45.

dnslists = <list of domain names and other data>

    This condition checks for entries in DNS black lists. These are also known
    as "RBL lists", after the original Realtime Blackhole List, but note that
    the use of the lists at mail-abuse.org now carries a charge. There are too
    many different variants of this condition to describe briefly here. See
    sections 44.18-44.18.10 for details.

domains = <domain list>

    This condition is relevant only in a RCPT ACL. It checks that the domain of
    the recipient address is in the domain list. If percent-hack processing is
    enabled, it is done before this test is done. If the check succeeds with a
    lookup, the result of the lookup is placed in $domain_data until the next 
    domains test.

    Note carefully (because many people seem to fall foul of this): you cannot
    use domains in a DATA ACL.

encrypted = <string list>

    If the SMTP connection is not encrypted, the condition is false. Otherwise,
    the name of the cipher suite in use is tested against the list. To test for
    encryption without testing for any specific cipher suite(s), set

    encrypted = *

hosts = <host list>

    This condition tests that the calling host matches the host list. If you
    have name lookups or wildcarded host names and IP addresses in the same
    host list, you should normally put the IP addresses first. For example, you
    could have:

    accept hosts = 10.9.8.7 : dbm;/etc/friendly/hosts

    The lookup in this example uses the host name for its key. This is implied
    by the lookup type "dbm". (For a host address lookup you would use
    "net-dbm" and it wouldn't matter which way round you had these two items.)

    The reason for the problem with host names lies in the left-to-right way
    that Exim processes lists. It can test IP addresses without doing any DNS
    lookups, but when it reaches an item that requires a host name, it fails if
    it cannot find a host name to compare with the pattern. If the above list
    is given in the opposite order, the accept statement fails for a host whose
    name cannot be found, even if its IP address is 10.9.8.7.

    If you really do want to do the name check first, and still recognize the
    IP address even if the name lookup fails, you can rewrite the ACL like
    this:

    accept hosts = dbm;/etc/friendly/hosts
    accept hosts = 10.9.8.7

    The default action on failing to find the host name is to assume that the
    host is not in the list, so the first accept statement fails. The second
    statement can then check the IP address.

    If a hosts condition is satisfied by means of a lookup, the result of the
    lookup is made available in the $host_data variable. This allows you, for
    example, to set up a statement like this:

    deny  hosts = net-lsearch;/some/file
          message = $host_data

    which gives a custom error message for each denied host.

local_parts = <local part list>

    This condition is relevant only in a RCPT ACL. It checks that the local
    part of the recipient address is in the list. If percent-hack processing is
    enabled, it is done before this test. If the check succeeds with a lookup,
    the result of the lookup is placed in $local_part_data, which remains set
    until the next local_parts test.

malware = <option>

    This condition is available only when Exim is compiled with the
    content-scanning extension and only after a DATA command. It causes the
    incoming message to be scanned for viruses. For details, see chapter 45.

mime_regex = <list of regular expressions>

    This condition is available only when Exim is compiled with the
    content-scanning extension, and it is allowed only in the ACL defined by 
    acl_smtp_mime. It causes the current MIME part to be scanned for a match
    with any of the regular expressions. For details, see chapter 45.

ratelimit = <parameters>

    This condition can be used to limit the rate at which a user or host
    submits messages. Details are given in section 44.20.

recipients = <address list>

    This condition is relevant only in a RCPT ACL. It checks the entire
    recipient address against a list of recipients.

regex = <list of regular expressions>

    This condition is available only when Exim is compiled with the
    content-scanning extension, and is available only in the DATA, MIME, and
    non-SMTP ACLs. It causes the incoming message to be scanned for a match
    with any of the regular expressions. For details, see chapter 45.

seen = <parameters>

    This condition can be used to test if a situation has been previously met,
    for example for greylisting. Details are given in section 44.19.

sender_domains = <domain list>

    This condition tests the domain of the sender of the message against the
    given domain list. Note: The domain of the sender address is in
    $sender_address_domain. It is not put in $domain during the testing of this
    condition. This is an exception to the general rule for testing domain
    lists. It is done this way so that, if this condition is used in an ACL for
    a RCPT command, the recipient's domain (which is in $domain) can be used to
    influence the sender checking.

    Warning: It is a bad idea to use this condition on its own as a control on
    relaying, because sender addresses are easily, and commonly, forged.

senders = <address list>

    This condition tests the sender of the message against the given list. To
    test for a bounce message, which has an empty sender, set

    senders = :

    Warning: It is a bad idea to use this condition on its own as a control on
    relaying, because sender addresses are easily, and commonly, forged.

spam = <username>

    This condition is available only when Exim is compiled with the
    content-scanning extension. It causes the incoming message to be scanned by
    SpamAssassin. For details, see chapter 45.

verify = certificate

    This condition is true in an SMTP session if the session is encrypted, and
    a certificate was received from the client, and the certificate was
    verified. The server requests a certificate only if the client matches 
    tls_verify_hosts or tls_try_verify_hosts (see chapter 43).

verify = csa

    This condition checks whether the sending host (the client) is authorized
    to send email. Details of how this works are given in section 44.26.

verify = header_names_ascii

    This condition is relevant only in an ACL that is run after a message has
    been received. This usually means an ACL specified by acl_smtp_data or 
    acl_not_smtp. It checks all header names (not the content) to make sure
    there are no non-ASCII characters, also excluding control characters. The
    allowable characters are decimal ASCII values 33 through 126.

    Exim itself will handle headers with non-ASCII characters, but it can cause
    problems for downstream applications, so this option will allow their
    detection and rejection in the DATA ACL's.

verify = header_sender/<options>

    This condition is relevant only in an ACL that is run after a message has
    been received, that is, in an ACL specified by acl_smtp_data or 
    acl_not_smtp. It checks that there is a verifiable address in at least one
    of the Sender:, Reply-To:, or From: header lines. Such an address is
    loosely thought of as a "sender" address (hence the name of the test).
    However, an address that appears in one of these headers need not be an
    address that accepts bounce messages; only sender addresses in envelopes
    are required to accept bounces. Therefore, if you use the callout option on
    this check, you might want to arrange for a non-empty address in the MAIL
    command.

    Details of address verification and the options are given later, starting
    at section 44.21 (callouts are described in section 44.22). You can combine
    this condition with the senders condition to restrict it to bounce messages
    only:

    deny    senders = :
           !verify  = header_sender
            message = A valid sender header is required for bounces

verify = header_syntax

    This condition is relevant only in an ACL that is run after a message has
    been received, that is, in an ACL specified by acl_smtp_data or 
    acl_not_smtp. It checks the syntax of all header lines that can contain
    lists of addresses (Sender:, From:, Reply-To:, To:, Cc:, and Bcc:),
    returning true if there are no problems. Unqualified addresses (local parts
    without domains) are permitted only in locally generated messages and from
    hosts that match sender_unqualified_hosts or recipient_unqualified_hosts,
    as appropriate.

    Note that this condition is a syntax check only. However, a common spamming
    ploy used to be to send syntactically invalid headers such as

    To: @

    and this condition can be used to reject such messages, though they are not
    as common as they used to be.

verify = helo

    This condition is true if a HELO or EHLO command has been received from the
    client host, and its contents have been verified. If there has been no
    previous attempt to verify the HELO/EHLO contents, it is carried out when
    this condition is encountered. See the description of the helo_verify_hosts
    and helo_try_verify_hosts options for details of how to request
    verification independently of this condition, and for detail of the
    verification.

    For SMTP input that does not come over TCP/IP (the -bs command line
    option), this condition is always true.

verify = not_blind/<options>

    This condition checks that there are no blind (bcc) recipients in the
    message. Every envelope recipient must appear either in a To: header line
    or in a Cc: header line for this condition to be true. Local parts are
    checked case-sensitively; domains are checked case-insensitively. If 
    Resent-To: or Resent-Cc: header lines exist, they are also checked. This
    condition can be used only in a DATA or non-SMTP ACL.

    There is one possible option, "case_insensitive". If this is present then
    local parts are checked case-insensitively.

    There are, of course, many legitimate messages that make use of blind (bcc)
    recipients. This check should not be used on its own for blocking messages.

verify = recipient/<options>

    This condition is relevant only after a RCPT command. It verifies the
    current recipient. Details of address verification are given later,
    starting at section 44.21. After a recipient has been verified, the value
    of $address_data is the last value that was set while routing the address.
    This applies even if the verification fails. When an address that is being
    verified is redirected to a single address, verification continues with the
    new address, and in that case, the subsequent value of $address_data is the
    value for the child address.

verify = reverse_host_lookup/<options>

    This condition ensures that a verified host name has been looked up from
    the IP address of the client host. (This may have happened already if the
    host name was needed for checking a host list, or if the host matched 
    host_lookup.) Verification ensures that the host name obtained from a
    reverse DNS lookup, or one of its aliases, does, when it is itself looked
    up in the DNS, yield the original IP address.

    There is one possible option, "defer_ok". If this is present and a DNS
    operation returns a temporary error, the verify condition succeeds.

    If this condition is used for a locally generated message (that is, when
    there is no client host involved), it always succeeds.

verify = sender/<options>

    This condition is relevant only after a MAIL or RCPT command, or after a
    message has been received (the acl_smtp_data or acl_not_smtp ACLs). If the
    message's sender is empty (that is, this is a bounce message), the
    condition is true. Otherwise, the sender address is verified.

    If there is data in the $address_data variable at the end of routing, its
    value is placed in $sender_address_data at the end of verification. This
    value can be used in subsequent conditions and modifiers in the same ACL
    statement. It does not persist after the end of the current statement. If
    you want to preserve the value for longer, you can save it in an ACL
    variable.

    Details of verification are given later, starting at section 44.21. Exim
    caches the result of sender verification, to avoid doing it more than once
    per message.

verify = sender=<address>/<options>

    This is a variation of the previous option, in which a modified address is
    verified as a sender.

    Note that '/' is legal in local-parts; if the address may have such (eg. is
    generated from the received message) they must be protected from the
    options parsing by doubling:

    verify = sender=${listquote{/}{${address:$h_sender:}}}


44.18 Using DNS lists
---------------------

In its simplest form, the dnslists condition tests whether the calling host is
on at least one of a number of DNS lists by looking up the inverted IP address
in one or more DNS domains. (Note that DNS list domains are not mail domains,
so the "+" syntax for named lists doesn't work - it is used for special options
instead.) For example, if the calling host's IP address is 192.168.62.43, and
the ACL statement is

deny dnslists = blackholes.mail-abuse.org : \
                dialups.mail-abuse.org

the following records are looked up:

43.62.168.192.blackholes.mail-abuse.org
43.62.168.192.dialups.mail-abuse.org

As soon as Exim finds an existing DNS record, processing of the list stops.
Thus, multiple entries on the list provide an "or" conjunction. If you want to
test that a host is on more than one list (an "and" conjunction), you can use
two separate conditions:

deny dnslists = blackholes.mail-abuse.org
     dnslists = dialups.mail-abuse.org

If a DNS lookup times out or otherwise fails to give a decisive answer, Exim
behaves as if the host does not match the list item, that is, as if the DNS
record does not exist. If there are further items in the DNS list, they are
processed.

This is usually the required action when dnslists is used with deny (which is
the most common usage), because it prevents a DNS failure from blocking mail.
However, you can change this behaviour by putting one of the following special
items in the list:

    +include_unknown behave as if the item is on the list
    +exclude_unknown behave as if the item is not on the list (default)
    +defer_unknown   give a temporary error

Each of these applies to any subsequent items on the list. For example:

deny dnslists = +defer_unknown : foo.bar.example

Testing the list of domains stops as soon as a match is found. If you want to
warn for one list and block for another, you can use two different statements:

deny  dnslists = blackholes.mail-abuse.org
warn  dnslists = dialups.mail-abuse.org
      message  = X-Warn: sending host is on dialups list

DNS list lookups are cached by Exim for the duration of the SMTP session (but
limited by the DNS return TTL value), so a lookup based on the IP address is
done at most once for any incoming connection (assuming long-enough TTL). Exim
does not share information between multiple incoming connections (but your
local name server cache should be active).

There are a number of DNS lists to choose from, some commercial, some free, or
free for small deployments. An overview can be found at https://
en.wikipedia.org/wiki/Comparison_of_DNS_blacklists.

44.18.1 Specifying the IP address for a DNS list lookup

By default, the IP address that is used in a DNS list lookup is the IP address
of the calling host. However, you can specify another IP address by listing it
after the domain name, introduced by a slash. For example:

deny dnslists = black.list.tld/192.168.1.2

This feature is not very helpful with explicit IP addresses; it is intended for
use with IP addresses that are looked up, for example, the IP addresses of the
MX hosts or nameservers of an email sender address. For an example, see section
44.18.3 below.

44.18.2 DNS lists keyed on domain names

There are some lists that are keyed on domain names rather than inverted IP
addresses (see, e.g., the domain based zones link at http://
www.rfc-ignorant.org/). No reversing of components is used with these lists.
You can change the name that is looked up in a DNS list by listing it after the
domain name, introduced by a slash. For example,

deny  dnslists = dsn.rfc-ignorant.org/$sender_address_domain
      message  = Sender's domain is listed at $dnslist_domain

This particular example is useful only in ACLs that are obeyed after the RCPT
or DATA commands, when a sender address is available. If (for example) the
message's sender is user@tld.example the name that is looked up by this example
is

tld.example.dsn.rfc-ignorant.org

A single dnslists condition can contain entries for both names and IP
addresses. For example:

deny dnslists = sbl.spamhaus.org : \
                dsn.rfc-ignorant.org/$sender_address_domain

The first item checks the sending host's IP address; the second checks a domain
name. The whole condition is true if either of the DNS lookups succeeds.

44.18.3 Multiple explicit keys for a DNS list

The syntax described above for looking up explicitly-defined values (either
names or IP addresses) in a DNS blacklist is a simplification. After the domain
name for the DNS list, what follows the slash can in fact be a list of items.
As with all lists in Exim, the default separator is a colon. However, because
this is a sublist within the list of DNS blacklist domains, it is necessary
either to double the separators like this:

dnslists = black.list.tld/name.1::name.2

or to change the separator character, like this:

dnslists = black.list.tld/<;name.1;name.2

If an item in the list is an IP address, it is inverted before the DNS
blacklist domain is appended. If it is not an IP address, no inversion occurs.
Consider this condition:

dnslists = black.list.tld/<;192.168.1.2;a.domain

The DNS lookups that occur are:

2.1.168.192.black.list.tld
a.domain.black.list.tld

Once a DNS record has been found (that matches a specific IP return address, if
specified - see section 44.18.6), no further lookups are done. If there is a
temporary DNS error, the rest of the sublist of domains or IP addresses is
tried. A temporary error for the whole dnslists item occurs only if no other
DNS lookup in this sublist succeeds. In other words, a successful lookup for
any of the items in the sublist overrides a temporary error for a previous
item.

The ability to supply a list of items after the slash is in some sense just a
syntactic convenience. These two examples have the same effect:

dnslists = black.list.tld/a.domain : black.list.tld/b.domain
dnslists = black.list.tld/a.domain::b.domain

However, when the data for the list is obtained from a lookup, the second form
is usually much more convenient. Consider this example:

deny dnslists = sbl.spamhaus.org/<|${lookup dnsdb {>|a=<|\
                                   ${lookup dnsdb {>|mxh=\
                                   $sender_address_domain} }} }
     message  = The mail servers for the domain \
                $sender_address_domain \
                are listed at $dnslist_domain ($dnslist_value); \
                see $dnslist_text.

Note the use of ">|" in the dnsdb lookup to specify the separator for multiple
DNS records. The inner dnsdb lookup produces a list of MX hosts and the outer
dnsdb lookup finds the IP addresses for these hosts. The result of expanding
the condition might be something like this:

dnslists = sbl.spamhaus.org/<|192.168.2.3|192.168.5.6|...

Thus, this example checks whether or not the IP addresses of the sender
domain's mail servers are on the Spamhaus black list.

The key that was used for a successful DNS list lookup is put into the variable
$dnslist_matched (see section 44.18.5).

44.18.4 Data returned by DNS lists

DNS lists are constructed using address records in the DNS. The original RBL
just used the address 127.0.0.1 on the right hand side of each record, but the
RBL+ list and some other lists use a number of values with different meanings.
The values used on the RBL+ list are:

    127.1.0.1 RBL
    127.1.0.2 DUL
    127.1.0.3 DUL and RBL
    127.1.0.4 RSS
    127.1.0.5 RSS and RBL
    127.1.0.6 RSS and DUL
    127.1.0.7 RSS and DUL and RBL

Section 44.18.6 below describes how you can distinguish between different
values. Some DNS lists may return more than one address record; see section
44.18.8 for details of how they are checked.

Values returned by a properly running DBSBL should be in the 127.0.0.0/8 range.
If a DNSBL operator loses control of the domain, lookups on it may start
returning other addresses. Because of this, Exim now ignores returned values
outside the 127/8 region.

44.18.5 Variables set from DNS lists

When an entry is found in a DNS list, the variable $dnslist_domain contains the
name of the overall domain that matched (for example, "spamhaus.example"),
$dnslist_matched contains the key within that domain (for example,
"192.168.5.3"), and $dnslist_value contains the data from the DNS record. When
the key is an IP address, it is not reversed in $dnslist_matched (though it is,
of course, in the actual lookup). In simple cases, for example:

deny dnslists = spamhaus.example

the key is also available in another variable (in this case,
$sender_host_address). In more complicated cases, however, this is not true.
For example, using a data lookup (as described in section 44.18.3) might
generate a dnslists lookup like this:

deny dnslists = spamhaus.example/<|192.168.1.2|192.168.6.7|...

If this condition succeeds, the value in $dnslist_matched might be
"192.168.6.7" (for example).

If more than one address record is returned by the DNS lookup, all the IP
addresses are included in $dnslist_value, separated by commas and spaces. The
variable $dnslist_text contains the contents of any associated TXT record. For
lists such as RBL+ the TXT record for a merged entry is often not very
meaningful. See section 44.18.9 for a way of obtaining more information.

You can use the DNS list variables in message or log_message modifiers - even
if these appear before the condition in the ACL, they are not expanded until
after it has failed. For example:

deny    hosts = !+local_networks
        message = $sender_host_address is listed \
                  at $dnslist_domain
        dnslists = rbl-plus.mail-abuse.example

44.18.6 Additional matching conditions for DNS lists

You can add an equals sign and an IP address after a dnslists domain name in
order to restrict its action to DNS records with a matching right hand side.
For example,

deny dnslists = rblplus.mail-abuse.org=127.0.0.2

rejects only those hosts that yield 127.0.0.2. Without this additional data,
any address record is considered to be a match. For the moment, we assume that
the DNS lookup returns just one record. Section 44.18.8 describes how multiple
records are handled.

More than one IP address may be given for checking, using a comma as a
separator. These are alternatives - if any one of them matches, the dnslists
condition is true. For example:

deny  dnslists = a.b.c=127.0.0.2,127.0.0.3

If you want to specify a constraining address list and also specify names or IP
addresses to be looked up, the constraining address list must be specified
first. For example:

deny dnslists = dsn.rfc-ignorant.org\
                =127.0.0.2/$sender_address_domain

If the character "&" is used instead of "=", the comparison for each listed IP
address is done by a bitwise "and" instead of by an equality test. In other
words, the listed addresses are used as bit masks. The comparison is true if
all the bits in the mask are present in the address that is being tested. For
example:

dnslists = a.b.c&0.0.0.3

matches if the address is x.x.x.3, x.x.x.7, x.x.x.11, etc. If you want to test
whether one bit or another bit is present (as opposed to both being present),
you must use multiple values. For example:

dnslists = a.b.c&0.0.0.1,0.0.0.2

matches if the final component of the address is an odd number or two times an
odd number.

44.18.7 Negated DNS matching conditions

You can supply a negative list of IP addresses as part of a dnslists condition.
Whereas

deny  dnslists = a.b.c=127.0.0.2,127.0.0.3

means "deny if the host is in the black list at the domain a.b.c and the IP
address yielded by the list is either 127.0.0.2 or 127.0.0.3",

deny  dnslists = a.b.c!=127.0.0.2,127.0.0.3

means "deny if the host is in the black list at the domain a.b.c and the IP
address yielded by the list is not 127.0.0.2 and not 127.0.0.3". In other
words, the result of the test is inverted if an exclamation mark appears before
the "=" (or the "&") sign.

Note: This kind of negation is not the same as negation in a domain, host, or
address list (which is why the syntax is different).

If you are using just one list, the negation syntax does not gain you much. The
previous example is precisely equivalent to

deny  dnslists = a.b.c
     !dnslists = a.b.c=127.0.0.2,127.0.0.3

However, if you are using multiple lists, the negation syntax is clearer.
Consider this example:

deny  dnslists = sbl.spamhaus.org : \
                 list.dsbl.org : \
                 dnsbl.njabl.org!=127.0.0.3 : \
                 relays.ordb.org

Using only positive lists, this would have to be:

deny  dnslists = sbl.spamhaus.org : \
                 list.dsbl.org
deny  dnslists = dnsbl.njabl.org
     !dnslists = dnsbl.njabl.org=127.0.0.3
deny  dnslists = relays.ordb.org

which is less clear, and harder to maintain.

Negation can also be used with a bitwise-and restriction. The dnslists
condition with only be trus if a result is returned by the lookup which, anded
with the restriction, is all zeroes. For example:

deny dnslists = zen.spamhaus.org!&0.255.255.0

44.18.8 Handling multiple DNS records from a DNS list

A DNS lookup for a dnslists condition may return more than one DNS record,
thereby providing more than one IP address. When an item in a dnslists list is
followed by "=" or "&" and a list of IP addresses, in order to restrict the
match to specific results from the DNS lookup, there are two ways in which the
checking can be handled. For example, consider the condition:

dnslists = a.b.c=127.0.0.1

What happens if the DNS lookup for the incoming IP address yields both
127.0.0.1 and 127.0.0.2 by means of two separate DNS records? Is the condition
true because at least one given value was found, or is it false because at
least one of the found values was not listed? And how does this affect negated
conditions? Both possibilities are provided for with the help of additional
separators "==" and "=&".

  * If "=" or "&" is used, the condition is true if any one of the looked up IP
    addresses matches one of the listed addresses. For the example above, the
    condition is true because 127.0.0.1 matches.

  * If "==" or "=&" is used, the condition is true only if every one of the
    looked up IP addresses matches one of the listed addresses. If the
    condition is changed to:

    dnslists = a.b.c==127.0.0.1

    and the DNS lookup yields both 127.0.0.1 and 127.0.0.2, the condition is
    false because 127.0.0.2 is not listed. You would need to have:

    dnslists = a.b.c==127.0.0.1,127.0.0.2

    for the condition to be true.

When "!" is used to negate IP address matching, it inverts the result, giving
the precise opposite of the behaviour above. Thus:

  * If "!=" or "!&" is used, the condition is true if none of the looked up IP
    addresses matches one of the listed addresses. Consider:

    dnslists = a.b.c!&0.0.0.1

    If the DNS lookup yields both 127.0.0.1 and 127.0.0.2, the condition is
    false because 127.0.0.1 matches.

  * If "!==" or "!=&" is used, the condition is true if there is at least one
    looked up IP address that does not match. Consider:

    dnslists = a.b.c!=&0.0.0.1

    If the DNS lookup yields both 127.0.0.1 and 127.0.0.2, the condition is
    true, because 127.0.0.2 does not match. You would need to have:

    dnslists = a.b.c!=&0.0.0.1,0.0.0.2

    for the condition to be false.

When the DNS lookup yields only a single IP address, there is no difference
between "=" and "==" and between "&" and "=&".

44.18.9 Detailed information from merged DNS lists

When the facility for restricting the matching IP values in a DNS list is used,
the text from the TXT record that is set in $dnslist_text may not reflect the
true reason for rejection. This happens when lists are merged and the IP
address in the A record is used to distinguish them; unfortunately there is
only one TXT record. One way round this is not to use merged lists, but that
can be inefficient because it requires multiple DNS lookups where one would do
in the vast majority of cases when the host of interest is not on any of the
lists.

A less inefficient way of solving this problem is available. If two domain
names, comma-separated, are given, the second is used first to do an initial
check, making use of any IP value restrictions that are set. If there is a
match, the first domain is used, without any IP value restrictions, to get the
TXT record. As a byproduct of this, there is also a check that the IP being
tested is indeed on the first list. The first domain is the one that is put in
$dnslist_domain. For example:

deny   dnslists = \
         sbl.spamhaus.org,sbl-xbl.spamhaus.org=127.0.0.2 : \
         dul.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.10
       message  = \
         rejected because $sender_host_address is blacklisted \
         at $dnslist_domain\n$dnslist_text

For the first blacklist item, this starts by doing a lookup in 
sbl-xbl.spamhaus.org and testing for a 127.0.0.2 return. If there is a match,
it then looks in sbl.spamhaus.org, without checking the return value, and as
long as something is found, it looks for the corresponding TXT record. If there
is no match in sbl-xbl.spamhaus.org, nothing more is done. The second blacklist
item is processed similarly.

If you are interested in more than one merged list, the same list must be given
several times, but because the results of the DNS lookups are cached, the DNS
calls themselves are not repeated. For example:

deny dnslists = \
         http.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.2 : \
         socks.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.3 : \
         misc.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.4 : \
         dul.dnsbl.sorbs.net,dnsbl.sorbs.net=127.0.0.10

In this case there is one lookup in dnsbl.sorbs.net, and if none of the IP
values matches (or if no record is found), this is the only lookup that is
done. Only if there is a match is one of the more specific lists consulted.

44.18.10 DNS lists and IPv6

If Exim is asked to do a dnslist lookup for an IPv6 address, it inverts it
nibble by nibble. For example, if the calling host's IP address is
3ffe:ffff:836f:0a00:000a:0800:200a:c031, Exim might look up

1.3.0.c.a.0.0.2.0.0.8.0.a.0.0.0.0.0.a.0.f.6.3.8.
  f.f.f.f.e.f.f.3.blackholes.mail-abuse.org

(split over two lines here to fit on the page). Unfortunately, some of the DNS
lists contain wildcard records, intended for IPv4, that interact badly with
IPv6. For example, the DNS entry

*.3.some.list.example.    A    127.0.0.1

is probably intended to put the entire 3.0.0.0/8 IPv4 network on the list.
Unfortunately, it also matches the entire 3::/4 IPv6 network.

You can exclude IPv6 addresses from DNS lookups by making use of a suitable 
condition condition, as in this example:

deny   condition = ${if isip4{$sender_host_address}}
       dnslists  = some.list.example

If an explicit key is being used for a DNS lookup and it may be an IPv6 address
you should specify alternate list separators for both the outer (DNS list name)
list and inner (lookup keys) list:

       dnslists = <; dnsbl.example.com/<|$acl_m_addrslist


44.19 Previously seen user and hosts
------------------------------------

The seen ACL condition can be used to test whether a situation has been
previously met. It uses a hints database to record a timestamp against a key.
The syntax of the condition is:

seen = <optional flag><time interval> / <options>

For example,

defer   seen = -5m / key=${sender_host_address}_$local_part@$domain

in a RCPT ACL will implement simple greylisting.

The parameters for the condition are a possible minus sign, then an interval,
then, slash-separated, a list of options. The interval is taken as an offset
before the current time, and used for the test. If the interval is preceded by
a minus sign then the condition returns whether a record is found which is
before the test time. Otherwise, the condition returns whether one is found
which is since the test time.

Options are read in order with later ones overriding earlier ones.

The default key is $sender_host_address. An explicit key can be set using a key
=value option.

If a readonly option is given then no record create or update is done. If a 
write option is given then a record create or update is always done. An update
is done if the test is for "since". If none of those hold and there was no
existing record, a record is created.

Creates and updates are marked with the current time.

Finally, a "before" test which succeeds, and for which the record is old
enough, will be refreshed with a timestamp of the test time. This can prevent
tidying of the database from removing the entry. The interval for this is, by
default, 10 days. An explicit interval can be set using a refresh=value option.

Note that "seen" should be added to the list of hints databases for maintenance
if this ACL condition is used.


44.20 Rate limiting incoming messages
-------------------------------------

The ratelimit ACL condition can be used to measure and control the rate at
which clients can send email. This is more powerful than the smtp_ratelimit_*
options, because those options control the rate of commands in a single SMTP
session only, whereas the ratelimit condition works across all connections
(concurrent and sequential) from the same client host. The syntax of the 
ratelimit condition is:

ratelimit = <m> / <p> / <options> / <key>

If the average client sending rate is less than m messages per time period p
then the condition is false; otherwise it is true.

As a side-effect, the ratelimit condition sets the expansion variable
$sender_rate to the client's computed rate, $sender_rate_limit to the
configured value of m, and $sender_rate_period to the configured value of p.

The parameter p is the smoothing time constant, in the form of an Exim time
interval, for example, "8h" for eight hours. A larger time constant means that
it takes Exim longer to forget a client's past behaviour. The parameter m is
the maximum number of messages that a client is permitted to send in each time
interval. It also specifies the number of messages permitted in a fast burst.
By increasing both m and p but keeping m/p constant, you can allow a client to
send more messages in a burst without changing its long-term sending rate
limit. Conversely, if m and p are both small, messages must be sent at an even
rate.

There is a script in util/ratelimit.pl which extracts sending rates from log
files, to assist with choosing appropriate settings for m and p when deploying
the ratelimit ACL condition. The script prints usage instructions when it is
run with no arguments.

The key is used to look up the data for calculating the client's average
sending rate. This data is stored in Exim's spool directory, alongside the
retry and other hints databases. The default key is $sender_host_address, which
means Exim computes the sending rate of each client host IP address. By
changing the key you can change how Exim identifies clients for the purpose of
ratelimiting. For example, to limit the sending rate of each authenticated
user, independent of the computer they are sending from, set the key to
$authenticated_id. You must ensure that the lookup key is meaningful; for
example, $authenticated_id is only meaningful if the client has authenticated
(which you can check with the authenticated ACL condition).

The lookup key does not have to identify clients: If you want to limit the rate
at which a recipient receives messages, you can use the key
"$local_part@$domain" with the per_rcpt option (see below) in a RCPT ACL.

Each ratelimit condition can have up to four options. A per_* option specifies
what Exim measures the rate of, for example, messages or recipients or bytes.
You can adjust the measurement using the unique= and/or count= options. You can
also control when Exim updates the recorded rate using a strict, leaky, or 
readonly option. The options are separated by a slash, like the other
parameters. They may appear in any order.

Internally, Exim appends the smoothing constant p onto the lookup key with any
options that alter the meaning of the stored data. The limit m is not stored,
so you can alter the configured maximum rate and Exim will still remember
clients' past behaviour. If you change the per_* mode or add or remove the 
unique= option, the lookup key changes so Exim will forget past behaviour. The
lookup key is not affected by changes to the update mode and the count= option.

44.20.1 Ratelimit options for what is being measured

per_conn

    This option limits the client's connection rate. It is not normally used in
    the acl_not_smtp, acl_not_smtp_mime, or acl_not_smtp_start ACLs.

per_mail

    This option limits the client's rate of sending messages. This is the
    default if none of the per_* options is specified. It can be used in 
    acl_smtp_mail, acl_smtp_rcpt, acl_smtp_predata, acl_smtp_mime, 
    acl_smtp_data, or acl_not_smtp.

per_byte

    This option limits the sender's email bandwidth. It can be used in the same
    ACLs as the per_mail option, though it is best to use this option in the 
    acl_smtp_mime, acl_smtp_data or acl_not_smtp ACLs; if it is used in an
    earlier ACL, Exim relies on the SIZE parameter given by the client in its
    MAIL command, which may be inaccurate or completely missing. You can follow
    the limit m in the configuration with K, M, or G to specify limits in
    kilobytes, megabytes, or gigabytes, respectively.

per_rcpt

    This option causes Exim to limit the rate at which recipients are accepted.
    It can be used in the acl_smtp_rcpt, acl_smtp_predata, acl_smtp_mime, or 
    acl_smtp_data ACLs. In acl_smtp_rcpt the rate is updated one recipient at a
    time; in the other ACLs the rate is updated with the total (accepted)
    recipient count in one go. Note that in either case the rate limiting
    engine will see a message with many recipients as a large high-speed burst.

per_addr

    This option is like the per_rcpt option, except it counts the number of
    different recipients that the client has sent messages to in the last time
    period. That is, if the client repeatedly sends messages to the same
    recipient, its measured rate is not increased. This option can only be used
    in acl_smtp_rcpt.

per_cmd

    This option causes Exim to recompute the rate every time the condition is
    processed. This can be used to limit the rate of any SMTP command. If it is
    used in multiple ACLs it can limit the aggregate rate of multiple different
    commands.

count

    This option can be used to alter how much Exim adds to the client's
    measured rate. A value is required, after an equals sign. For example, the 
    per_byte option is equivalent to "per_mail/count=$message_size". If there
    is no count= option, Exim increases the measured rate by one (except for
    the per_rcpt option in ACLs other than acl_smtp_rcpt). The count does not
    have to be an integer.

unique

    This option is described in section 44.20.4 below.

44.20.2 Ratelimit update modes

You can specify one of three options with the ratelimit condition to control
when its database is updated. This section describes the readonly mode, and the
next section describes the strict and leaky modes.

If the ratelimit condition is used in readonly mode, Exim looks up a
previously-computed rate to check against the limit.

For example, you can test the client's sending rate and deny it access (when it
is too fast) in the connect ACL. If the client passes this check then it can go
on to send a message, in which case its recorded rate will be updated in the
MAIL ACL. Subsequent connections from the same client will check this new rate.

acl_check_connect:
 deny ratelimit = 100 / 5m / readonly
      log_message = RATE CHECK: $sender_rate/$sender_rate_period \
                  (max $sender_rate_limit)
# ...
acl_check_mail:
 warn ratelimit = 100 / 5m / strict
      log_message = RATE UPDATE: $sender_rate/$sender_rate_period \
                  (max $sender_rate_limit)

If Exim encounters multiple ratelimit conditions with the same key when
processing a message then it may increase the client's measured rate more than
it should. For example, this will happen if you check the per_rcpt option in
both acl_smtp_rcpt and acl_smtp_data. However it's OK to check the same 
ratelimit condition multiple times in the same ACL. You can avoid any multiple
update problems by using the readonly option on later ratelimit checks.

The per_* options described above do not make sense in some ACLs. If you use a 
per_* option in an ACL where it is not normally permitted then the update mode
defaults to readonly and you cannot specify the strict or leaky modes. In other
ACLs the default update mode is leaky (see the next section) so you must
specify the readonly option explicitly.

44.20.3 Ratelimit options for handling fast clients

If a client's average rate is greater than the maximum, the rate limiting
engine can react in two possible ways, depending on the presence of the strict
or leaky update modes. This is independent of the other counter-measures (such
as rejecting the message) that may be specified by the rest of the ACL.

The leaky (default) option means that the client's recorded rate is not updated
if it is above the limit. The effect of this is that Exim measures the client's
average rate of successfully sent email, up to the given limit. This is
appropriate if the countermeasure when the condition is true consists of
refusing the message, and is generally the better choice if you have clients
that retry automatically. If the action when true is anything more complex then
this option is likely not what is wanted.

The strict option means that the client's recorded rate is always updated. The
effect of this is that Exim measures the client's average rate of attempts to
send email, which can be much higher than the maximum it is actually allowed.
If the client is over the limit it may be subjected to counter-measures by the
ACL. It must slow down and allow sufficient time to pass that its computed rate
falls below the maximum before it can send email again. The time (the number of
smoothing periods) it must wait and not attempt to send mail can be calculated
with this formula:

        ln(peakrate/maxrate)

44.20.4 Limiting the rate of different events

The ratelimit unique= option controls a mechanism for counting the rate of
different events. For example, the per_addr option uses this mechanism to count
the number of different recipients that the client has sent messages to in the
last time period; it is equivalent to "per_rcpt/unique=$local_part@$domain".
You could use this feature to measure the rate that a client uses different
sender addresses with the options "per_mail/unique=$sender_address".

For each ratelimit key Exim stores the set of unique= values that it has seen
for that key. The whole set is thrown away when it is older than the rate
smoothing period p, so each different event is counted at most once per period.
In the leaky update mode, an event that causes the client to go over the limit
is not added to the set, in the same way that the client's recorded rate is not
updated in the same situation.

When you combine the unique= and readonly options, the specific unique= value
is ignored, and Exim just retrieves the client's stored rate.

The unique= mechanism needs more space in the ratelimit database than the other
ratelimit options in order to store the event set. The number of unique values
is potentially as large as the rate limit, so the extra space required
increases with larger limits.

The uniqueification is not perfect: there is a small probability that Exim will
think a new event has happened before. If the sender's rate is less than the
limit, Exim should be more than 99.9% correct. However in strict mode the
measured rate can go above the limit, in which case Exim may under-count events
by a significant margin. Fortunately, if the rate is high enough (2.7 times the
limit) that the false positive rate goes above 9%, then Exim will throw away
the over-full event set before the measured rate falls below the limit.
Therefore the only harm should be that exceptionally high sending rates are
logged incorrectly; any countermeasures you configure will be as effective as
intended.

44.20.5 Using rate limiting

Exim's other ACL facilities are used to define what counter-measures are taken
when the rate limit is exceeded. This might be anything from logging a warning
(for example, while measuring existing sending rates in order to define
policy), through time delays to slow down fast senders, up to rejecting the
message. For example:

# Log all senders' rates
warn ratelimit = 0 / 1h / strict
     log_message = Sender rate $sender_rate / $sender_rate_period

# Slow down fast senders; note the need to truncate $sender_rate
# at the decimal point.
warn ratelimit = 100 / 1h / per_rcpt / strict
     delay     = ${eval: ${sg{$sender_rate}{[.].*}{}} - \
                   $sender_rate_limit }s

# Keep authenticated users under control
deny authenticated = *
     ratelimit = 100 / 1d / strict / $authenticated_id

# System-wide rate limit
defer ratelimit = 10 / 1s / $primary_hostname
      message = Sorry, too busy. Try again later.

# Restrict incoming rate from each host, with a default
# set using a macro and special cases looked up in a table.
defer ratelimit = ${lookup {$sender_host_address} \
                   cdb {DB/ratelimits.cdb} \
                   {$value} {RATELIMIT} }
      message = Sender rate exceeds $sender_rate_limit \
               messages per $sender_rate_period

Warning: If you have a busy server with a lot of ratelimit tests, especially
with the per_rcpt option, you may suffer from a performance bottleneck caused
by locking on the ratelimit hints database. Apart from making your ACLs less
complicated, you can reduce the problem by using a RAM disk for Exim's hints
directory (usually /var/spool/exim/db/). However this means that Exim will lose
its hints data after a reboot (including retry hints, the callout cache, and
ratelimit data).


44.21 Address verification
--------------------------

Several of the verify conditions described in section 44.17 cause addresses to
be verified. Section 44.24 discusses the reporting of sender verification
failures. The verification conditions can be followed by options that modify
the verification process. The options are separated from the keyword and from
each other by slashes, and some of them contain parameters. For example:

verify = sender/callout
verify = recipient/defer_ok/callout=10s,defer_ok

The first stage of address verification, which always happens, is to run the
address through the routers, in "verify mode". Routers can detect the
difference between verification and routing for delivery, and their actions can
be varied by a number of generic options such as verify and verify_only (see
chapter 15). If routing fails, verification fails. The available options are as
follows:

  * If the callout option is specified, successful routing to one or more
    remote hosts is followed by a "callout" to those hosts as an additional
    check. Callouts and their sub-options are discussed in the next section.

  * If there is a defer error while doing verification routing, the ACL
    normally returns "defer". However, if you include defer_ok in the options,
    the condition is forced to be true instead. Note that this is a main
    verification option as well as a suboption for callouts.

  * The no_details option is covered in section 44.24, which discusses the
    reporting of sender address verification failures.

  * The success_on_redirect option causes verification always to succeed
    immediately after a successful redirection. By default, if a redirection
    generates just one address, that address is also verified. See further
    discussion in section 44.25.

  * If the quota option is specified for recipient verify, successful routing
    to an appendfile transport is followed by a call into the transport to
    evaluate the quota status for the recipient. No actual delivery is done,
    but verification will succeed if the quota is sufficient for the message
    (if the sender gave a message size) or not already exceeded (otherwise).

After an address verification failure, $acl_verify_message contains the error
message that is associated with the failure. It can be preserved by coding like
this:

warn  !verify = sender
       set acl_m0 = $acl_verify_message

If you are writing your own custom rejection message or log message when
denying access, you can use this variable to include information about the
verification failure. This variable is cleared at the end of processing the ACL
verb.

In addition, $sender_verify_failure or $recipient_verify_failure (as
appropriate) contains one of the following words:

  * qualify: The address was unqualified (no domain), and the message was
    neither local nor came from an exempted host.

  * route: Routing failed.

  * mail: Routing succeeded, and a callout was attempted; rejection occurred at
    or before the MAIL command (that is, on initial connection, HELO, or MAIL).

  * recipient: The RCPT command in a callout was rejected.

  * postmaster: The postmaster check in a callout was rejected.

  * quota: The quota check for a local recipient did non pass.

The main use of these variables is expected to be to distinguish between
rejections of MAIL and rejections of RCPT in callouts.

The above variables may also be set after a successful address verification to:

  * random: A random local-part callout succeeded


44.22 Callout verification
--------------------------

For non-local addresses, routing verifies the domain, but is unable to do any
checking of the local part. There are situations where some means of verifying
the local part is desirable. One way this can be done is to make an SMTP 
callback to a delivery host for the sender address or a callforward to a
subsequent host for a recipient address, to see if the host accepts the
address. We use the term callout to cover both cases. Note that for a sender
address, the callback is not to the client host that is trying to deliver the
message, but to one of the hosts that accepts incoming mail for the sender's
domain.

Exim does not do callouts by default. If you want them to happen, you must
request them by setting appropriate options on the verify condition, as
described below. This facility should be used with care, because it can add a
lot of resource usage to the cost of verifying an address. However, Exim does
cache the results of callouts, which helps to reduce the cost. Details of
caching are in section 44.22.2.

Recipient callouts are usually used only between hosts that are controlled by
the same administration. For example, a corporate gateway host could use
callouts to check for valid recipients on an internal mailserver. A successful
callout does not guarantee that a real delivery to the address would succeed;
on the other hand, a failing callout does guarantee that a delivery would fail.

If the callout option is present on a condition that verifies an address, a
second stage of verification occurs if the address is successfully routed to
one or more remote hosts. The usual case is routing by a dnslookup or a 
manualroute router, where the router specifies the hosts. However, if a router
that does not set up hosts routes to an smtp transport with a hosts setting,
the transport's hosts are used. If an smtp transport has hosts_override set,
its hosts are always used, whether or not the router supplies a host list.
Callouts are only supported on smtp transports.

The port that is used is taken from the transport, if it is specified and is a
remote transport. (For routers that do verification only, no transport need be
specified.) Otherwise, the default SMTP port is used. If a remote transport
specifies an outgoing interface, this is used; otherwise the interface is not
specified. Likewise, the text that is used for the HELO command is taken from
the transport's helo_data option; if there is no transport, the value of
$smtp_active_hostname is used.

For a sender callout check, Exim makes SMTP connections to the remote hosts, to
test whether a bounce message could be delivered to the sender address. The
following SMTP commands are sent:

HELO <local host name>
MAIL FROM:<>
RCPT TO:<the address to be tested>
QUIT

LHLO is used instead of HELO if the transport's protocol option is set to
"lmtp".

The callout may use EHLO, AUTH and/or STARTTLS given appropriate option
settings.

A recipient callout check is similar. By default, it also uses an empty address
for the sender. This default is chosen because most hosts do not make use of
the sender address when verifying a recipient. Using the same address means
that a single cache entry can be used for each recipient. Some sites, however,
do make use of the sender address when verifying. These are catered for by the 
use_sender and use_postmaster options, described in the next section.

If the response to the RCPT command is a 2xx code, the verification succeeds.
If it is 5xx, the verification fails. For any other condition, Exim tries the
next host, if any. If there is a problem with all the remote hosts, the ACL
yields "defer", unless the defer_ok parameter of the callout option is given,
in which case the condition is forced to succeed.

A callout may take a little time. For this reason, Exim normally flushes SMTP
output before performing a callout in an ACL, to avoid unexpected timeouts in
clients when the SMTP PIPELINING extension is in use. The flushing can be
disabled by using a control modifier to set no_callout_flush.

A recipient callout which gets a 2xx code will assign untainted values to the
$domain_data and $local_part_data variables, corresponding to the domain and
local parts of the recipient address.

44.22.1 Additional parameters for callouts

The callout option can be followed by an equals sign and a number of optional
parameters, separated by commas. For example:

verify = recipient/callout=10s,defer_ok

The old syntax, which had callout_defer_ok and check_postmaster as separate
verify options, is retained for backwards compatibility, but is now deprecated.
The additional parameters for callout are as follows:

<a time interval>

    This specifies the timeout that applies for the callout attempt to each
    host. For example:

    verify = sender/callout=5s

    The default is 30 seconds. The timeout is used for each response from the
    remote host. It is also used for the initial connection, unless overridden
    by the connect parameter.

connect = <time interval>

    This parameter makes it possible to set a different (usually smaller)
    timeout for making the SMTP connection. For example:

    verify = sender/callout=5s,connect=1s

    If not specified, this timeout defaults to the general timeout value.

defer_ok

    When this parameter is present, failure to contact any host, or any other
    kind of temporary error, is treated as success by the ACL. However, the
    cache is not updated in this circumstance.

fullpostmaster

    This operates like the postmaster option (see below), but if the check for 
    postmaster@domain fails, it tries just postmaster, without a domain, in
    accordance with the specification in RFC 2821. The RFC states that the
    unqualified address postmaster should be accepted.

mailfrom = <email address>

    When verifying addresses in header lines using the header_sender
    verification option, Exim behaves by default as if the addresses are
    envelope sender addresses from a message. Callout verification therefore
    tests to see whether a bounce message could be delivered, by using an empty
    address in the MAIL command. However, it is arguable that these addresses
    might never be used as envelope senders, and could therefore justifiably
    reject bounce messages (empty senders). The mailfrom callout parameter
    allows you to specify what address to use in the MAIL command. For example:

    require  verify = header_sender/callout=mailfrom=abcd@x.y.z

    This parameter is available only for the header_sender verification option.

maxwait = <time interval>

    This parameter sets an overall timeout for performing a callout
    verification. For example:

    verify = sender/callout=5s,maxwait=30s

    This timeout defaults to four times the callout timeout for individual SMTP
    commands. The overall timeout applies when there is more than one host that
    can be tried. The timeout is checked before trying the next host. This
    prevents very long delays if there are a large number of hosts and all are
    timing out (for example, when network connections are timing out).

no_cache

    When this parameter is given, the callout cache is neither read nor
    updated.

postmaster

    When this parameter is set, a successful callout check is followed by a
    similar check for the local part postmaster at the same domain. If this
    address is rejected, the callout fails (but see fullpostmaster above). The
    result of the postmaster check is recorded in a cache record; if it is a
    failure, this is used to fail subsequent callouts for the domain without a
    connection being made, until the cache record expires.

postmaster_mailfrom = <email address>

    The postmaster check uses an empty sender in the MAIL command by default.
    You can use this parameter to do a postmaster check using a different
    address. For example:

    require  verify = sender/callout=postmaster_mailfrom=abc@x.y.z

    If both postmaster and postmaster_mailfrom are present, the rightmost one
    overrides. The postmaster parameter is equivalent to this example:

    require  verify = sender/callout=postmaster_mailfrom=

    Warning: The caching arrangements for postmaster checking do not take
    account of the sender address. It is assumed that either the empty address
    or a fixed non-empty address will be used. All that Exim remembers is that
    the postmaster check for the domain succeeded or failed.

random

    When this parameter is set, before doing the normal callout check, Exim
    does a check for a "random" local part at the same domain. The local part
    is not really random - it is defined by the expansion of the option 
    callout_random_local_part, which defaults to

    $primary_hostname-$tod_epoch-testing

    The idea here is to try to determine whether the remote host accepts all
    local parts without checking. If it does, there is no point in doing
    callouts for specific local parts. If the "random" check succeeds, the
    result is saved in a cache record, and used to force the current and
    subsequent callout checks to succeed without a connection being made, until
    the cache record expires.

use_postmaster

    This parameter applies to recipient callouts only. For example:

    deny  !verify = recipient/callout=use_postmaster

    It causes a non-empty postmaster address to be used in the MAIL command
    when performing the callout for the recipient, and also for a "random"
    check if that is configured. The local part of the address is "postmaster"
    and the domain is the contents of $qualify_domain.

use_sender

    This option applies to recipient callouts only. For example:

    require  verify = recipient/callout=use_sender

    It causes the message's actual sender address to be used in the MAIL
    command when performing the callout, instead of an empty address. There is
    no need to use this option unless you know that the called hosts make use
    of the sender when checking recipients. If used indiscriminately, it
    reduces the usefulness of callout caching.

hold

    This option applies to recipient callouts only. For example:

    require  verify = recipient/callout=use_sender,hold

    It causes the connection to be held open and used for any further
    recipients and for eventual delivery (should that be done quickly). Doing
    this saves on TCP and SMTP startup costs, and TLS costs also when that is
    used for the connections. The advantage is only gained if there are no
    callout cache hits (which could be enforced by the no_cache option), if the
    use_sender option is used, if neither the random nor the use_postmaster
    option is used, and if no other callouts intervene.

If you use any of the parameters that set a non-empty sender for the MAIL
command (mailfrom, postmaster_mailfrom, use_postmaster, or use_sender), you
should think about possible loops. Recipient checking is usually done between
two hosts that are under the same management, and the host that receives the
callouts is not normally configured to do callouts itself. Therefore, it is
normally safe to use use_postmaster or use_sender in these circumstances.

However, if you use a non-empty sender address for a callout to an arbitrary
host, there is the likelihood that the remote host will itself initiate a
callout check back to your host. As it is checking what appears to be a message
sender, it is likely to use an empty address in MAIL, thus avoiding a callout
loop. However, to be on the safe side it would be best to set up your own ACLs
so that they do not do sender verification checks when the recipient is the
address you use for header sender or postmaster callout checking.

Another issue to think about when using non-empty senders for callouts is
caching. When you set mailfrom or use_sender, the cache record is keyed by the
sender/recipient combination; thus, for any given recipient, many more actual
callouts are performed than when an empty sender or postmaster is used.

44.22.2 Callout caching

Exim caches the results of callouts in order to reduce the amount of resources
used, unless you specify the no_cache parameter with the callout option. A
hints database called "callout" is used for the cache. Two different record
types are used: one records the result of a callout check for a specific
address, and the other records information that applies to the entire domain
(for example, that it accepts the local part postmaster).

When an original callout fails, a detailed SMTP error message is given about
the failure. However, for subsequent failures that use the cache data, this
message is not available.

The expiry times for negative and positive address cache records are
independent, and can be set by the global options callout_negative_expire
(default 2h) and callout_positive_expire (default 24h), respectively.

If a host gives a negative response to an SMTP connection, or rejects any
commands up to and including

MAIL FROM:<>

(but not including the MAIL command with a non-empty address), any callout
attempt is bound to fail. Exim remembers such failures in a domain cache
record, which it uses to fail callouts for the domain without making new
connections, until the domain record times out. There are two separate expiry
times for domain cache records: callout_domain_negative_expire (default 3h) and
callout_domain_positive_expire (default 7d).

Domain records expire when the negative expiry time is reached if callouts
cannot be made for the domain, or if the postmaster check failed. Otherwise,
they expire when the positive expiry time is reached. This ensures that, for
example, a host that stops accepting "random" local parts will eventually be
noticed.

The callout caching mechanism is based on the domain of the address that is
being tested. If the domain routes to several hosts, it is assumed that their
behaviour will be the same.


44.23 Quota caching
-------------------

Exim caches the results of quota verification in order to reduce the amount of
resources used. The "callout" hints database is used.

The default cache periods are five minutes for a positive (good) result and one
hour for a negative result. To change the periods the quota option can be
followed by an equals sign and a number of optional paramemters, separated by
commas. For example:

verify = recipient/quota=cachepos=1h,cacheneg=1d

Possible parameters are:

cachepos = <time interval>

    Set the lifetime for a positive cache entry. A value of zero seconds is
    legitimate.

cacheneg = <time interval>

    As above, for a negative entry.

no_cache

    Set both positive and negative lifetimes to zero.


44.24 Sender address verification reporting
-------------------------------------------

See section 44.21 for a general discussion of verification. When sender
verification fails in an ACL, the details of the failure are given as
additional output lines before the 550 response to the relevant SMTP command
(RCPT or DATA). For example, if sender callout is in use, you might see:

MAIL FROM:<xyz@abc.example>
250 OK
RCPT TO:<pqr@def.example>
550-Verification failed for <xyz@abc.example>
550-Called:   192.168.34.43
550-Sent:     RCPT TO:<xyz@abc.example>
550-Response: 550 Unknown local part xyz in <xyz@abc.example>
550 Sender verification failed

If more than one RCPT command fails in the same way, the details are given only
for the first of them. However, some administrators do not want to send out
this much information. You can suppress the details by adding "/no_details" to
the ACL statement that requests sender verification. For example:

verify = sender/no_details


44.25 Redirection while verifying
---------------------------------

A dilemma arises when a local address is redirected by aliasing or forwarding
during verification: should the generated addresses themselves be verified, or
should the successful expansion of the original address be enough to verify it?
By default, Exim takes the following pragmatic approach:

  * When an incoming address is redirected to just one child address,
    verification continues with the child address, and if that fails to verify,
    the original verification also fails.

  * When an incoming address is redirected to more than one child address,
    verification does not continue. A success result is returned.

This seems the most reasonable behaviour for the common use of aliasing as a
way of redirecting different local parts to the same mailbox. It means, for
example, that a pair of alias entries of the form

A.Wol:   aw123
aw123:   :fail: Gone away, no forwarding address

work as expected, with both local parts causing verification failure. When a
redirection generates more than one address, the behaviour is more like a
mailing list, where the existence of the alias itself is sufficient for
verification to succeed.

It is possible, however, to change the default behaviour so that all successful
redirections count as successful verifications, however many new addresses are
generated. This is specified by the success_on_redirect verification option.
For example:

require verify = recipient/success_on_redirect/callout=10s

In this example, verification succeeds if a router generates a new address, and
the callout does not occur, because no address was routed to a remote host.

When verification is being tested via the -bv option, the treatment of
redirections is as just described, unless the -v or any debugging option is
also specified. In that case, full verification is done for every generated
address and a report is output for each of them.


44.26 Client SMTP authorization (CSA)
-------------------------------------

Client SMTP Authorization is a system that allows a site to advertise which
machines are and are not permitted to send email. This is done by placing
special SRV records in the DNS; these are looked up using the client's HELO
domain. At the time of writing, CSA is still an Internet Draft. Client SMTP
Authorization checks in Exim are performed by the ACL condition:

verify = csa

This fails if the client is not authorized. If there is a DNS problem, or if no
valid CSA SRV record is found, or if the client is authorized, the condition
succeeds. These three cases can be distinguished using the expansion variable
$csa_status, which can take one of the values "fail", "defer", "unknown", or
"ok". The condition does not itself defer because that would be likely to cause
problems for legitimate email.

The error messages produced by the CSA code include slightly more detail. If
$csa_status is "defer", this may be because of problems looking up the CSA SRV
record, or problems looking up the CSA target address record. There are four
reasons for $csa_status being "fail":

  * The client's host name is explicitly not authorized.

  * The client's IP address does not match any of the CSA target IP addresses.

  * The client's host name is authorized but it has no valid target IP
    addresses (for example, the target's addresses are IPv6 and the client is
    using IPv4).

  * The client's host name has no CSA SRV record but a parent domain has
    asserted that all subdomains must be explicitly authorized.

The csa verification condition can take an argument which is the domain to use
for the DNS query. The default is:

verify = csa/$sender_helo_name

This implementation includes an extension to CSA. If the query domain is an
address literal such as [192.0.2.95], or if it is a bare IP address, Exim
searches for CSA SRV records in the reverse DNS as if the HELO domain was (for
example) 95.2.0.192.in-addr.arpa. Therefore it is meaningful to say:

verify = csa/$sender_host_address

In fact, this is the check that Exim performs if the client does not say HELO.
This extension can be turned off by setting the main configuration option 
dns_csa_use_reverse to be false.

If a CSA SRV record is not found for the domain itself, a search is performed
through its parent domains for a record which might be making assertions about
subdomains. The maximum depth of this search is limited using the main
configuration option dns_csa_search_limit, which is 5 by default. Exim does not
look for CSA SRV records in a top level domain, so the default settings handle
HELO domains as long as seven (hostname.five.four.three.two.one.com). This
encompasses the vast majority of legitimate HELO domains.

The dnsdb lookup also has support for CSA. Although dnsdb also supports direct
SRV lookups, this is not sufficient because of the extra parent domain search
behaviour of CSA, and (as with PTR lookups) dnsdb also turns IP addresses into
lookups in the reverse DNS space. The result of a successful lookup such as:

${lookup dnsdb {csa=$sender_helo_name}}

has two space-separated fields: an authorization code and a target host name.
The authorization code can be "Y" for yes, "N" for no, "X" for explicit
authorization required but absent, or "?" for unknown.


44.27 Bounce address tag validation
-----------------------------------

Bounce address tag validation (BATV) is a scheme whereby the envelope senders
of outgoing messages have a cryptographic, timestamped "tag" added to them.
Genuine incoming bounce messages should therefore always be addressed to
recipients that have a valid tag. This scheme is a way of detecting unwanted
bounce messages caused by sender address forgeries (often called "collateral
spam"), because the recipients of such messages do not include valid tags.

There are two expansion items to help with the implementation of the BATV
"prvs" (private signature) scheme in an Exim configuration. This scheme signs
the original envelope sender address by using a simple key to add a hash of the
address and some time-based randomizing information. The prvs expansion item
creates a signed address, and the prvscheck expansion item checks one. The
syntax of these expansion items is described in section 11.5. The validity
period on signed addresses is seven days.

As an example, suppose the secret per-address keys are stored in an MySQL
database. A query to look up the key for an address could be defined as a macro
like this:

PRVSCHECK_SQL = ${lookup mysql{SELECT secret FROM batv_prvs \
                WHERE sender='${quote_mysql:$prvscheck_address}'\
                }{$value}}

Suppose also that the senders who make use of BATV are defined by an address
list called batv_senders. Then, in the ACL for RCPT commands, you could use
this:

# Bounces: drop unsigned addresses for BATV senders
deny senders = :
     recipients = +batv_senders
     message = This address does not send an unsigned reverse path

# Bounces: In case of prvs-signed address, check signature.
deny senders = :
     condition  = ${prvscheck {$local_part@$domain}\
                  {PRVSCHECK_SQL}{1}}
     !condition = $prvscheck_result
     message = Invalid reverse path signature.

The first statement rejects recipients for bounce messages that are addressed
to plain BATV sender addresses, because it is known that BATV senders do not
send out messages with plain sender addresses. The second statement rejects
recipients that are prvs-signed, but with invalid signatures (either because
the key is wrong, or the signature has timed out).

A non-prvs-signed address is not rejected by the second statement, because the 
prvscheck expansion yields an empty string if its first argument is not a
prvs-signed address, thus causing the condition condition to be false. If the
first argument is a syntactically valid prvs-signed address, the yield is the
third string (in this case "1"), whether or not the cryptographic and timeout
checks succeed. The $prvscheck_result variable contains the result of the
checks (empty for failure, "1" for success).

There is one more issue you must consider when implementing prvs-signing: you
have to ensure that the routers accept prvs-signed addresses and deliver them
correctly. The easiest way to handle this is to use a redirect router to remove
the signature with a configuration along these lines:

batv_redirect:
  driver = redirect
  data = ${prvscheck {$local_part@$domain}{PRVSCHECK_SQL}}

This works because, if the third argument of prvscheck is empty, the result of
the expansion of a prvs-signed address is the decoded value of the original
address. This router should probably be the first of your routers that handles
local addresses.

To create BATV-signed addresses in the first place, a transport of this form
can be used:

external_smtp_batv:
  driver = smtp
  return_path = ${prvs {$return_path} \
                       {${lookup mysql{SELECT \
                       secret FROM batv_prvs WHERE \
                       sender='${quote_mysql:$sender_address}'} \
                       {$value}fail}}}

If no key can be found for the existing return path, no signing takes place.


44.28 Using an ACL to control relaying
--------------------------------------

An MTA is said to relay a message if it receives it from some host and delivers
it directly to another host as a result of a remote address contained within
it. Redirecting a local address via an alias or forward file and then passing
the message on to another host is not relaying, but a redirection as a result
of the "percent hack" is.

Two kinds of relaying exist, which are termed "incoming" and "outgoing". A host
which is acting as a gateway or an MX backup is concerned with incoming
relaying from arbitrary hosts to a specific set of domains. On the other hand,
a host which is acting as a smart host for a number of clients is concerned
with outgoing relaying from those clients to the Internet at large. Often the
same host is fulfilling both functions, but in principle these two kinds of
relaying are entirely independent. What is not wanted is the transmission of
mail from arbitrary remote hosts through your system to arbitrary domains.

You can implement relay control by means of suitable statements in the ACL that
runs for each RCPT command. For convenience, it is often easiest to use Exim's
named list facility to define the domains and hosts involved. For example,
suppose you want to do the following:

  * Deliver a number of domains to mailboxes on the local host (or process them
    locally in some other way). Let's say these are my.dom1.example and 
    my.dom2.example.

  * Relay mail for a number of other domains for which you are the secondary
    MX. These might be friend1.example and friend2.example.

  * Relay mail from the hosts on your local LAN, to whatever domains are
    involved. Suppose your LAN is 192.168.45.0/24.

In the main part of the configuration, you put the following definitions:

domainlist local_domains    = my.dom1.example : my.dom2.example
domainlist relay_to_domains = friend1.example : friend2.example
hostlist   relay_from_hosts = 192.168.45.0/24

Now you can use these definitions in the ACL that is run for every RCPT
command:

acl_check_rcpt:
  accept domains = +local_domains : +relay_to_domains
  accept hosts   = +relay_from_hosts

The first statement accepts any RCPT command that contains an address in the
local or relay domains. For any other domain, control passes to the second
statement, which accepts the command only if it comes from one of the relay
hosts. In practice, you will probably want to make your ACL more sophisticated
than this, for example, by including sender and recipient verification. The
default configuration includes a more comprehensive example, which is described
in chapter 7.


44.29 Checking a relay configuration
------------------------------------

You can check the relay characteristics of your configuration in the same way
that you can test any ACL behaviour for an incoming SMTP connection, by using
the -bh option to run a fake SMTP session with which you interact.



===============================================================================
45. CONTENT SCANNING AT ACL TIME

The extension of Exim to include content scanning at ACL time, formerly known
as "exiscan", was originally implemented as a patch by Tom Kistner. The code
was integrated into the main source for Exim release 4.50, and Tom continues to
maintain it. Most of the wording of this chapter is taken from Tom's
specification.

It is also possible to scan the content of messages at other times. The
local_scan() function (see chapter 46) allows for content scanning after all
the ACLs have run. A transport filter can be used to scan messages at delivery
time (see the transport_filter option, described in chapter 24).

If you want to include the ACL-time content-scanning features when you compile
Exim, you need to arrange for WITH_CONTENT_SCAN to be defined in your Local/
Makefile. When you do that, the Exim binary is built with:

  * Two additional ACLs (acl_smtp_mime and acl_not_smtp_mime) that are run for
    all MIME parts for SMTP and non-SMTP messages, respectively.

  * Additional ACL conditions and modifiers: decode, malware, mime_regex, regex
    , and spam. These can be used in the ACL that is run at the end of message
    reception (the acl_smtp_data ACL).

  * An additional control feature ("no_mbox_unspool") that saves spooled copies
    of messages, or parts of messages, for debugging purposes.

  * Additional expansion variables that are set in the new ACL and by the new
    conditions.

  * Two new main configuration options: av_scanner and spamd_address.

Content-scanning is continually evolving, and new features are still being
added. While such features are still unstable and liable to incompatible
changes, they are made available in Exim by setting options whose names begin
EXPERIMENTAL_ in Local/Makefile. Such features are not documented in this
manual. You can find out about them by reading the file called doc/
experimental.txt.

All the content-scanning facilities work on a MBOX copy of the message that is
temporarily created in a file called:

<spool_directory>/scan/<message_id>/<message_id>.eml

The .eml extension is a friendly hint to virus scanners that they can expect an
MBOX-like structure inside that file. The file is created when the first
content scanning facility is called. Subsequent calls to content scanning
conditions open the same file again. The directory is recursively removed when
the acl_smtp_data ACL has finished running, unless

control = no_mbox_unspool

has been encountered. When the MIME ACL decodes files, they are put into the
same directory by default.


45.1 Scanning for viruses
-------------------------

The malware ACL condition lets you connect virus scanner software to Exim. It
supports a "generic" interface to scanners called via the shell, and
specialized interfaces for "daemon" type virus scanners, which are resident in
memory and thus are much faster.

Since message data needs to have arrived, the condition may be only called in
ACL defined by acl_smtp_data, acl_smtp_data_prdr, acl_smtp_mime or 
acl_smtp_dkim

A timeout of 2 minutes is applied to a scanner call (by default); if it expires
then a defer action is taken.

You can set the av_scanner option in the main part of the configuration to
specify which scanner to use, together with any additional options that are
needed. The basic syntax is as follows:

av_scanner = <scanner-type>:<option1>:<option2>:[...]

If you do not set av_scanner, it defaults to

av_scanner = sophie:/var/run/sophie

If the value of av_scanner starts with a dollar character, it is expanded
before use. The usual list-parsing of the content (see 6.20) applies. The
following scanner types are supported in this release, though individual ones
can be included or not at build time:

avast

    This is the scanner daemon of Avast. It has been tested with Avast Core
    Security (currently at version 2.2.0). You can get a trial version at 
    https://www.avast.com or for Linux at https://www.avast.com/
    linux-server-antivirus. This scanner type takes one option, which can be
    either a full path to a UNIX socket, or host and port specifiers separated
    by white space. The host may be a name or an IP address; the port is either
    a single number or a pair of numbers with a dash between. A list of options
    may follow. These options are interpreted on the Exim's side of the malware
    scanner, or are given on separate lines to the daemon as options before the
    main scan command.

    If "pass_unscanned" is set, any files the Avast scanner can't scan (e.g.
    decompression bombs, or invalid archives) are considered clean. Use with
    care.

    For example:

    av_scanner = avast:/var/run/avast/scan.sock:FLAGS -fullfiles:SENSITIVITY -pup
    av_scanner = avast:/var/run/avast/scan.sock:pass_unscanned:FLAGS -fullfiles:SENSITIVITY -pup
    av_scanner = avast:192.168.2.22 5036

    If you omit the argument, the default path /var/run/avast/scan.sock is
    used. If you use a remote host, you need to make Exim's spool directory
    available to it, as the scanner is passed a file path, not file contents.
    For information about available commands and their options you may use

    $ socat UNIX:/var/run/avast/scan.sock STDIO:
        FLAGS
        SENSITIVITY
        PACK

    If the scanner returns a temporary failure (e.g. license issues, or
    permission problems), the message is deferred and a paniclog entry is
    written. The usual "defer_ok" option is available.

aveserver

    This is the scanner daemon of Kaspersky Version 5. You can get a trial
    version at https://www.kaspersky.com/. This scanner type takes one option,
    which is the path to the daemon's UNIX socket. The default is shown in this
    example:

    av_scanner = aveserver:/var/run/aveserver

clamd

    This daemon-type scanner is GPL and free. You can get it at https://
    www.clamav.net/. Some older versions of clamd do not seem to unpack MIME
    containers, so it used to be recommended to unpack MIME attachments in the
    MIME ACL. This is no longer believed to be necessary.

    The options are a list of server specifiers, which may be a UNIX socket
    specification, a TCP socket specification, or a (global) option.

    A socket specification consists of a space-separated list. For a Unix
    socket the first element is a full path for the socket, for a TCP socket
    the first element is the IP address and the second a port number, Any
    further elements are per-server (non-global) options. These per-server
    options are supported:

    retry=<timespec>        Retry on connect fail

    The "retry" option specifies a time after which a single retry for a failed
    connect is made. The default is to not retry.

    If a Unix socket file is specified, only one server is supported.

    Examples:

    av_scanner = clamd:/opt/clamd/socket
    av_scanner = clamd:192.0.2.3 1234
    av_scanner = clamd:192.0.2.3 1234:local
    av_scanner = clamd:192.0.2.3 1234 retry=10s
    av_scanner = clamd:192.0.2.3 1234 : 192.0.2.4 1234

    If the value of av_scanner points to a UNIX socket file or contains the
    "local" option, then the ClamAV interface will pass a filename containing
    the data to be scanned, which should normally result in less I/O happening
    and be more efficient. Normally in the TCP case, the data is streamed to
    ClamAV as Exim does not assume that there is a common filesystem with the
    remote host.

    The final example shows that multiple TCP targets can be specified. Exim
    will randomly use one for each incoming email (i.e. it load balances them).
    Note that only TCP targets may be used if specifying a list of scanners; a
    UNIX socket cannot be mixed in with TCP targets. If one of the servers
    becomes unavailable, Exim will try the remaining one(s) until it finds one
    that works. When a clamd server becomes unreachable, Exim will log a
    message. Exim does not keep track of scanner state between multiple
    messages, and the scanner selection is random, so the message will get
    logged in the mainlog for each email that the down scanner gets chosen
    first (message wrapped to be readable):

    2013-10-09 14:30:39 1VTumd-0000Y8-BQ malware acl condition:
       clamd: connection to localhost, port 3310 failed
       (Connection refused)

    If the option is unset, the default is /tmp/clamd. Thanks to David Saez for
    contributing the code for this scanner.

cmdline

    This is the keyword for the generic command line scanner interface. It can
    be used to attach virus scanners that are invoked from the shell. This
    scanner type takes 3 mandatory options:

     1. The full path and name of the scanner binary, with all command line
        options, and a placeholder ("%s") for the directory to scan.

     2. A regular expression to match against the STDOUT and STDERR output of
        the virus scanner. If the expression matches, a virus was found. You
        must make absolutely sure that this expression matches on "virus
        found". This is called the "trigger" expression.

     3. Another regular expression, containing exactly one pair of parentheses,
        to match the name of the virus found in the scanners output. This is
        called the "name" expression.

    For example, Sophos Sweep reports a virus on a line like this:

    Virus 'W32/Magistr-B' found in file ./those.bat

    For the trigger expression, we can match the phrase "found in file". For
    the name expression, we want to extract the W32/Magistr-B string, so we can
    match for the single quotes left and right of it. Altogether, this makes
    the configuration setting:

    av_scanner = cmdline:\
                 /path/to/sweep -ss -all -rec -archive %s:\
                 found in file:'(.+)'

drweb

    The DrWeb daemon scanner (https://www.sald.ru/) interface takes one option,
    either a full path to a UNIX socket, or host and port specifiers separated
    by white space. The host may be a name or an IP address; the port is either
    a single number or a pair of numbers with a dash between. For example:

    av_scanner = drweb:/var/run/drwebd.sock
    av_scanner = drweb:192.168.2.20 31337

    If you omit the argument, the default path /usr/local/drweb/run/drwebd.sock
    is used. Thanks to Alex Miller for contributing the code for this scanner.

f-protd

    The f-protd scanner is accessed via HTTP over TCP. One argument is taken,
    being a space-separated hostname and port number (or port-range). For
    example:

    av_scanner = f-protd:localhost 10200-10204

    If you omit the argument, the default values shown above are used.

f-prot6d

    The f-prot6d scanner is accessed using the FPSCAND protocol over TCP. One
    argument is taken, being a space-separated hostname and port number. For
    example:

    av_scanner = f-prot6d:localhost 10200

    If you omit the argument, the default values show above are used.

fsecure

    The F-Secure daemon scanner (https://www.f-secure.com/) takes one argument
    which is the path to a UNIX socket. For example:

    av_scanner = fsecure:/path/to/.fsav

    If no argument is given, the default is /var/run/.fsav. Thanks to Johan
    Thelmen for contributing the code for this scanner.

kavdaemon

    This is the scanner daemon of Kaspersky Version 4. This version of the
    Kaspersky scanner is outdated. Please upgrade (see aveserver above). This
    scanner type takes one option, which is the path to the daemon's UNIX
    socket. For example:

    av_scanner = kavdaemon:/opt/AVP/AvpCtl

    The default path is /var/run/AvpCtl.

mksd

    This was a daemon type scanner that is aimed mainly at Polish users, though
    some documentation was available in English. The history can be shown at 
    https://en.wikipedia.org/wiki/Mks_vir and this appears to be a candidate
    for removal from Exim, unless we are informed of other virus scanners which
    use the same protocol to integrate. The only option for this scanner type
    is the maximum number of processes used simultaneously to scan the
    attachments, provided that mksd has been run with at least the same number
    of child processes. For example:

    av_scanner = mksd:2

    You can safely omit this option (the default value is 1).

sock

    This is a general-purpose way of talking to simple scanner daemons running
    on the local machine. There are four options: an address (which may be an
    IP address and port, or the path of a Unix socket), a commandline to send
    (may include a single %s which will be replaced with the path to the mail
    file to be scanned), an RE to trigger on from the returned data, and an RE
    to extract malware_name from the returned data. For example:

    av_scanner = sock:127.0.0.1 6001:%s:(SPAM|VIRUS):(.*)$

    Note that surrounding whitespace is stripped from each option, meaning
    there is no way to specify a trailing newline. The socket specifier and
    both regular-expressions are required. Default for the commandline is %s\n
    (note this does have a trailing newline); specify an empty element to get
    this.

sophie

    Sophie is a daemon that uses Sophos' libsavi library to scan for viruses.
    You can get Sophie at http://sophie.sourceforge.net/. The only option for
    this scanner type is the path to the UNIX socket that Sophie uses for
    client communication. For example:

    av_scanner = sophie:/tmp/sophie

    The default path is /var/run/sophie, so if you are using this, you can omit
    the option.

When av_scanner is correctly set, you can use the malware condition in the DATA
ACL. Note: You cannot use the malware condition in the MIME ACL.

The av_scanner option is expanded each time malware is called. This makes it
possible to use different scanners. See further below for an example. The 
malware condition caches its results, so when you use it multiple times for the
same message, the actual scanning process is only carried out once. However,
using expandable items in av_scanner disables this caching, in which case each
use of the malware condition causes a new scan of the message.

The malware condition takes a right-hand argument that is expanded before use
and taken as a list, slash-separated by default. The first element can then be
one of

  * "true", "*", or "1", in which case the message is scanned for viruses. The
    condition succeeds if a virus was found, and fail otherwise. This is the
    recommended usage.

  * "false" or "0" or an empty string, in which case no scanning is done and
    the condition fails immediately.

  * A regular expression, in which case the message is scanned for viruses. The
    condition succeeds if a virus is found and its name matches the regular
    expression. This allows you to take special actions on certain types of
    virus. Note that "/" characters in the RE must be doubled due to the
    list-processing, unless the separator is changed (in the usual way 6.21).

You can append a "defer_ok" element to the malware argument list to accept
messages even if there is a problem with the virus scanner. Otherwise, such a
problem causes the ACL to defer.

You can append a "tmo=<val>" element to the malware argument list to specify a
non-default timeout. The default is two minutes. For example:

malware = * / defer_ok / tmo=10s

A timeout causes the ACL to defer.

When a connection is made to the scanner the expansion variable
$callout_address is set to record the actual address used.

When a virus is found, the condition sets up an expansion variable called
$malware_name that contains the name of the virus. You can use it in a message
modifier that specifies the error returned to the sender, and/or in logging
data.

Beware the interaction of Exim's message_size_limit with any size limits
imposed by your anti-virus scanner.

Here is a very simple scanning example:

deny malware = *
     message = This message contains malware ($malware_name)

The next example accepts messages when there is a problem with the scanner:

deny malware = */defer_ok
     message = This message contains malware ($malware_name)

The next example shows how to use an ACL variable to scan with both sophie and
aveserver. It assumes you have set:

av_scanner = $acl_m0

in the main Exim configuration.

deny set acl_m0 = sophie
     malware = *
     message = This message contains malware ($malware_name)

deny set acl_m0 = aveserver
     malware = *
     message = This message contains malware ($malware_name)


45.2 Scanning with SpamAssassin and Rspamd
------------------------------------------

The spam ACL condition calls SpamAssassin's spamd daemon to get a spam score
and a report for the message. Support is also provided for Rspamd.

For more information about installation and configuration of SpamAssassin or
Rspamd refer to their respective websites at https://spamassassin.apache.org/
and https://www.rspamd.com/

SpamAssassin can be installed with CPAN by running:

perl -MCPAN -e 'install Mail::SpamAssassin'

SpamAssassin has its own set of configuration files. Please review its
documentation to see how you can tweak it. The default installation should work
nicely, however.

By default, SpamAssassin listens on 127.0.0.1, TCP port 783 and if you intend
to use an instance running on the local host you do not need to set 
spamd_address. If you intend to use another host or port for SpamAssassin, you
must set the spamd_address option in the global part of the Exim configuration
as follows (example):

spamd_address = 192.168.99.45 783

The SpamAssassin protocol relies on a TCP half-close from the client. If your
SpamAssassin client side is running a Linux system with an iptables firewall,
consider setting net.netfilter.nf_conntrack_tcp_timeout_close_wait to at least
the timeout, Exim uses when waiting for a response from the SpamAssassin server
(currently defaulting to 120s). With a lower value the Linux connection
tracking may consider your half-closed connection as dead too soon.

To use Rspamd (which by default listens on all local addresses on TCP port
11333) you should add variant=rspamd after the address/port pair, for example:

spamd_address = 127.0.0.1 11333 variant=rspamd

As of version 2.60, SpamAssassin also supports communication over UNIX sockets.
If you want to us these, supply spamd_address with an absolute filename instead
of an address/port pair:

spamd_address = /var/run/spamd_socket

You can have multiple spamd servers to improve scalability. These can reside on
other hardware reachable over the network. To specify multiple spamd servers,
put multiple address/port pairs in the spamd_address option, separated with
colons (the separator can be changed in the usual way 6.21):

spamd_address = 192.168.2.10 783 : \
                192.168.2.11 783 : \
                192.168.2.12 783

Up to 32 spamd servers are supported. When a server fails to respond to the
connection attempt, all other servers are tried until one succeeds. If no
server responds, the spam condition defers.

Unix and TCP socket specifications may be mixed in any order. Each element of
the list is a list itself, space-separated by default and changeable in the
usual way (6.21); take care to not double the separator.

For TCP socket specifications a host name or IP (v4 or v6, but subject to
list-separator quoting rules) address can be used, and the port can be one or a
dash-separated pair. In the latter case, the range is tried in strict order.

Elements after the first for Unix sockets, or second for TCP socket, are
options. The supported options are:

pri=<priority>      Selection priority
weight=<value>      Selection bias
time=<start>-<end>  Use only between these times of day
retry=<timespec>    Retry on connect fail
tmo=<timespec>      Connection time limit
variant=rspamd      Use Rspamd rather than SpamAssassin protocol

The "pri" option specifies a priority for the server within the list, higher
values being tried first. The default priority is 1.

The "weight" option specifies a selection bias. Within a priority set servers
are queried in a random fashion, weighted by this value. The default value for
selection bias is 1.

Time specifications for the "time" option are <hour>.<minute>.<second> in the
local time zone; each element being one or more digits. Either the seconds or
both minutes and seconds, plus the leading "." characters, may be omitted and
will be taken as zero.

Timeout specifications for the "retry" and "tmo" options are the usual Exim
time interval standard, e.g. "20s" or "1m".

The "tmo" option specifies an overall timeout for communication. The default
value is two minutes.

The "retry" option specifies a time after which a single retry for a failed
connect is made. The default is to not retry.

The spamd_address variable is expanded before use if it starts with a dollar
sign. In this case, the expansion may return a string that is used as the list
so that multiple spamd servers can be the result of an expansion.

When a connection is made to the server the expansion variable $callout_address
is set to record the actual address used.


45.3 Calling SpamAssassin from an Exim ACL
------------------------------------------

Here is a simple example of the use of the spam condition in a DATA ACL:

deny spam = joe
     message = This message was classified as SPAM

The right-hand side of the spam condition specifies a name. This is relevant if
you have set up multiple SpamAssassin profiles. If you do not want to scan
using a specific profile, but rather use the SpamAssassin system-wide default
profile, you can scan for an unknown name, or simply use "nobody". Rspamd does
not use this setting. However, you must put something on the right-hand side.

The name allows you to use per-domain or per-user antispam profiles in
principle, but this is not straightforward in practice, because a message may
have multiple recipients, not necessarily all in the same domain. Because the 
spam condition has to be called from a DATA-time ACL in order to be able to
read the contents of the message, the variables $local_part and $domain are not
set. Careful enforcement of single-recipient messages (e.g. by responding with
defer in the recipient ACL for all recipients after the first), or the use of
PRDR, are needed to use this feature.

The right-hand side of the spam condition is expanded before being used, so you
can put lookups or conditions there. When the right-hand side evaluates to "0"
or "false", no scanning is done and the condition fails immediately.

Scanning with SpamAssassin uses a lot of resources. If you scan every message,
large ones may cause significant performance degradation. As most spam messages
are quite small, it is recommended that you do not scan the big ones. For
example:

deny condition = ${if < {$message_size}{10K}}
     spam = nobody
     message = This message was classified as SPAM

The spam condition returns true if the threshold specified in the user's
SpamAssassin profile has been matched or exceeded. If you want to use the spam
condition for its side effects (see the variables below), you can make it
always return "true" by appending ":true" to the username.

When the spam condition is run, it sets up a number of expansion variables.
Except for $spam_report, these variables are saved with the received message so
are available for use at delivery time.

$spam_score

    The spam score of the message, for example, "3.4" or "30.5". This is useful
    for inclusion in log or reject messages.

$spam_score_int

    The spam score of the message, multiplied by ten, as an integer value. For
    example "34" or "305". It may appear to disagree with $spam_score because
    $spam_score is rounded and $spam_score_int is truncated. The integer value
    is useful for numeric comparisons in conditions.

$spam_bar

    A string consisting of a number of "+" or "-" characters, representing the
    integer part of the spam score value. A spam score of 4.4 would have a
    $spam_bar value of "++++". This is useful for inclusion in warning headers,
    since MUAs can match on such strings. The maximum length of the spam bar is
    50 characters.

$spam_report

    A multiline text table, containing the full SpamAssassin report for the
    message. Useful for inclusion in headers or reject messages. This variable
    is only usable in a DATA-time ACL. Beware that SpamAssassin may return
    non-ASCII characters, especially when running in country-specific locales,
    which are not legal unencoded in headers.

$spam_action

    For SpamAssassin either 'reject' or 'no action' depending on the spam score
    versus threshold. For Rspamd, the recommended action.

The spam condition caches its results unless expansion in spamd_address was
used. If you call it again with the same user name, it does not scan again, but
rather returns the same values as before.

The spam condition returns DEFER if there is any error while running the
message through SpamAssassin or if the expansion of spamd_address failed. If
you want to treat DEFER as FAIL (to pass on to the next ACL statement block),
append "/defer_ok" to the right-hand side of the spam condition, like this:

deny spam    = joe/defer_ok
     message = This message was classified as SPAM

This causes messages to be accepted even if there is a problem with spamd.

Here is a longer, commented example of the use of the spam condition:

# put headers in all messages (no matter if spam or not)
warn  spam = nobody:true
      add_header = X-Spam-Score: $spam_score ($spam_bar)
      add_header = X-Spam-Report: $spam_report

# add second subject line with *SPAM* marker when message
# is over threshold
warn  spam = nobody
      add_header = Subject: *SPAM* $h_Subject:

# reject spam at high scores (> 12)
deny  spam = nobody:true
      condition = ${if >{$spam_score_int}{120}{1}{0}}
      message = This message scored $spam_score spam points.


45.4 Scanning MIME parts
------------------------

The acl_smtp_mime global option specifies an ACL that is called once for each
MIME part of an SMTP message, including multipart types, in the sequence of
their position in the message. Similarly, the acl_not_smtp_mime option
specifies an ACL that is used for the MIME parts of non-SMTP messages. These
options may both refer to the same ACL if you want the same processing in both
cases.

These ACLs are called (possibly many times) just before the acl_smtp_data ACL
in the case of an SMTP message, or just before the acl_not_smtp ACL in the case
of a non-SMTP message. However, a MIME ACL is called only if the message
contains a Content-Type: header line. When a call to a MIME ACL does not yield
"accept", ACL processing is aborted and the appropriate result code is sent to
the client. In the case of an SMTP message, the acl_smtp_data ACL is not called
when this happens.

You cannot use the malware or spam conditions in a MIME ACL; these can only be
used in the DATA or non-SMTP ACLs. However, you can use the regex condition to
match against the raw MIME part. You can also use the mime_regex condition to
match against the decoded MIME part (see section 45.5).

At the start of a MIME ACL, a number of variables are set from the header
information for the relevant MIME part. These are described below. The contents
of the MIME part are not by default decoded into a disk file except for MIME
parts whose content-type is "message/rfc822". If you want to decode a MIME part
into a disk file, you can use the decode condition. The general syntax is:

decode = [/<path>/]<filename>

The right hand side is expanded before use. After expansion, the value can be:

 1. "0" or "false", in which case no decoding is done.

 2. The string "default". In that case, the file is put in the temporary
    "default" directory <spool_directory>/scan/<message_id>/ with a sequential
    filename consisting of the message id and a sequence number. The full path
    and name is available in $mime_decoded_filename after decoding.

 3. A full path name starting with a slash. If the full name is an existing
    directory, it is used as a replacement for the default directory. The
    filename is then sequentially assigned. If the path does not exist, it is
    used as the full path and filename.

 4. If the string does not start with a slash, it is used as the filename, and
    the default path is then used.

The decode condition normally succeeds. It is only false for syntax errors or
unusual circumstances such as memory shortages. You can easily decode a file
with its original, proposed filename using

decode = $mime_filename

However, you should keep in mind that $mime_filename might contain anything. If
you place files outside of the default path, they are not automatically
unlinked.

For RFC822 attachments (these are messages attached to messages, with a
content-type of "message/rfc822"), the ACL is called again in the same manner
as for the primary message, only that the $mime_is_rfc822 expansion variable is
set (see below). Attached messages are always decoded to disk before being
checked, and the files are unlinked once the check is done.

The MIME ACL supports the regex and mime_regex conditions. These can be used to
match regular expressions against raw and decoded MIME parts, respectively.
They are described in section 45.5.

The following list describes all expansion variables that are available in the
MIME ACL:

$mime_anomaly_level, $mime_anomaly_text

    If there are problems decoding, these variables contain information on the
    detected issue.

$mime_boundary

    If the current part is a multipart (see $mime_is_multipart below), it
    should have a boundary string, which is stored in this variable. If the
    current part has no boundary parameter in the Content-Type: header, this
    variable contains the empty string.

$mime_charset

    This variable contains the character set identifier, if one was found in
    the Content-Type: header. Examples for charset identifiers are:

    us-ascii
    gb2312 (Chinese)
    iso-8859-1

    Please note that this value is not normalized, so you should do matches
    case-insensitively.

$mime_content_description

    This variable contains the normalized content of the Content-Description:
    header. It can contain a human-readable description of the parts content.
    Some implementations repeat the filename for attachments here, but they are
    usually only used for display purposes.

$mime_content_disposition

    This variable contains the normalized content of the Content-Disposition:
    header. You can expect strings like "attachment" or "inline" here.

$mime_content_id

    This variable contains the normalized content of the Content-ID: header.
    This is a unique ID that can be used to reference a part from another part.

$mime_content_size

    This variable is set only after the decode modifier (see above) has been
    successfully run. It contains the size of the decoded part in kilobytes.
    The size is always rounded up to full kilobytes, so only a completely empty
    part has a $mime_content_size of zero.

$mime_content_transfer_encoding

    This variable contains the normalized content of the 
    Content-transfer-encoding: header. This is a symbolic name for an encoding
    type. Typical values are "base64" and "quoted-printable".

$mime_content_type

    If the MIME part has a Content-Type: header, this variable contains its
    value, lowercased, and without any options (like "name" or "charset"). Here
    are some examples of popular MIME types, as they may appear in this
    variable:

    text/plain
    text/html
    application/octet-stream
    image/jpeg
    audio/midi

    If the MIME part has no Content-Type: header, this variable contains the
    empty string.

$mime_decoded_filename

    This variable is set only after the decode modifier (see above) has been
    successfully run. It contains the full path and filename of the file
    containing the decoded data.

$mime_filename

    This is perhaps the most important of the MIME variables. It contains a
    proposed filename for an attachment, if one was found in either the 
    Content-Type: or Content-Disposition: headers. The filename will be RFC2047
    or RFC2231 decoded, but no additional sanity checks are done. If no
    filename was found, this variable contains the empty string.

$mime_is_coverletter

    This variable attempts to differentiate the "cover letter" of an e-mail
    from attached data. It can be used to clamp down on flashy or unnecessarily
    encoded content in the cover letter, while not restricting attachments at
    all.

    The variable contains 1 (true) for a MIME part believed to be part of the
    cover letter, and 0 (false) for an attachment. At present, the algorithm is
    as follows:

     1. The outermost MIME part of a message is always a cover letter.

     2. If a multipart/alternative or multipart/related MIME part is a cover
        letter, so are all MIME subparts within that multipart.

     3. If any other multipart is a cover letter, the first subpart is a cover
        letter, and the rest are attachments.

     4. All parts contained within an attachment multipart are attachments.

    As an example, the following will ban "HTML mail" (including that sent with
    alternative plain text), while allowing HTML files to be attached. HTML
    coverletter mail attached to non-HTML coverletter mail will also be
    allowed:

    deny !condition = $mime_is_rfc822
         condition = $mime_is_coverletter
         condition = ${if eq{$mime_content_type}{text/html}{1}{0}}
         message = HTML mail is not accepted here

$mime_is_multipart

    This variable has the value 1 (true) when the current part has the main
    type "multipart", for example, "multipart/alternative" or "multipart/
    mixed". Since multipart entities only serve as containers for other parts,
    you may not want to carry out specific actions on them.

$mime_is_rfc822

    This variable has the value 1 (true) if the current part is not a part of
    the checked message itself, but part of an attached message. Attached
    message decoding is fully recursive.

$mime_part_count

    This variable is a counter that is raised for each processed MIME part. It
    starts at zero for the very first part (which is usually a multipart). The
    counter is per-message, so it is reset when processing RFC822 attachments
    (see $mime_is_rfc822). The counter stays set after acl_smtp_mime is
    complete, so you can use it in the DATA ACL to determine the number of MIME
    parts of a message. For non-MIME messages, this variable contains the value
    -1.


45.5 Scanning with regular expressions
--------------------------------------

You can specify your own custom regular expression matches on the full body of
the message, or on individual MIME parts.

The regex condition takes one or more regular expressions as arguments and
matches them against the full message (when called in the DATA ACL) or a raw
MIME part (when called in the MIME ACL). The regex condition matches linewise,
with a maximum line length of 32K characters. That means you cannot have
multiline matches with the regex condition.

The mime_regex condition can be called only in the MIME ACL. It matches up to
32K of decoded content (the whole content at once, not linewise). If the part
has not been decoded with the decode modifier earlier in the ACL, it is decoded
automatically when mime_regex is executed (using default path and filename
values). If the decoded data is larger than 32K, only the first 32K characters
are checked.

The regular expressions are passed as a colon-separated list. To include a
literal colon, you must double it. Since the whole right-hand side string is
expanded before being used, you must also escape dollar signs and backslashes
with more backslashes, or use the "\N" facility to disable expansion. Here is a
simple example that contains two regular expressions:

deny regex = [Mm]ortgage : URGENT BUSINESS PROPOSAL
     message = contains blacklisted regex ($regex_match_string)

The conditions returns true if any one of the regular expressions matches. The
$regex_match_string expansion variable is then set up and contains the matching
regular expression. The expansion variables $regex1 $regex2 etc are set to any
substrings captured by the regular expression.

Warning: With large messages, these conditions can be fairly CPU-intensive.



===============================================================================
46. ADDING A LOCAL SCAN FUNCTION TO EXIM

In these days of email worms, viruses, and ever-increasing spam, some sites
want to apply a lot of checking to messages before accepting them.

The content scanning extension (chapter 45) has facilities for passing messages
to external virus and spam scanning software. You can also do a certain amount
in Exim itself through string expansions and the condition condition in the ACL
that runs after the SMTP DATA command or the ACL for non-SMTP messages (see
chapter 44), but this has its limitations.

To allow for further customization to a site's own requirements, there is the
possibility of linking Exim with a private message scanning function, written
in C. If you want to run code that is written in something other than C, you
can of course use a little C stub to call it.

The local scan function is run once for every incoming message, at the point
when Exim is just about to accept the message. It can therefore be used to
control non-SMTP messages from local processes as well as messages arriving via
SMTP.

Exim applies a timeout to calls of the local scan function, and there is an
option called local_scan_timeout for setting it. The default is 5 minutes. Zero
means "no timeout". Exim also sets up signal handlers for SIGSEGV, SIGILL,
SIGFPE, and SIGBUS before calling the local scan function, so that the most
common types of crash are caught. If the timeout is exceeded or one of those
signals is caught, the incoming message is rejected with a temporary error if
it is an SMTP message. For a non-SMTP message, the message is dropped and Exim
ends with a non-zero code. The incident is logged on the main and reject logs.


46.1 Building Exim to use a local scan function
-----------------------------------------------

To make use of the local scan function feature, you must tell Exim where your
function is before building Exim, by setting both HAVE_LOCAL_SCAN and
LOCAL_SCAN_SOURCE in your Local/Makefile. A recommended place to put it is in
the Local directory, so you might set

HAVE_LOCAL_SCAN=yes
LOCAL_SCAN_SOURCE=Local/local_scan.c

for example. The function must be called local_scan(); the source file(s) for
it should first #define LOCAL_SCAN and then #include "local_scan.h". It is
called by Exim after it has received a message, when the success return code is
about to be sent. This is after all the ACLs have been run. The return code
from your function controls whether the message is actually accepted or not.
There is a commented template function (that just accepts the message) in the
file _src/local_scan.c_.

If you want to make use of Exim's runtime configuration file to set options for
your local_scan() function, you must also set

LOCAL_SCAN_HAS_OPTIONS=yes

in Local/Makefile (see section 46.3 below).


46.2 API for local_scan()
-------------------------

You must include this line near the start of your code:

#define LOCAL_SCAN
#include "local_scan.h"

This header file defines a number of variables and other values, and the
prototype for the function itself. Exim is coded to use unsigned char values
almost exclusively, and one of the things this header defines is a shorthand
for "unsigned char" called "uschar". It also makes available the following
macro definitions, to simplify casting character strings and pointers to
character strings:

#define CS   (char *)
#define CCS  (const char *)
#define CSS  (char **)
#define US   (unsigned char *)
#define CUS  (const unsigned char *)
#define USS  (unsigned char **)

The function prototype for local_scan() is:

extern int local_scan(int fd, uschar **return_text);

The arguments are as follows:

  * fd is a file descriptor for the file that contains the body of the message
    (the -D file). The file is open for reading and writing, but updating it is
    not recommended. Warning: You must not close this file descriptor.

    The descriptor is positioned at character 19 of the file, which is the
    first character of the body itself, because the first 19 characters are the
    message id followed by "-D" and a newline. If you rewind the file, you
    should use the macro SPOOL_DATA_START_OFFSET to reset to the start of the
    data, just in case this changes in some future version.

  * return_text is an address which you can use to return a pointer to a text
    string at the end of the function. The value it points to on entry is NULL.

The function must return an int value which is one of the following macros:

"LOCAL_SCAN_ACCEPT"

    The message is accepted. If you pass back a string of text, it is saved
    with the message, and made available in the variable $local_scan_data. No
    newlines are permitted (if there are any, they are turned into spaces) and
    the maximum length of text is 1000 characters.

"LOCAL_SCAN_ACCEPT_FREEZE"

    This behaves as LOCAL_SCAN_ACCEPT, except that the accepted message is
    queued without immediate delivery, and is frozen.

"LOCAL_SCAN_ACCEPT_QUEUE"

    This behaves as LOCAL_SCAN_ACCEPT, except that the accepted message is
    queued without immediate delivery.

"LOCAL_SCAN_REJECT"

    The message is rejected; the returned text is used as an error message
    which is passed back to the sender and which is also logged. Newlines are
    permitted - they cause a multiline response for SMTP rejections, but are
    converted to "\n" in log lines. If no message is given, "Administrative
    prohibition" is used.

"LOCAL_SCAN_TEMPREJECT"

    The message is temporarily rejected; the returned text is used as an error
    message as for LOCAL_SCAN_REJECT. If no message is given, "Temporary local
    problem" is used.

"LOCAL_SCAN_REJECT_NOLOGHDR"

    This behaves as LOCAL_SCAN_REJECT, except that the header of the rejected
    message is not written to the reject log. It has the effect of unsetting
    the rejected_header log selector for just this rejection. If 
    rejected_header is already unset (see the discussion of the log_selection
    option in section 53.15), this code is the same as LOCAL_SCAN_REJECT.

"LOCAL_SCAN_TEMPREJECT_NOLOGHDR"

    This code is a variation of LOCAL_SCAN_TEMPREJECT in the same way that
    LOCAL_SCAN_REJECT_NOLOGHDR is a variation of LOCAL_SCAN_REJECT.

If the message is not being received by interactive SMTP, rejections are
reported by writing to stderr or by sending an email, as configured by the -oe
command line options.


46.3 Configuration options for local_scan()
-------------------------------------------

It is possible to have option settings in the main configuration file that set
values in static variables in the local_scan() module. If you want to do this,
you must have the line

LOCAL_SCAN_HAS_OPTIONS=yes

in your Local/Makefile when you build Exim. (This line is in OS/
Makefile-Default, commented out). Then, in the local_scan() source file, you
must define static variables to hold the option values, and a table to define
them.

The table must be a vector called local_scan_options, of type "optionlist".
Each entry is a triplet, consisting of a name, an option type, and a pointer to
the variable that holds the value. The entries must appear in alphabetical
order. Following local_scan_options you must also define a variable called 
local_scan_options_count that contains the number of entries in the table. Here
is a short example, showing two kinds of option:

static int my_integer_option = 42;
static uschar *my_string_option = US"a default string";

optionlist local_scan_options[] = {
  { "my_integer", opt_int,       &my_integer_option },
  { "my_string",  opt_stringptr, &my_string_option }
};

int local_scan_options_count =
  sizeof(local_scan_options)/sizeof(optionlist);

The values of the variables can now be changed from Exim's runtime
configuration file by including a local scan section as in this example:

begin local_scan
my_integer = 99
my_string = some string of text...

The available types of option data are as follows:

opt_bool

    This specifies a boolean (true/false) option. The address should point to a
    variable of type "BOOL", which will be set to TRUE or FALSE, which are
    macros that are defined as "1" and "0", respectively. If you want to detect
    whether such a variable has been set at all, you can initialize it to
    TRUE_UNSET. (BOOL variables are integers underneath, so can hold more than
    two values.)

opt_fixed

    This specifies a fixed point number, such as is used for load averages. The
    address should point to a variable of type "int". The value is stored
    multiplied by 1000, so, for example, 1.4142 is truncated and stored as
    1414.

opt_int

    This specifies an integer; the address should point to a variable of type
    "int". The value may be specified in any of the integer formats accepted by
    Exim.

opt_mkint

    This is the same as opt_int, except that when such a value is output in a 
    -bP listing, if it is an exact number of kilobytes or megabytes, it is
    printed with the suffix K or M.

opt_octint

    This also specifies an integer, but the value is always interpreted as an
    octal integer, whether or not it starts with the digit zero, and it is
    always output in octal.

opt_stringptr

    This specifies a string value; the address must be a pointer to a variable
    that points to a string (for example, of type "uschar *").

opt_time

    This specifies a time interval value. The address must point to a variable
    of type "int". The value that is placed there is a number of seconds.

If the -bP command line option is followed by "local_scan", Exim prints out the
values of all the local_scan() options.


46.4 Available Exim variables
-----------------------------

The header local_scan.h gives you access to a number of C variables. These are
the only ones that are guaranteed to be maintained from release to release.
Note, however, that you can obtain the value of any Exim expansion variable,
including $recipients, by calling expand_string(). The exported C variables are
as follows:

int body_linecount

    This variable contains the number of lines in the message's body. It is not
    valid if the spool_wireformat option is used.

int body_zerocount

    This variable contains the number of binary zero bytes in the message's
    body. It is not valid if the spool_wireformat option is used.

unsigned int debug_selector

    This variable is set to zero when no debugging is taking place. Otherwise,
    it is a bitmap of debugging selectors. Two bits are identified for use in
    local_scan(); they are defined as macros:

      o The "D_v" bit is set when -v was present on the command line. This is a
        testing option that is not privileged - any caller may set it. All the
        other selector bits can be set only by admin users.

      o The "D_local_scan" bit is provided for use by local_scan(); it is set
        by the "+local_scan" debug selector. It is not included in the default
        set of debugging bits.

    Thus, to write to the debugging output only when "+local_scan" has been
    selected, you should use code like this:

    if ((debug_selector & D_local_scan) != 0)
      debug_printf("xxx", ...);

uschar *expand_string_message

    After a failing call to expand_string() (returned value NULL), the variable
    expand_string_message contains the error message, zero-terminated.

header_line *header_list

    A pointer to a chain of header lines. The header_line structure is
    discussed below.

header_line *header_last

    A pointer to the last of the header lines.

const uschar *headers_charset

    The value of the headers_charset configuration option.

BOOL host_checking

    This variable is TRUE during a host checking session that is initiated by
    the -bh command line option.

uschar *interface_address

    The IP address of the interface that received the message, as a string.
    This is NULL for locally submitted messages.

int interface_port

    The port on which this message was received. When testing with the -bh
    command line option, the value of this variable is -1 unless a port has
    been specified via the -oMi option.

uschar *message_id

    This variable contains Exim's message id for the incoming message (the
    value of $message_exim_id) as a zero-terminated string.

uschar *received_protocol

    The name of the protocol by which the message was received.

int recipients_count

    The number of accepted recipients.

recipient_item *recipients_list

    The list of accepted recipients, held in a vector of length 
    recipients_count. The recipient_item structure is discussed below. You can
    add additional recipients by calling receive_add_recipient() (see below).
    You can delete recipients by removing them from the vector and adjusting
    the value in recipients_count. In particular, by setting recipients_count
    to zero you remove all recipients. If you then return the value
    "LOCAL_SCAN_ACCEPT", the message is accepted, but immediately blackholed.
    To replace the recipients, you can set recipients_count to zero and then
    call receive_add_recipient() as often as needed.

uschar *sender_address

    The envelope sender address. For bounce messages this is the empty string.

uschar *sender_host_address

    The IP address of the sending host, as a string. This is NULL for
    locally-submitted messages.

uschar *sender_host_authenticated

    The name of the authentication mechanism that was used, or NULL if the
    message was not received over an authenticated SMTP connection.

uschar *sender_host_name

    The name of the sending host, if known.

int sender_host_port

    The port on the sending host.

BOOL smtp_input

    This variable is TRUE for all SMTP input, including BSMTP.

BOOL smtp_batched_input

    This variable is TRUE for BSMTP input.

int store_pool

    The contents of this variable control which pool of memory is used for new
    requests. See section 46.8 for details.


46.5 Structure of header lines
------------------------------

The header_line structure contains the members listed below. You can add
additional header lines by calling the header_add() function (see below). You
can cause header lines to be ignored (deleted) by setting their type to *.

struct header_line *next

    A pointer to the next header line, or NULL for the last line.

int type

    A code identifying certain headers that Exim recognizes. The codes are
    printing characters, and are documented in chapter 57 of this manual.
    Notice in particular that any header line whose type is * is not
    transmitted with the message. This flagging is used for header lines that
    have been rewritten, or are to be removed (for example, Envelope-sender:
    header lines.) Effectively, * means "deleted".

int slen

    The number of characters in the header line, including the terminating and
    any internal newlines.

uschar *text

    A pointer to the text of the header. It always ends with a newline,
    followed by a zero byte. Internal newlines are preserved.


46.6 Structure of recipient items
---------------------------------

The recipient_item structure contains these members:

uschar *address

    This is a pointer to the recipient address as it was received.

int pno

    This is used in later Exim processing when top level addresses are created
    by the one_time option. It is not relevant at the time local_scan() is run
    and must always contain -1 at this stage.

uschar *errors_to

    If this value is not NULL, bounce messages caused by failing to deliver to
    the recipient are sent to the address it contains. In other words, it
    overrides the envelope sender for this one recipient. (Compare the 
    errors_to generic router option.) If a local_scan() function sets an 
    errors_to field to an unqualified address, Exim qualifies it using the
    domain from qualify_recipient. When local_scan() is called, the errors_to
    field is NULL for all recipients.


46.7 Available Exim functions
-----------------------------

The header local_scan.h gives you access to a number of Exim functions. These
are the only ones that are guaranteed to be maintained from release to release:

pid_t child_open
(uschar **argv, uschar **envp, int newumask, int *infdptr, int *outfdptr,
  BOOL make_leader)

    This function creates a child process that runs the command specified by 
    argv. The environment for the process is specified by envp, which can be
    NULL if no environment variables are to be passed. A new umask is supplied
    for the process in newumask.

    Pipes to the standard input and output of the new process are set up and
    returned to the caller via the infdptr and outfdptr arguments. The standard
    error is cloned to the standard output. If there are any file descriptors
    "in the way" in the new process, they are closed. If the final argument is
    TRUE, the new process is made into a process group leader.

    The function returns the pid of the new process, or -1 if things go wrong.

int child_close(pid_t pid, int timeout)

    This function waits for a child process to terminate, or for a timeout (in
    seconds) to expire. A timeout value of zero means wait as long as it takes.
    The return value is as follows:

      o >= 0

        The process terminated by a normal exit and the value is the process
        ending status.

      o < 0 and > -256

        The process was terminated by a signal and the value is the negation of
        the signal number.

      o -256

        The process timed out.

      o -257

        The was some other error in wait(); errno is still set.

pid_t child_open_exim(int *fd)

    This function provide you with a means of submitting a new message to Exim.
    (Of course, you can also call /usr/sbin/sendmail yourself if you want, but
    this packages it all up for you.) The function creates a pipe, forks a
    subprocess that is running

    exim -t -oem -oi -f <>

    and returns to you (via the "int *" argument) a file descriptor for the
    pipe that is connected to the standard input. The yield of the function is
    the PID of the subprocess. You can then write a message to the file
    descriptor, with recipients in To:, Cc:, and/or Bcc: header lines.

    When you have finished, call child_close() to wait for the process to
    finish and to collect its ending status. A timeout value of zero is usually
    fine in this circumstance. Unless you have made a mistake with the
    recipient addresses, you should get a return code of zero.

pid_t child_open_exim2(int *fd, uschar *sender, uschar *sender_authentication)

    This function is a more sophisticated version of child_open(). The command
    that it runs is:

    exim -t -oem -oi -f sender -oMas sender_authentication

    The third argument may be NULL, in which case the -oMas option is omitted.

void debug_printf(char *, ...)

    This is Exim's debugging function, with arguments as for (printf(). The
    output is written to the standard error stream. If no debugging is
    selected, calls to debug_printf() have no effect. Normally, you should make
    calls conditional on the "local_scan" debug selector by coding like this:

    if ((debug_selector & D_local_scan) != 0)
      debug_printf("xxx", ...);

uschar *expand_string(uschar *string)

    This is an interface to Exim's string expansion code. The return value is
    the expanded string, or NULL if there was an expansion failure. The C
    variable expand_string_message contains an error message after an expansion
    failure. If expansion does not change the string, the return value is the
    pointer to the input string. Otherwise, the return value points to a new
    block of memory that was obtained by a call to store_get(). See section
    46.8 below for a discussion of memory handling.

void header_add(int type, char *format, ...)

    This function allows you to an add additional header line at the end of the
    existing ones. The first argument is the type, and should normally be a
    space character. The second argument is a format string and any number of
    substitution arguments as for sprintf(). You may include internal newlines
    if you want, and you must ensure that the string ends with a newline.

void header_add_at_position
(BOOL after, uschar *name, BOOL topnot, int type, char *format,   ...)

    This function adds a new header line at a specified point in the header
    chain. The header itself is specified as for header_add().

    If name is NULL, the new header is added at the end of the chain if after
    is true, or at the start if after is false. If name is not NULL, the header
    lines are searched for the first non-deleted header that matches the name.
    If one is found, the new header is added before it if after is false. If 
    after is true, the new header is added after the found header and any
    adjacent subsequent ones with the same name (even if marked "deleted"). If
    no matching non-deleted header is found, the topnot option controls where
    the header is added. If it is true, addition is at the top; otherwise at
    the bottom. Thus, to add a header after all the Received: headers, or at
    the top if there are no Received: headers, you could use

    header_add_at_position(TRUE, US"Received", TRUE,
      ' ', "X-xxx: ...");

    Normally, there is always at least one non-deleted Received: header, but
    there may not be if received_header_text expands to an empty string.

void header_remove(int occurrence, uschar *name)

    This function removes header lines. If occurrence is zero or negative, all
    occurrences of the header are removed. If occurrence is greater than zero,
    that particular instance of the header is removed. If no header(s) can be
    found that match the specification, the function does nothing.

BOOL header_testname(header_line *hdr, uschar *name, int length, BOOL notdel)

    This function tests whether the given header has the given name. It is not
    just a string comparison, because white space is permitted between the name
    and the colon. If the notdel argument is true, a false return is forced for
    all "deleted" headers; otherwise they are not treated specially. For
    example:

    if (header_testname(h, US"X-Spam", 6, TRUE)) ...

uschar *lss_b64encode(uschar *cleartext, int length)

    This function base64-encodes a string, which is passed by address and
    length. The text may contain bytes of any value, including zero. The result
    is passed back in dynamic memory that is obtained by calling store_get().
    It is zero-terminated.

int lss_b64decode(uschar *codetext, uschar **cleartext)

    This function decodes a base64-encoded string. Its arguments are a
    zero-terminated base64-encoded string and the address of a variable that is
    set to point to the result, which is in dynamic memory. The length of the
    decoded string is the yield of the function. If the input is invalid base64
    data, the yield is -1. A zero byte is added to the end of the output string
    to make it easy to interpret as a C string (assuming it contains no zeros
    of its own). The added zero byte is not included in the returned count.

int lss_match_domain(uschar *domain, uschar *list)

    This function checks for a match in a domain list. Domains are always
    matched caselessly. The return value is one of the following:

        "OK"    match succeeded
        "FAIL"  match failed
        "DEFER" match deferred

    DEFER is usually caused by some kind of lookup defer, such as the inability
    to contact a database.

int lss_match_local_part(uschar *localpart, uschar *list, BOOL caseless)

    This function checks for a match in a local part list. The third argument
    controls case-sensitivity. The return values are as for lss_match_domain().

int lss_match_address(uschar *address, uschar *list, BOOL caseless)

    This function checks for a match in an address list. The third argument
    controls the case-sensitivity of the local part match. The domain is always
    matched caselessly. The return values are as for lss_match_domain().

int lss_match_host(uschar *host_name, uschar *host_address, uschar *list)

    This function checks for a match in a host list. The most common usage is
    expected to be

    lss_match_host(sender_host_name, sender_host_address, ...)

    An empty address field matches an empty item in the host list. If the host
    name is NULL, the name corresponding to $sender_host_address is
    automatically looked up if a host name is required to match an item in the
    list. The return values are as for lss_match_domain(), but in addition, 
    lss_match_host() returns ERROR in the case when it had to look up a host
    name, but the lookup failed.

void log_write(unsigned int selector, int which, char *format, ...)

    This function writes to Exim's log files. The first argument should be zero
    (it is concerned with log_selector). The second argument can be "LOG_MAIN"
    or "LOG_REJECT" or "LOG_PANIC" or the inclusive "or" of any combination of
    them. It specifies to which log or logs the message is written. The
    remaining arguments are a format and relevant insertion arguments. The
    string should not contain any newlines, not even at the end.

void receive_add_recipient(uschar *address, int pno)

    This function adds an additional recipient to the message. The first
    argument is the recipient address. If it is unqualified (has no domain), it
    is qualified with the qualify_recipient domain. The second argument must
    always be -1.

    This function does not allow you to specify a private errors_to address (as
    described with the structure of recipient_item above), because it pre-dates
    the addition of that field to the structure. However, it is easy to add
    such a value afterwards. For example:

     receive_add_recipient(US"monitor@mydom.example", -1);
     recipients_list[recipients_count-1].errors_to =
       US"postmaster@mydom.example";

BOOL receive_remove_recipient(uschar *recipient)

    This is a convenience function to remove a named recipient from the list of
    recipients. It returns true if a recipient was removed, and false if no
    matching recipient could be found. The argument must be a complete email
    address.

uschar rfc2047_decode
(uschar *string, BOOL lencheck, uschar *target, int zeroval, int *lenptr,
  uschar **error)

    This function decodes strings that are encoded according to RFC 2047.
    Typically these are the contents of header lines. First, each "encoded
    word" is decoded from the Q or B encoding into a byte-string. Then, if
    provided with the name of a charset encoding, and if the iconv() function
    is available, an attempt is made to translate the result to the named
    character set. If this fails, the binary string is returned with an error
    message.

    The first argument is the string to be decoded. If lencheck is TRUE, the
    maximum MIME word length is enforced. The third argument is the target
    encoding, or NULL if no translation is wanted.

    If a binary zero is encountered in the decoded string, it is replaced by
    the contents of the zeroval argument. For use with Exim headers, the value
    must not be 0 because header lines are handled as zero-terminated strings.

    The function returns the result of processing the string, zero-terminated;
    if lenptr is not NULL, the length of the result is set in the variable to
    which it points. When zeroval is 0, lenptr should not be NULL.

    If an error is encountered, the function returns NULL and uses the error
    argument to return an error message. The variable pointed to by error is
    set to NULL if there is no error; it may be set non-NULL even when the
    function returns a non-NULL value if decoding was successful, but there was
    a problem with translation.

int smtp_fflush(void)

    This function is used in conjunction with smtp_printf(), as described
    below.

void smtp_printf(char *,BOOL, ...)

    The arguments of this function are almost like printf(); it writes to the
    SMTP output stream. You should use this function only when there is an SMTP
    output stream, that is, when the incoming message is being received via
    interactive SMTP. This is the case when smtp_input is TRUE and 
    smtp_batched_input is FALSE. If you want to test for an incoming message
    from another host (as opposed to a local process that used the -bs command
    line option), you can test the value of sender_host_address, which is
    non-NULL when a remote host is involved.

    If an SMTP TLS connection is established, smtp_printf() uses the TLS output
    function, so it can be used for all forms of SMTP connection.

    The second argument is used to request that the data be buffered (when
    TRUE) or flushed (along with any previously buffered, when FALSE). This is
    advisory only, but likely to save on system-calls and packets sent when a
    sequence of calls to the function are made.

    The argument was added in Exim version 4.90 - changing the API/ABI. Nobody
    noticed until 4.93 was imminent, at which point the ABI version number was
    incremented.

    Strings that are written by smtp_printf() from within local_scan() must
    start with an appropriate response code: 550 if you are going to return
    LOCAL_SCAN_REJECT, 451 if you are going to return LOCAL_SCAN_TEMPREJECT,
    and 250 otherwise. Because you are writing the initial lines of a
    multi-line response, the code must be followed by a hyphen to indicate that
    the line is not the final response line. You must also ensure that the
    lines you write terminate with CRLF. For example:

    smtp_printf("550-this is some extra info\r\n");
    return LOCAL_SCAN_REJECT;

    Note that you can also create multi-line responses by including newlines in
    the data returned via the return_text argument. The added value of using 
    smtp_printf() is that, for instance, you could introduce delays between
    multiple output lines.

    The smtp_printf() function does not return any error indication, because it
    does not guarantee a flush of pending output, and therefore does not test
    the state of the stream. (In the main code of Exim, flushing and error
    detection is done when Exim is ready for the next SMTP input command.) If
    you want to flush the output and check for an error (for example, the
    dropping of a TCP/IP connection), you can call smtp_fflush(), which has no
    arguments. It flushes the output stream, and returns a non-zero value if
    there is an error.

void *store_get(int,BOOL)

    This function accesses Exim's internal store (memory) manager. It gets a
    new chunk of memory whose size is given by the first argument. The second
    argument should be given as TRUE if the memory will be used for data
    possibly coming from an attacker (eg. the message content), FALSE if it is
    locally-sourced. Exim bombs out if it ever runs out of memory. See the next
    section for a discussion of memory handling.

void *store_get_perm(int,BOOL)

    This function is like store_get(), but it always gets memory from the
    permanent pool. See the next section for a discussion of memory handling.

uschar *string_copy(uschar *string)

    See below.

uschar *string_copyn(uschar *string, int length)

    See below.

uschar *string_sprintf(char *format, ...)

    These three functions create strings using Exim's dynamic memory
    facilities. The first makes a copy of an entire string. The second copies
    up to a maximum number of characters, indicated by the second argument. The
    third uses a format and insertion arguments to create a new string. In each
    case, the result is a pointer to a new string in the current memory pool.
    See the next section for more discussion.


46.8 More about Exim's memory handling
--------------------------------------

No function is provided for freeing memory, because that is never needed. The
dynamic memory that Exim uses when receiving a message is automatically
recycled if another message is received by the same process (this applies only
to incoming SMTP connections - other input methods can supply only one message
at a time). After receiving the last message, a reception process terminates.

Because it is recycled, the normal dynamic memory cannot be used for holding
data that must be preserved over a number of incoming messages on the same SMTP
connection. However, Exim in fact uses two pools of dynamic memory; the second
one is not recycled, and can be used for this purpose.

If you want to allocate memory that remains available for subsequent messages
in the same SMTP connection, you should set

store_pool = POOL_PERM

before calling the function that does the allocation. There is no need to
restore the value if you do not need to; however, if you do want to revert to
the normal pool, you can either restore the previous value of store_pool or set
it explicitly to POOL_MAIN.

The pool setting applies to all functions that get dynamic memory, including 
expand_string(), store_get(), and the string_xxx() functions. There is also a
convenience function called store_get_perm() that gets a block of memory from
the permanent pool while preserving the value of store_pool.



===============================================================================
47. SYSTEM-WIDE MESSAGE FILTERING

The previous chapters (on ACLs and the local scan function) describe checks
that can be applied to messages before they are accepted by a host. There is
also a mechanism for checking messages once they have been received, but before
they are delivered. This is called the system filter.

The system filter operates in a similar manner to users' filter files, but it
is run just once per message (however many recipients the message has). It
should not normally be used as a substitute for routing, because deliver
commands in a system router provide new envelope recipient addresses. The
system filter must be an Exim filter. It cannot be a Sieve filter.

The system filter is run at the start of a delivery attempt, before any routing
is done. If a message fails to be completely delivered at the first attempt,
the system filter is run again at the start of every retry. If you want your
filter to do something only once per message, you can make use of the 
first_delivery condition in an if command in the filter to prevent it happening
on retries.

Warning: Because the system filter runs just once, variables that are specific
to individual recipient addresses, such as $local_part and $domain, are not
set, and the "personal" condition is not meaningful. If you want to run a
centrally-specified filter for each recipient address independently, you can do
so by setting up a suitable redirect router, as described in section 47.8
below.


47.1 Specifying a system filter
-------------------------------

The name of the file that contains the system filter must be specified by
setting system_filter. If you want the filter to run under a uid and gid other
than root, you must also set system_filter_user and system_filter_group as
appropriate. For example:

system_filter = /etc/mail/exim.filter
system_filter_user = exim

If a system filter generates any deliveries directly to files or pipes (via the
save or pipe commands), transports to handle these deliveries must be specified
by setting system_filter_file_transport and system_filter_pipe_transport,
respectively. Similarly, system_filter_reply_transport must be set to handle
any messages generated by the reply command.


47.2 Testing a system filter
----------------------------

You can run simple tests of a system filter in the same way as for a user
filter, but you should use -bF rather than -bf, so that features that are
permitted only in system filters are recognized.

If you want to test the combined effect of a system filter and a user filter,
you can use both -bF and -bf on the same command line.


47.3 Contents of a system filter
--------------------------------

The language used to specify system filters is the same as for users' filter
files. It is described in the separate end-user document Exim's interface to
mail filtering. However, there are some additional features that are available
only in system filters; these are described in subsequent sections. If they are
encountered in a user's filter file or when testing with -bf, they cause
errors.

There are two special conditions which, though available in users' filter
files, are designed for use in system filters. The condition first_delivery is
true only for the first attempt at delivering a message, and manually_thawed is
true only if the message has been frozen, and subsequently thawed by an admin
user. An explicit forced delivery counts as a manual thaw, but thawing as a
result of the auto_thaw setting does not.

Warning: If a system filter uses the first_delivery condition to specify an
"unseen" (non-significant) delivery, and that delivery does not succeed, it
will not be tried again. If you want Exim to retry an unseen delivery until it
succeeds, you should arrange to set it up every time the filter runs.

When a system filter finishes running, the values of the variables $n0 - $n9
are copied into $sn0 - $sn9 and are thereby made available to users' filter
files. Thus a system filter can, for example, set up "scores" to which users'
filter files can refer.


47.4 Additional variable for system filters
-------------------------------------------

The expansion variable $recipients, containing a list of all the recipients of
the message (separated by commas and white space), is available in system
filters. It is not available in users' filters for privacy reasons.


47.5 Defer, freeze, and fail commands for system filters
--------------------------------------------------------

There are three extra commands (defer, freeze and fail) which are always
available in system filters, but are not normally enabled in users' filters.
(See the allow_defer, allow_freeze and allow_fail options for the redirect
router.) These commands can optionally be followed by the word text and a
string containing an error message, for example:

fail text "this message looks like spam to me"

The keyword text is optional if the next character is a double quote.

The defer command defers delivery of the original recipients of the message.
The fail command causes all the original recipients to be failed, and a bounce
message to be created. The freeze command suspends all delivery attempts for
the original recipients. In all cases, any new deliveries that are specified by
the filter are attempted as normal after the filter has run.

The freeze command is ignored if the message has been manually unfrozen and not
manually frozen since. This means that automatic freezing by a system filter
can be used as a way of checking out suspicious messages. If a message is found
to be all right, manually unfreezing it allows it to be delivered.

The text given with a fail command is used as part of the bounce message as
well as being written to the log. If the message is quite long, this can fill
up a lot of log space when such failures are common. To reduce the size of the
log message, Exim interprets the text in a special way if it starts with the
two characters "<<" and contains ">>" later. The text between these two strings
is written to the log, and the rest of the text is used in the bounce message.
For example:

fail "<<filter test 1>>Your message is rejected \
     because it contains attachments that we are \
     not prepared to receive."

Take great care with the fail command when basing the decision to fail on the
contents of the message, because the bounce message will of course include the
contents of the original message and will therefore trigger the fail command
again (causing a mail loop) unless steps are taken to prevent this. Testing the
error_message condition is one way to prevent this. You could use, for example

if $message_body contains "this is spam" and not error_message
then fail text "spam is not wanted here" endif

though of course that might let through unwanted bounce messages. The
alternative is clever checking of the body and/or headers to detect bounces
generated by the filter.

The interpretation of a system filter file ceases after a defer, freeze, or 
fail command is obeyed. However, any deliveries that were set up earlier in the
filter file are honoured, so you can use a sequence such as

mail ...
freeze

to send a specified message when the system filter is freezing (or deferring or
failing) a message. The normal deliveries for the message do not, of course,
take place.


47.6 Adding and removing headers in a system filter
---------------------------------------------------

Two filter commands that are available only in system filters are:

headers add <string>
headers remove <string>

The argument for the headers add is a string that is expanded and then added to
the end of the message's headers. It is the responsibility of the filter
maintainer to make sure it conforms to RFC 2822 syntax. Leading white space is
ignored, and if the string is otherwise empty, or if the expansion is forced to
fail, the command has no effect.

You can use "\n" within the string, followed by white space, to specify
continued header lines. More than one header may be added in one command by
including "\n" within the string without any following white space. For
example:

headers add "X-header-1: ....\n  \
             continuation of X-header-1 ...\n\
             X-header-2: ...."

Note that the header line continuation white space after the first newline must
be placed before the backslash that continues the input string, because white
space after input continuations is ignored.

The argument for headers remove is a colon-separated list of header names. This
command applies only to those headers that are stored with the message; those
that are added at delivery time (such as Envelope-To: and Return-Path:) cannot
be removed by this means. If there is more than one header with the same name,
they are all removed.

The headers command in a system filter makes an immediate change to the set of
header lines that was received with the message (with possible additions from
ACL processing). Subsequent commands in the system filter operate on the
modified set, which also forms the basis for subsequent message delivery.
Unless further modified during routing or transporting, this set of headers is
used for all recipients of the message.

During routing and transporting, the variables that refer to the contents of
header lines refer only to those lines that are in this set. Thus, header lines
that are added by a system filter are visible to users' filter files and to all
routers and transports. This contrasts with the manipulation of header lines by
routers and transports, which is not immediate, but which instead is saved up
until the message is actually being written (see section 48.6).

If the message is not delivered at the first attempt, header lines that were
added by the system filter are stored with the message, and so are still
present at the next delivery attempt. Header lines that were removed are still
present, but marked "deleted" so that they are not transported with the
message. For this reason, it is usual to make the headers command conditional
on first_delivery so that the set of header lines is not modified more than
once.

Because header modification in a system filter acts immediately, you have to
use an indirect approach if you want to modify the contents of a header line.
For example:

headers add "Old-Subject: $h_subject:"
headers remove "Subject"
headers add "Subject: new subject (was: $h_old-subject:)"
headers remove "Old-Subject"


47.7 Setting an errors address in a system filter
-------------------------------------------------

In a system filter, if a deliver command is followed by

errors_to <some address>

in order to change the envelope sender (and hence the error reporting) for that
delivery, any address may be specified. (In a user filter, only the current
user's address can be set.) For example, if some mail is being monitored, you
might use

unseen deliver monitor@spying.example errors_to root@local.example

to take a copy which would not be sent back to the normal error reporting
address if its delivery failed.


47.8 Per-address filtering
--------------------------

In contrast to the system filter, which is run just once per message for each
delivery attempt, it is also possible to set up a system-wide filtering
operation that runs once for each recipient address. In this case, variables
such as $local_part_data and $domain_data can be used, and indeed, the choice
of filter file could be made dependent on them. This is an example of a router
which implements such a filter:

central_filter:
  check_local_user
  driver = redirect
  domains = +local_domains
  file = /central/filters/$local_part_data
  no_verify
  allow_filter
  allow_freeze

The filter is run in a separate process under its own uid. Therefore, either 
check_local_user must be set (as above), in which case the filter is run as the
local user, or the user option must be used to specify which user to use. If
both are set, user overrides.

Care should be taken to ensure that none of the commands in the filter file
specify a significant delivery if the message is to go on to be delivered to
its intended recipient. The router will not then claim to have dealt with the
address, so it will be passed on to subsequent routers to be delivered in the
normal way.



===============================================================================
48. MESSAGE PROCESSING

Exim performs various transformations on the sender and recipient addresses of
all messages that it handles, and also on the messages' header lines. Some of
these are optional and configurable, while others always take place. All of
this processing, except rewriting as a result of routing, and the addition or
removal of header lines while delivering, happens when a message is received,
before it is placed on Exim's queue.

Some of the automatic processing takes place by default only for
"locally-originated" messages. This adjective is used to describe messages that
are not received over TCP/IP, but instead are passed to an Exim process on its
standard input. This includes the interactive "local SMTP" case that is set up
by the -bs command line option.

Note: Messages received over TCP/IP on the loopback interface (127.0.0.1 or
::1) are not considered to be locally-originated. Exim does not treat the
loopback interface specially in any way.

If you want the loopback interface to be treated specially, you must ensure
that there are appropriate entries in your ACLs.


48.1 Submission mode for non-local messages
-------------------------------------------

Processing that happens automatically for locally-originated messages (unless 
suppress_local_fixups is set) can also be requested for messages that are
received over TCP/IP. The term "submission mode" is used to describe this
state. Submission mode is set by the modifier

control = submission

in a MAIL, RCPT, or pre-data ACL for an incoming message (see sections 44.12
and 44.13). This makes Exim treat the message as a local submission, and is
normally used when the source of the message is known to be an MUA running on a
client host (as opposed to an MTA). For example, to set submission mode for
messages originating on the IPv4 loopback interface, you could include the
following in the MAIL ACL:

warn  hosts = 127.0.0.1
      control = submission

There are some options that can be used when setting submission mode. A slash
is used to separate options. For example:

control = submission/sender_retain

Specifying sender_retain has the effect of setting local_sender_retain true and
local_from_check false for the current incoming message. The first of these
allows an existing Sender: header in the message to remain, and the second
suppresses the check to ensure that From: matches the authenticated sender.
With this setting, Exim still fixes up messages by adding Date: and Message-ID:
header lines if they are missing, but makes no attempt to check sender
authenticity in header lines.

When sender_retain is not set, a submission mode setting may specify a domain
to be used when generating a From: or Sender: header line. For example:

control = submission/domain=some.domain

The domain may be empty. How this value is used is described in sections 48.5.7
and 48.5.12. There is also a name option that allows you to specify the user's
full name for inclusion in a created Sender: or From: header line. For example:

accept authenticated = *
       control = submission/domain=wonderland.example/\
                            name=${lookup {$authenticated_id} \
                                   lsearch {/etc/exim/namelist}}

Because the name may contain any characters, including slashes, the name option
must be given last. The remainder of the string is used as the name. For the
example above, if /etc/exim/namelist contains:

bigegg:  Humpty Dumpty

then when the sender has authenticated as bigegg, the generated Sender: line
would be:

Sender: Humpty Dumpty <bigegg@wonderland.example>

By default, submission mode forces the return path to the same address as is
used to create the Sender: header. However, if sender_retain is specified, the
return path is also left unchanged.

Note: The changes caused by submission mode take effect after the predata ACL.
This means that any sender checks performed before the fix-ups use the
untrusted sender address specified by the user, not the trusted sender address
specified by submission mode. Although this might be slightly unexpected, it
does mean that you can configure ACL checks to spot that a user is trying to
spoof another's address.


48.2 Line endings
-----------------

RFC 2821 specifies that CRLF (two characters: carriage-return, followed by
linefeed) is the line ending for messages transmitted over the Internet using
SMTP over TCP/IP. However, within individual operating systems, different
conventions are used. For example, Unix-like systems use just LF, but others
use CRLF or just CR.

Exim was designed for Unix-like systems, and internally, it stores messages
using the system's convention of a single LF as a line terminator. When
receiving a message, all line endings are translated to this standard format.
Originally, it was thought that programs that passed messages directly to an
MTA within an operating system would use that system's convention. Experience
has shown that this is not the case; for example, there are Unix applications
that use CRLF in this circumstance. For this reason, and for compatibility with
other MTAs, the way Exim handles line endings for all messages is now as
follows:

  * CR is treated as a line ending; if it is immediately followed by LF, the LF
    is ignored.

  * The sequence "CR, dot, CR" does not terminate an incoming SMTP message, nor
    a local message in the state where a line containing only a dot is a
    terminator.

  * If a bare CR is encountered within a header line, an extra space is added
    after the line terminator so as not to end the header line. The reasoning
    behind this is that bare CRs in header lines are most likely either to be
    mistakes, or people trying to play silly games.

  * If the first header line received in a message ends with CRLF, a subsequent
    bare LF in a header line is treated in the same way as a bare CR in a
    header line and a bare LF in a body line is replaced with a space.
  
  * If the first header line received in a message does not end with CRLF,
    a subsequent LF not preceded by CR is treated as a line ending.


48.3 Unqualified addresses
--------------------------

By default, Exim expects every envelope address it receives from an external
host to be fully qualified. Unqualified addresses cause negative responses to
SMTP commands. However, because SMTP is used as a means of transporting
messages from MUAs running on personal workstations, there is sometimes a
requirement to accept unqualified addresses from specific hosts or IP networks.

Exim has two options that separately control which hosts may send unqualified
sender or recipient addresses in SMTP commands, namely sender_unqualified_hosts
and recipient_unqualified_hosts. In both cases, if an unqualified address is
accepted, it is qualified by adding the value of qualify_domain or 
qualify_recipient, as appropriate.

Unqualified addresses in header lines are automatically qualified for messages
that are locally originated, unless the -bnq option is given on the command
line. For messages received over SMTP, unqualified addresses in header lines
are qualified only if unqualified addresses are permitted in SMTP commands. In
other words, such qualification is also controlled by sender_unqualified_hosts
and recipient_unqualified_hosts,


48.4 The UUCP From line
-----------------------

Messages that have come from UUCP (and some other applications) often begin
with a line containing the envelope sender and a timestamp, following the word
"From". Examples of two common formats are:

From a.oakley@berlin.mus Fri Jan  5 12:35 GMT 1996
From f.butler@berlin.mus Fri, 7 Jan 97 14:00:00 GMT

This line precedes the RFC 2822 header lines. For compatibility with Sendmail,
Exim recognizes such lines at the start of messages that are submitted to it
via the command line (that is, on the standard input). It does not recognize
such lines in incoming SMTP messages, unless the sending host matches 
ignore_fromline_hosts or the -bs option was used for a local message and 
ignore_fromline_local is set. The recognition is controlled by a regular
expression that is defined by the uucp_from_pattern option, whose default value
matches the two common cases shown above and puts the address that follows
"From" into $1.

When the caller of Exim for a non-SMTP message that contains a "From" line is a
trusted user, the message's sender address is constructed by expanding the
contents of uucp_sender_address, whose default value is "$1". This is then
parsed as an RFC 2822 address. If there is no domain, the local part is
qualified with qualify_domain unless it is the empty string. However, if the
command line -f option is used, it overrides the "From" line.

If the caller of Exim is not trusted, the "From" line is recognized, but the
sender address is not changed. This is also the case for incoming SMTP messages
that are permitted to contain "From" lines.

Only one "From" line is recognized. If there is more than one, the second is
treated as a data line that starts the body of the message, as it is not valid
as a header line. This also happens if a "From" line is present in an incoming
SMTP message from a source that is not permitted to send them.


48.5 Header lines
-----------------

48.5.1 Resent- header lines

RFC 2822 makes provision for sets of header lines starting with the string
"Resent-" to be added to a message when it is resent by the original recipient
to somebody else. These headers are Resent-Date:, Resent-From:, Resent-Sender:,
Resent-To:, Resent-Cc:, Resent-Bcc: and Resent-Message-ID:. The RFC says:

    Resent fields are strictly informational. They MUST NOT be used in the
    normal processing of replies or other such automatic actions on messages.

This leaves things a bit vague as far as other processing actions such as
address rewriting are concerned. Exim treats Resent- header lines as follows:

  * A Resent-From: line that just contains the login id of the submitting user
    is automatically rewritten in the same way as From: (see below).

  * If there's a rewriting rule for a particular header line, it is also
    applied to Resent- header lines of the same type. For example, a rule that
    rewrites From: also rewrites Resent-From:.

  * For local messages, if Sender: is removed on input, Resent-Sender: is also
    removed.

  * For a locally-submitted message, if there are any Resent- header lines but
    no Resent-Date:, Resent-From:, or Resent-Message-Id:, they are added as
    necessary. It is the contents of Resent-Message-Id: (rather than 
    Message-Id:) which are included in log lines in this case.

  * The logic for adding Sender: is duplicated for Resent-Sender: when any 
    Resent- header lines are present.

48.5.2 Auto-Submitted:

Whenever Exim generates an autoreply, a bounce, or a delay warning message, it
includes the header line:

Auto-Submitted: auto-replied

48.5.3 Bcc:

If Exim is called with the -t option, to take recipient addresses from a
message's header, it removes any Bcc: header line that may exist (after
extracting its addresses). If -t is not present on the command line, any
existing Bcc: is not removed.

48.5.4 Date:

If a locally-generated or submission-mode message has no Date: header line,
Exim adds one, using the current date and time, unless the 
suppress_local_fixups control has been specified.

48.5.5 Delivery-date:

Delivery-date: header lines are not part of the standard RFC 2822 header set.
Exim can be configured to add them to the final delivery of messages. (See the
generic delivery_date_add transport option.) They should not be present in
messages in transit. If the delivery_date_remove configuration option is set
(the default), Exim removes Delivery-date: header lines from incoming messages.

48.5.6 Envelope-to:

Envelope-to: header lines are not part of the standard RFC 2822 header set.
Exim can be configured to add them to the final delivery of messages. (See the
generic envelope_to_add transport option.) They should not be present in
messages in transit. If the envelope_to_remove configuration option is set (the
default), Exim removes Envelope-to: header lines from incoming messages.

48.5.7 From:

If a submission-mode message does not contain a From: header line, Exim adds
one if either of the following conditions is true:

  * The envelope sender address is not empty (that is, this is not a bounce
    message). The added header line copies the envelope sender address.

  * The SMTP session is authenticated and $authenticated_id is not empty.

     1. If no domain is specified by the submission control, the local part is
        $authenticated_id and the domain is $qualify_domain.

     2. If a non-empty domain is specified by the submission control, the local
        part is $authenticated_id, and the domain is the specified domain.

     3. If an empty domain is specified by the submission control,
        $authenticated_id is assumed to be the complete address.

A non-empty envelope sender takes precedence.

If a locally-generated incoming message does not contain a From: header line,
and the suppress_local_fixups control is not set, Exim adds one containing the
sender's address. The calling user's login name and full name are used to
construct the address, as described in section 48.7. They are obtained from the
password data by calling getpwuid() (but see the unknown_login configuration
option). The address is qualified with qualify_domain.

For compatibility with Sendmail, if an incoming, non-SMTP message has a From:
header line containing just the unqualified login name of the calling user,
this is replaced by an address containing the user's login name and full name
as described in section 48.7.

48.5.8 Message-ID:

If a locally-generated or submission-mode incoming message does not contain a 
Message-ID: or Resent-Message-ID: header line, and the suppress_local_fixups
control is not set, Exim adds a suitable header line to the message. If there
are any Resent-: headers in the message, it creates Resent-Message-ID:. The id
is constructed from Exim's internal message id, preceded by the letter E to
ensure it starts with a letter, and followed by @ and the primary host name.
Additional information can be included in this header line by setting the 
message_id_header_text and/or message_id_header_domain options.

48.5.9 Received:

A Received: header line is added at the start of every message. The contents
are defined by the received_header_text configuration option, and Exim
automatically adds a semicolon and a timestamp to the configured string.

The Received: header is generated as soon as the message's header lines have
been received. At this stage, the timestamp in the Received: header line is the
time that the message started to be received. This is the value that is seen by
the DATA ACL and by the local_scan() function.

Once a message is accepted, the timestamp in the Received: header line is
changed to the time of acceptance, which is (apart from a small delay while the
-H spool file is written) the earliest time at which delivery could start.

48.5.10 References:

Messages created by the autoreply transport include a References: header line.
This is constructed according to the rules that are described in section 3.64
of RFC 2822 (which states that replies should contain such a header line), and
section 3.14 of RFC 3834 (which states that automatic responses are not
different in this respect). However, because some mail processing software does
not cope well with very long header lines, no more than 12 message IDs are
copied from the References: header line in the incoming message. If there are
more than 12, the first one and then the final 11 are copied, before adding the
message ID of the incoming message.

48.5.11 Return-path:

Return-path: header lines are defined as something an MTA may insert when it
does the final delivery of messages. (See the generic return_path_add transport
option.) Therefore, they should not be present in messages in transit. If the 
return_path_remove configuration option is set (the default), Exim removes 
Return-path: header lines from incoming messages.

48.5.12 Sender:

For a locally-originated message from an untrusted user, Exim may remove an
existing Sender: header line, and it may add a new one. You can modify these
actions by setting the local_sender_retain option true, the local_from_check
option false, or by using the suppress_local_fixups control setting.

When a local message is received from an untrusted user and local_from_check is
true (the default), and the suppress_local_fixups control has not been set, a
check is made to see if the address given in the From: header line is the
correct (local) sender of the message. The address that is expected has the
login name as the local part and the value of qualify_domain as the domain.
Prefixes and suffixes for the local part can be permitted by setting 
local_from_prefix and local_from_suffix appropriately. If From: does not
contain the correct sender, a Sender: line is added to the message.

If you set local_from_check false, this checking does not occur. However, the
removal of an existing Sender: line still happens, unless you also set 
local_sender_retain to be true. It is not possible to set both of these options
true at the same time.

By default, no processing of Sender: header lines is done for messages received
over TCP/IP or for messages submitted by trusted users. However, when a message
is received over TCP/IP in submission mode, and sender_retain is not specified
on the submission control, the following processing takes place:

First, any existing Sender: lines are removed. Then, if the SMTP session is
authenticated, and $authenticated_id is not empty, a sender address is created
as follows:

  * If no domain is specified by the submission control, the local part is
    $authenticated_id and the domain is $qualify_domain.

  * If a non-empty domain is specified by the submission control, the local
    part is $authenticated_id, and the domain is the specified domain.

  * If an empty domain is specified by the submission control,
    $authenticated_id is assumed to be the complete address.

This address is compared with the address in the From: header line. If they are
different, a Sender: header line containing the created address is added.
Prefixes and suffixes for the local part in From: can be permitted by setting 
local_from_prefix and local_from_suffix appropriately.

Note: Whenever a Sender: header line is created, the return path for the
message (the envelope sender address) is changed to be the same address, except
in the case of submission mode when sender_retain is specified.


48.6 Adding and removing header lines in routers and transports
---------------------------------------------------------------

When a message is delivered, the addition and removal of header lines can be
specified in a system filter, or on any of the routers and transports that
process the message. Section 47.6 contains details about modifying headers in a
system filter. Header lines can also be added in an ACL as a message is
received (see section 44.15).

In contrast to what happens in a system filter, header modifications that are
specified on routers and transports apply only to the particular recipient
addresses that are being processed by those routers and transports. These
changes do not actually take place until a copy of the message is being
transported. Therefore, they do not affect the basic set of header lines, and
they do not affect the values of the variables that refer to header lines.

Note: In particular, this means that any expansions in the configuration of the
transport cannot refer to the modified header lines, because such expansions
all occur before the message is actually transported.

For both routers and transports, the argument of a headers_add option must be
in the form of one or more RFC 2822 header lines, separated by newlines (coded
as "\n"). For example:

headers_add = X-added-header: added by $primary_hostname\n\
              X-added-second: another added header line

Exim does not check the syntax of these added header lines.

Multiple headers_add options for a single router or transport can be specified;
the values will append to a single list of header lines. Each header-line is
separately expanded.

The argument of a headers_remove option must consist of a colon-separated list
of header names. This is confusing, because header names themselves are often
terminated by colons. In this case, the colons are the list separators, not
part of the names. For example:

headers_remove = return-receipt-to:acknowledge-to

Multiple headers_remove options for a single router or transport can be
specified; the arguments will append to a single header-names list. Each item
is separately expanded. Note that colons in complex expansions which are used
to form all or part of a headers_remove list will act as list separators.

When headers_add or headers_remove is specified on a router, items are expanded
at routing time, and then associated with all addresses that are accepted by
that router, and also with any new addresses that it generates. If an address
passes through several routers as a result of aliasing or forwarding, the
changes are cumulative.

However, this does not apply to multiple routers that result from the use of
the unseen option. Any header modifications that were specified by the "unseen"
router or its predecessors apply only to the "unseen" delivery.

Addresses that end up with different headers_add or headers_remove settings
cannot be delivered together in a batch, so a transport is always dealing with
a set of addresses that have the same header-processing requirements.

The transport starts by writing the original set of header lines that arrived
with the message, possibly modified by the system filter. As it writes out
these lines, it consults the list of header names that were attached to the
recipient address(es) by headers_remove options in routers, and it also
consults the transport's own headers_remove option. Header lines whose names
are on either of these lists are not written out. If there are multiple
instances of any listed header, they are all skipped.

After the remaining original header lines have been written, new header lines
that were specified by routers' headers_add options are written, in the order
in which they were attached to the address. These are followed by any header
lines specified by the transport's headers_add option.

This way of handling header line modifications in routers and transports has
the following consequences:

  * The original set of header lines, possibly modified by the system filter,
    remains "visible", in the sense that the $header_xxx variables refer to it,
    at all times.

  * Header lines that are added by a router's headers_add option are not
    accessible by means of the $header_xxx expansion syntax in subsequent
    routers or the transport.

  * Conversely, header lines that are specified for removal by headers_remove
    in a router remain visible to subsequent routers and the transport.

  * Headers added to an address by headers_add in a router cannot be removed by
    a later router or by a transport.

  * An added header can refer to the contents of an original header that is to
    be removed, even it has the same name as the added header. For example:

    headers_remove = subject
    headers_add = Subject: new subject (was: $h_subject:)

Warning: The headers_add and headers_remove options cannot be used for a 
redirect router that has the one_time option set.


48.7 Constructed addresses
--------------------------

When Exim constructs a sender address for a locally-generated message, it uses
the form

<user name>  <login@qualify_domain>

For example:

Zaphod Beeblebrox <zaphod@end.univ.example>

The user name is obtained from the -F command line option if set, or otherwise
by looking up the calling user by getpwuid() and extracting the "gecos" field
from the password entry. If the "gecos" field contains an ampersand character,
this is replaced by the login name with the first letter upper cased, as is
conventional in a number of operating systems. See the gecos_name option for a
way to tailor the handling of the "gecos" field. The unknown_username option
can be used to specify user names in cases when there is no password file
entry.

In all cases, the user name is made to conform to RFC 2822 by quoting all or
parts of it if necessary. In addition, if it contains any non-printing
characters, it is encoded as described in RFC 2047, which defines a way of
including non-ASCII characters in header lines. The value of the 
headers_charset option specifies the name of the encoding that is used (the
characters are assumed to be in this encoding). The setting of 
print_topbitchars controls whether characters with the top bit set (that is,
with codes greater than 127) count as printing characters or not.


48.8 Case of local parts
------------------------

RFC 2822 states that the case of letters in the local parts of addresses cannot
be assumed to be non-significant. Exim preserves the case of local parts of
addresses, but by default it uses a lower-cased form when it is routing,
because on most Unix systems, usernames are in lower case and case-insensitive
routing is required. However, any particular router can be made to use the
original case for local parts by setting the caseful_local_part generic router
option.

If you must have mixed-case user names on your system, the best way to proceed,
assuming you want case-independent handling of incoming email, is to set up
your first router to convert incoming local parts in your domains to the
correct case by means of a file lookup. For example:

correct_case:
  driver = redirect
  domains = +local_domains
  data = ${lookup{$local_part}cdb\
              {/etc/usercased.cdb}{$value}fail}\
              @$domain

For this router, the local part is forced to lower case by the default action (
caseful_local_part is not set). The lower-cased local part is used to look up a
new local part in the correct case. If you then set caseful_local_part on any
subsequent routers which process your domains, they will operate on local parts
with the correct case in a case-sensitive manner.


48.9 Dots in local parts
------------------------

RFC 2822 forbids empty components in local parts. That is, an unquoted local
part may not begin or end with a dot, nor have two consecutive dots in the
middle. However, it seems that many MTAs do not enforce this, so Exim permits
empty components for compatibility.


48.10 Rewriting addresses
-------------------------

Rewriting of sender and recipient addresses, and addresses in headers, can
happen automatically, or as the result of configuration options, as described
in chapter 31. The headers that may be affected by this are Bcc:, Cc:, From:, 
Reply-To:, Sender:, and To:.

Automatic rewriting includes qualification, as mentioned above. The other case
in which it can happen is when an incomplete non-local domain is given. The
routing process may cause this to be expanded into the full domain name. For
example, a header such as

To: hare@teaparty

might get rewritten as

To: hare@teaparty.wonderland.fict.example

Rewriting as a result of routing is the one kind of message processing that
does not happen at input time, as it cannot be done until the address has been
routed.

Strictly, one should not do any deliveries of a message until all its addresses
have been routed, in case any of the headers get changed as a result of
routing. However, doing this in practice would hold up many deliveries for
unreasonable amounts of time, just because one address could not immediately be
routed. Exim therefore does not delay other deliveries when routing of one or
more addresses is deferred.



===============================================================================
49. SMTP PROCESSING

Exim supports a number of different ways of using the SMTP protocol, and its
LMTP variant, which is an interactive protocol for transferring messages into a
closed mail store application. This chapter contains details of how SMTP is
processed. For incoming mail, the following are available:

  * SMTP over TCP/IP (Exim daemon or inetd);

  * SMTP over the standard input and output (the -bs option);

  * Batched SMTP on the standard input (the -bS option).

For mail delivery, the following are available:

  * SMTP over TCP/IP (the smtp transport);

  * LMTP over TCP/IP (the smtp transport with the protocol option set to
    "lmtp");

  * LMTP over a pipe to a process running in the local host (the lmtp
    transport);

  * Batched SMTP to a file or pipe (the appendfile and pipe transports with the
    use_bsmtp option set).

Batched SMTP is the name for a process in which batches of messages are stored
in or read from files (or pipes), in a format in which SMTP commands are used
to contain the envelope information.


49.1 Outgoing SMTP and LMTP over TCP/IP
---------------------------------------

Outgoing SMTP and LMTP over TCP/IP is implemented by the smtp transport. The 
protocol option selects which protocol is to be used, but the actual processing
is the same in both cases.

If, in response to its EHLO command, Exim is told that the SIZE extension is
supported, it adds SIZE=<n> to each subsequent MAIL command. The value of <n>
is the message size plus the value of the size_addition option (default 1024)
to allow for additions to the message such as per-transport header lines, or
changes made in a transport filter. If size_addition is set negative, the use
of SIZE is suppressed.

If the remote server advertises support for PIPELINING, Exim uses the
pipelining extension to SMTP (RFC 2197) to reduce the number of TCP/IP packets
required for the transaction.

If the remote server advertises support for the STARTTLS command, and Exim was
built to support TLS encryption, it tries to start a TLS session unless the
server matches hosts_avoid_tls. See chapter 43 for more details. Either a match
in that or hosts_verify_avoid_tls apply when the transport is called for
verification.

If the remote server advertises support for the AUTH command, Exim scans the
authenticators configuration for any suitable client settings, as described in
chapter 33.

Responses from the remote host are supposed to be terminated by CR followed by
LF. However, there are known to be hosts that do not send CR characters, so in
order to be able to interwork with such hosts, Exim treats LF on its own as a
line terminator.

If a message contains a number of different addresses, all those with the same
characteristics (for example, the same envelope sender) that resolve to the
same set of hosts, in the same order, are sent in a single SMTP transaction,
even if they are for different domains, unless there are more than the setting
of the max_rcpts option in the smtp transport allows, in which case they are
split into groups containing no more than max_rcpts addresses each. If 
remote_max_parallel is greater than one, such groups may be sent in parallel
sessions. The order of hosts with identical MX values is not significant when
checking whether addresses can be batched in this way.

When the smtp transport suffers a temporary failure that is not
message-related, Exim updates its transport-specific database, which contains
records indexed by host name that remember which messages are waiting for each
particular host. It also updates the retry database with new retry times.

Exim's retry hints are based on host name plus IP address, so if one address of
a multi-homed host is broken, it will soon be skipped most of the time. See the
next section for more detail about error handling.

When a message is successfully delivered over a TCP/IP SMTP connection, Exim
looks in the hints database for the transport to see if there are any queued
messages waiting for the host to which it is connected. If it finds one, it
creates a new Exim process using the -MC option (which can only be used by a
process running as root or the Exim user) and passes the TCP/IP socket to it so
that it can deliver another message using the same socket. The new process does
only those deliveries that are routed to the connected host, and may in turn
pass the socket on to a third process, and so on.

The connection_max_messages option of the smtp transport can be used to limit
the number of messages sent down a single TCP/IP connection.

The second and subsequent messages delivered down an existing connection are
identified in the main log by the addition of an asterisk after the closing
square bracket of the IP address.

49.1.1 Errors in outgoing SMTP

Three different kinds of error are recognized for outgoing SMTP: host errors,
message errors, and recipient errors.

Host errors

    A host error is not associated with a particular message or with a
    particular recipient of a message. The host errors are:

      o Connection refused or timed out,

      o Any error response code on connection,

      o Any error response code to EHLO or HELO,

      o Loss of connection at any time, except after ".",

      o I/O errors at any time,

      o Timeouts during the session, other than in response to MAIL, RCPT or
        the "." at the end of the data.

    For a host error, a permanent error response on connection, or in response
    to EHLO, causes all addresses routed to the host to be failed. Any other
    host error causes all addresses to be deferred, and retry data to be
    created for the host. It is not tried again, for any message, until its
    retry time arrives. If the current set of addresses are not all delivered
    in this run (to some alternative host), the message is added to the list of
    those waiting for this host, so if it is still undelivered when a
    subsequent successful delivery is made to the host, it will be sent down
    the same SMTP connection.

Message errors

    A message error is associated with a particular message when sent to a
    particular host, but not with a particular recipient of the message. The
    message errors are:

      o Any error response code to MAIL, DATA, or the "." that terminates the
        data,

      o Timeout after MAIL,

      o Timeout or loss of connection after the "." that terminates the data. A
        timeout after the DATA command itself is treated as a host error, as is
        loss of connection at any other time.

    For a message error, a permanent error response (5xx) causes all addresses
    to be failed, and a delivery error report to be returned to the sender. A
    temporary error response (4xx), or one of the timeouts, causes all
    addresses to be deferred. Retry data is not created for the host, but
    instead, a retry record for the combination of host plus message id is
    created. The message is not added to the list of those waiting for this
    host. This ensures that the failing message will not be sent to this host
    again until the retry time arrives. However, other messages that are routed
    to the host are not affected, so if it is some property of the message that
    is causing the error, it will not stop the delivery of other mail.

    If the remote host specified support for the SIZE parameter in its response
    to EHLO, Exim adds SIZE=nnn to the MAIL command, so an over-large message
    will cause a message error because the error arrives as a response to MAIL.

Recipient errors

    A recipient error is associated with a particular recipient of a message.
    The recipient errors are:

      o Any error response to RCPT,

      o Timeout after RCPT.

    For a recipient error, a permanent error response (5xx) causes the
    recipient address to be failed, and a bounce message to be returned to the
    sender. A temporary error response (4xx) or a timeout causes the failing
    address to be deferred, and routing retry data to be created for it. This
    is used to delay processing of the address in subsequent queue runs, until
    its routing retry time arrives. This applies to all messages, but because
    it operates only in queue runs, one attempt will be made to deliver a new
    message to the failing address before the delay starts to operate. This
    ensures that, if the failure is really related to the message rather than
    the recipient ("message too big for this recipient" is a possible example),
    other messages have a chance of getting delivered. If a delivery to the
    address does succeed, the retry information gets cleared, so all stuck
    messages get tried again, and the retry clock is reset.

    The message is not added to the list of those waiting for this host. Use of
    the host for other messages is unaffected, and except in the case of a
    timeout, other recipients are processed independently, and may be
    successfully delivered in the current SMTP session. After a timeout it is
    of course impossible to proceed with the session, so all addresses get
    deferred. However, those other than the one that failed do not suffer any
    subsequent retry delays. Therefore, if one recipient is causing trouble,
    the others have a chance of getting through when a subsequent delivery
    attempt occurs before the failing recipient's retry time.

In all cases, if there are other hosts (or IP addresses) available for the
current set of addresses (for example, from multiple MX records), they are
tried in this run for any undelivered addresses, subject of course to their own
retry data. In other words, recipient error retry data does not take effect
until the next delivery attempt.

Some hosts have been observed to give temporary error responses to every MAIL
command at certain times ("insufficient space" has been seen). It would be nice
if such circumstances could be recognized, and defer data for the host itself
created, but this is not possible within the current Exim design. What actually
happens is that retry data for every (host, message) combination is created.

The reason that timeouts after MAIL and RCPT are treated specially is that
these can sometimes arise as a result of the remote host's verification
procedures. Exim makes this assumption, and treats them as if a temporary error
response had been received. A timeout after "." is treated specially because it
is known that some broken implementations fail to recognize the end of the
message if the last character of the last line is a binary zero. Thus, it is
helpful to treat this case as a message error.

Timeouts at other times are treated as host errors, assuming a problem with the
host, or the connection to it. If a timeout after MAIL, RCPT, or "." is really
a connection problem, the assumption is that at the next try the timeout is
likely to occur at some other point in the dialogue, causing it then to be
treated as a host error.

There is experimental evidence that some MTAs drop the connection after the
terminating "." if they do not like the contents of the message for some
reason, in contravention of the RFC, which indicates that a 5xx response should
be given. That is why Exim treats this case as a message rather than a host
error, in order not to delay other messages to the same host.


49.2 Incoming SMTP messages over TCP/IP
---------------------------------------

Incoming SMTP messages can be accepted in one of two ways: by running a
listening daemon, or by using inetd. In the latter case, the entry in /etc/
inetd.conf should be like this:

smtp stream tcp nowait exim /opt/exim/bin/exim in.exim -bs

Exim distinguishes between this case and the case of a locally running user
agent using the -bs option by checking whether or not the standard input is a
socket. When it is, either the port must be privileged (less than 1024), or the
caller must be root or the Exim user. If any other user passes a socket with an
unprivileged port number, Exim prints a message on the standard error stream
and exits with an error code.

By default, Exim does not make a log entry when a remote host connects or
disconnects (either via the daemon or inetd), unless the disconnection is
unexpected. It can be made to write such log entries by setting the 
smtp_connection log selector.

Commands from the remote host are supposed to be terminated by CR followed by
LF. However, there are known to be hosts that do not send CR characters. In
order to be able to interwork with such hosts, Exim treats LF on its own as a
line terminator. Furthermore, because common code is used for receiving
messages from all sources, a CR on its own is also interpreted as a line
terminator. However, the sequence "CR, dot, CR" does not terminate incoming
SMTP data.

One area that sometimes gives rise to problems concerns the EHLO or HELO
commands. Some clients send syntactically invalid versions of these commands,
which Exim rejects by default. (This is nothing to do with verifying the data
that is sent, so helo_verify_hosts is not relevant.) You can tell Exim not to
apply a syntax check by setting helo_accept_junk_hosts to match the broken
hosts that send invalid commands.

The amount of disk space available is checked whenever SIZE is received on a
MAIL command, independently of whether message_size_limit or check_spool_space
is configured, unless smtp_check_spool_space is set false. A temporary error is
given if there is not enough space. If check_spool_space is set, the check is
for that amount of space plus the value given with SIZE, that is, it checks
that the addition of the incoming message will not reduce the space below the
threshold.

When a message is successfully received, Exim includes the local message id in
its response to the final "." that terminates the data. If the remote host logs
this text it can help with tracing what has happened to a message.

The Exim daemon can limit the number of simultaneous incoming connections it is
prepared to handle (see the smtp_accept_max option). It can also limit the
number of simultaneous incoming connections from a single remote host (see the 
smtp_accept_max_per_host option). Additional connection attempts are rejected
using the SMTP temporary error code 421.

The Exim daemon does not rely on the SIGCHLD signal to detect when a subprocess
has finished, as this can get lost at busy times. Instead, it looks for
completed subprocesses every time it wakes up. Provided there are other things
happening (new incoming calls, starts of queue runs), completed processes will
be noticed and tidied away. On very quiet systems you may sometimes see a
"defunct" Exim process hanging about. This is not a problem; it will be noticed
when the daemon next wakes up.

When running as a daemon, Exim can reserve some SMTP slots for specific hosts,
and can also be set up to reject SMTP calls from non-reserved hosts at times of
high system load - for details see the smtp_accept_reserve, smtp_load_reserve,
and smtp_reserve_hosts options. The load check applies in both the daemon and 
inetd cases.

Exim normally starts a delivery process for each message received, though this
can be varied by means of the -odq command line option and the queue_only, 
queue_only_file, and queue_only_load options. The number of simultaneously
running delivery processes started in this way from SMTP input can be limited
by the smtp_accept_queue and smtp_accept_queue_per_connection options. When
either limit is reached, subsequently received messages are just put on the
input queue without starting a delivery process.

The controls that involve counts of incoming SMTP calls (smtp_accept_max, 
smtp_accept_queue, smtp_accept_reserve) are not available when Exim is started
up from the inetd daemon, because in that case each connection is handled by an
entirely independent Exim process. Control by load average is, however,
available with inetd.

Exim can be configured to verify addresses in incoming SMTP commands as they
are received. See chapter 44 for details. It can also be configured to rewrite
addresses at this time - before any syntax checking is done. See section 31.6.3
.

Exim can also be configured to limit the rate at which a client host submits
MAIL and RCPT commands in a single SMTP session. See the smtp_ratelimit_hosts
option.

49.2.1 Unrecognized SMTP commands

If Exim receives more than smtp_max_unknown_commands unrecognized SMTP commands
during a single SMTP connection, it drops the connection after sending the
error response to the last command. The default value for 
smtp_max_unknown_commands is 3. This is a defence against some kinds of abuse
that subvert web servers into making connections to SMTP ports; in these
circumstances, a number of non-SMTP lines are sent first.

49.2.2 Syntax and protocol errors in SMTP commands

A syntax error is detected if an SMTP command is recognized, but there is
something syntactically wrong with its data, for example, a malformed email
address in a RCPT command. Protocol errors include invalid command sequencing
such as RCPT before MAIL. If Exim receives more than smtp_max_synprot_errors
such commands during a single SMTP connection, it drops the connection after
sending the error response to the last command. The default value for 
smtp_max_synprot_errors is 3. This is a defence against broken clients that
loop sending bad commands (yes, it has been seen).

49.2.3 Use of non-mail SMTP commands

The "non-mail" SMTP commands are those other than MAIL, RCPT, and DATA. Exim
counts such commands, and drops the connection if there are too many of them in
a single SMTP session. This action catches some denial-of-service attempts and
things like repeated failing AUTHs, or a mad client looping sending EHLO. The
global option smtp_accept_max_nonmail defines what "too many" means. Its
default value is 10.

When a new message is expected, one occurrence of RSET is not counted. This
allows a client to send one RSET between messages (this is not necessary, but
some clients do it). Exim also allows one uncounted occurrence of HELO or EHLO,
and one occurrence of STARTTLS between messages. After starting up a TLS
session, another EHLO is expected, and so it too is not counted.

The first occurrence of AUTH in a connection, or immediately following STARTTLS
is also not counted. Otherwise, all commands other than MAIL, RCPT, DATA, and
QUIT are counted.

You can control which hosts are subject to the limit set by 
smtp_accept_max_nonmail by setting smtp_accept_max_nonmail_hosts. The default
value is "*", which makes the limit apply to all hosts. This option means that
you can exclude any specific badly-behaved hosts that you have to live with.

49.2.4 The VRFY and EXPN commands

When Exim receives a VRFY or EXPN command on a TCP/IP connection, it runs the
ACL specified by acl_smtp_vrfy or acl_smtp_expn (as appropriate) in order to
decide whether the command should be accepted or not.

When no ACL is defined for VRFY, or if it rejects without setting an explicit
response code, the command is accepted (with a 252 SMTP response code) in order
to support awkward clients that do a VRFY before every RCPT. When VRFY is
accepted, it runs exactly the same code as when Exim is called with the -bv
option, and returns 250/451/550 SMTP response codes.

If no ACL for EXPN is defined, the command is rejected. When EXPN is accepted,
a single-level expansion of the address is done. EXPN is treated as an "address
test" (similar to the -bt option) rather than a verification (the -bv option).
If an unqualified local part is given as the argument to EXPN, it is qualified
with qualify_domain. Rejections of VRFY and EXPN commands are logged on the
main and reject logs, and VRFY verification failures are logged in the main log
for consistency with RCPT failures.

49.2.5 The ETRN command

RFC 1985 describes an ESMTP command called ETRN that is designed to overcome
the security problems of the TURN command (which has fallen into disuse). When
Exim receives an ETRN command on a TCP/IP connection, it runs the ACL specified
by acl_smtp_etrn in order to decide whether the command should be accepted or
not. If no ACL is defined, the command is rejected.

The ETRN command is concerned with "releasing" messages that are awaiting
delivery to certain hosts. As Exim does not organize its message queue by host,
the only form of ETRN that is supported by default is the one where the text
starts with the "#" prefix, in which case the remainder of the text is specific
to the SMTP server. A valid ETRN command causes a run of Exim with the -R
option to happen, with the remainder of the ETRN text as its argument. For
example,

ETRN #brigadoon

runs the command

exim -R brigadoon

which causes a delivery attempt on all messages with undelivered addresses
containing the text "brigadoon". When smtp_etrn_serialize is set (the default),
Exim prevents the simultaneous execution of more than one queue run for the
same argument string as a result of an ETRN command. This stops a misbehaving
client from starting more than one queue runner at once.

Exim implements the serialization by means of a hints database in which a
record is written whenever a process is started by ETRN, and deleted when the
process completes. However, Exim does not keep the SMTP session waiting for the
ETRN process to complete. Once ETRN is accepted, the client is sent a "success"
return code. Obviously there is scope for hints records to get left lying
around if there is a system or program crash. To guard against this, Exim
ignores any records that are more than six hours old.

For more control over what ETRN does, the smtp_etrn_command option can used.
This specifies a command that is run whenever ETRN is received, whatever the
form of its argument. For example:

smtp_etrn_command = /etc/etrn_command $domain \
                    $sender_host_address

The string is split up into arguments which are independently expanded. The
expansion variable $domain is set to the argument of the ETRN command, and no
syntax checking is done on the contents of this argument. Exim does not wait
for the command to complete, so its status code is not checked. Exim runs under
its own uid and gid when receiving incoming SMTP, so it is not possible for it
to change them before running the command.


49.3 Incoming local SMTP
------------------------

Some user agents use SMTP to pass messages to their local MTA using the
standard input and output, as opposed to passing the envelope on the command
line and writing the message to the standard input. This is supported by the 
-bs option. This form of SMTP is handled in the same way as incoming messages
over TCP/IP (including the use of ACLs), except that the envelope sender given
in a MAIL command is ignored unless the caller is trusted. In an ACL you can
detect this form of SMTP input by testing for an empty host identification. It
is common to have this as the first line in the ACL that runs for RCPT
commands:

accept hosts = :

This accepts SMTP messages from local processes without doing any other tests.


49.4 Outgoing batched SMTP
--------------------------

Both the appendfile and pipe transports can be used for handling batched SMTP.
Each has an option called use_bsmtp which causes messages to be output in BSMTP
format. No SMTP responses are possible for this form of delivery. All it is
doing is using SMTP commands as a way of transmitting the envelope along with
the message.

The message is written to the file or pipe preceded by the SMTP commands MAIL
and RCPT, and followed by a line containing a single dot. Lines in the message
that start with a dot have an extra dot added. The SMTP command HELO is not
normally used. If it is required, the message_prefix option can be used to
specify it.

Because appendfile and pipe are both local transports, they accept only one
recipient address at a time by default. However, you can arrange for them to
handle several addresses at once by setting the batch_max option. When this is
done for BSMTP, messages may contain multiple RCPT commands. See chapter 25 for
more details.

When one or more addresses are routed to a BSMTP transport by a router that
sets up a host list, the name of the first host on the list is available to the
transport in the variable $host. Here is an example of such a transport and
router:

begin routers
route_append:
  driver = manualroute
  transport = smtp_appendfile
  route_list = domain.example  batch.host.example

begin transports
smtp_appendfile:
  driver = appendfile
  directory = /var/bsmtp/$host
  batch_max = 1000
  use_bsmtp
  user = exim

This causes messages addressed to domain.example to be written in BSMTP format
to /var/bsmtp/batch.host.example, with only a single copy of each message
(unless there are more than 1000 recipients).


49.5 Incoming batched SMTP
--------------------------

The -bS command line option causes Exim to accept one or more messages by
reading SMTP on the standard input, but to generate no responses. If the caller
is trusted, the senders in the MAIL commands are believed; otherwise the sender
is always the caller of Exim. Unqualified senders and receivers are not
rejected (there seems little point) but instead just get qualified. HELO and
EHLO act as RSET; VRFY, EXPN, ETRN and HELP, act as NOOP; QUIT quits.

Minimal policy checking is done for BSMTP input. Only the non-SMTP ACL is run
in the same way as for non-SMTP local input.

If an error is detected while reading a message, including a missing "." at the
end, Exim gives up immediately. It writes details of the error to the standard
output in a stylized way that the calling program should be able to make some
use of automatically, for example:

554 Unexpected end of file
Transaction started in line 10
Error detected in line 14

It writes a more verbose version, for human consumption, to the standard error
file, for example:

An error was detected while processing a file of BSMTP input.
The error message was:

501 '>' missing at end of address

The SMTP transaction started in line 10.
The error was detected in line 12.
The SMTP command at fault was:

rcpt to:<malformed@in.com.plete

1 previous message was successfully processed.
The rest of the batch was abandoned.

The return code from Exim is zero only if there were no errors. It is 1 if some
messages were accepted before an error was detected, and 2 if no messages were
accepted.



===============================================================================
50. CUSTOMIZING BOUNCE AND WARNING MESSAGES

When a message fails to be delivered, or remains in the queue for more than a
configured amount of time, Exim sends a message to the original sender, or to
an alternative configured address. The text of these messages is built into the
code of Exim, but it is possible to change it, either by adding a single
string, or by replacing each of the paragraphs by text supplied in a file.

The From: and To: header lines are automatically generated; you can cause a 
Reply-To: line to be added by setting the errors_reply_to option. Exim also
adds the line

Auto-Submitted: auto-generated

to all warning and bounce messages,


50.1 Customizing bounce messages
--------------------------------

If bounce_message_text is set, its contents are included in the default message
immediately after "This message was created automatically by mail delivery
software." The string is not expanded. It is not used if bounce_message_file is
set.

When bounce_message_file is set, it must point to a template file for
constructing error messages. The file consists of a series of text items,
separated by lines consisting of exactly four asterisks. If the file cannot be
opened, default text is used and a message is written to the main and panic
logs. If any text item in the file is empty, default text is used for that
item.

Each item of text that is read from the file is expanded, and there are two
expansion variables which can be of use here: $bounce_recipient is set to the
recipient of an error message while it is being created, and
$bounce_return_size_limit contains the value of the return_size_limit option,
rounded to a whole number.

The items must appear in the file in the following order:

  * The first item is included in the headers, and should include at least a 
    Subject: header. Exim does not check the syntax of these headers.

  * The second item forms the start of the error message. After it, Exim lists
    the failing addresses with their error messages.

  * The third item is used to introduce any text from pipe transports that is
    to be returned to the sender. It is omitted if there is no such text.

  * The fourth, fifth and sixth items will be ignored and may be empty. The
    fields exist for back-compatibility

The default state (bounce_message_file unset) is equivalent to the following
file, in which the sixth item is empty. The Subject: and some other lines have
been split in order to fit them on the page:

Subject: Mail delivery failed
  ${if eq{$sender_address}{$bounce_recipient}
  {: returning message to sender}}
****
This message was created automatically by mail delivery software.

A message ${if eq{$sender_address}{$bounce_recipient}
  {that you sent }{sent by

<$sender_address>

}}could not be delivered to all of its recipients.
This is a permanent error. The following address(es) failed:
****
The following text was generated during the delivery attempt(s):
****
------ This is a copy of the message, including all the headers.
  ------
****
------ The body of the message is $message_size characters long;
  only the first
------ $bounce_return_size_limit or so are included here.
****


50.2 Customizing warning messages
---------------------------------

The option warn_message_file can be pointed at a template file for use when
warnings about message delays are created. In this case there are only three
text sections:

  * The first item is included in the headers, and should include at least a 
    Subject: header. Exim does not check the syntax of these headers.

  * The second item forms the start of the warning message. After it, Exim
    lists the delayed addresses.

  * The third item then ends the message.

The default state is equivalent to the following file, except that some lines
have been split here, in order to fit them on the page:

Subject: Warning: message $message_exim_id delayed
  $warn_message_delay
****
This message was created automatically by mail delivery software.

A message ${if eq{$sender_address}{$warn_message_recipients}
{that you sent }{sent by

<$sender_address>

}}has not been delivered to all of its recipients after
more than $warn_message_delay in the queue on $primary_hostname.

The message identifier is:     $message_exim_id
The subject of the message is: $h_subject
The date of the message is:    $h_date

The following address(es) have not yet been delivered:
****
No action is required on your part. Delivery attempts will
continue for some time, and this warning may be repeated at
intervals if the message remains undelivered. Eventually the
mail delivery software will give up, and when that happens,
the message will be returned to you.

However, in the default state the subject and date lines are omitted if no
appropriate headers exist. During the expansion of this file,
$warn_message_delay is set to the delay time in one of the forms "<n> minutes"
or "<n> hours", and $warn_message_recipients contains a list of recipients for
the warning message. There may be more than one if there are multiple addresses
with different errors_to settings on the routers that handled them.



===============================================================================
51. SOME COMMON CONFIGURATION SETTINGS

This chapter discusses some configuration settings that seem to be fairly
common. More examples and discussion can be found in the Exim book.


51.1 Sending mail to a smart host
---------------------------------

If you want to send all mail for non-local domains to a "smart host", you
should replace the default dnslookup router with a router which does the
routing explicitly:

send_to_smart_host:
  driver = manualroute
  route_list = !+local_domains smart.host.name
  transport = remote_smtp

You can use the smart host's IP address instead of the name if you wish. If you
are using Exim only to submit messages to a smart host, and not for receiving
incoming messages, you can arrange for it to do the submission synchronously by
setting the mua_wrapper option (see chapter 52).


51.2 Using Exim to handle mailing lists
---------------------------------------

Exim can be used to run simple mailing lists, but for large and/or complicated
requirements, the use of additional specialized mailing list software such as
Majordomo or Mailman is recommended.

The redirect router can be used to handle mailing lists where each list is
maintained in a separate file, which can therefore be managed by an independent
manager. The domains router option can be used to run these lists in a separate
domain from normal mail. For example:

lists:
  driver = redirect
  domains = lists.example
  file = ${lookup {$local_part} dsearch,ret=full {/usr/lists}}
  forbid_pipe
  forbid_file
  errors_to = ${quote_local_part:$local_part-request}@lists.example
  no_more

This router is skipped for domains other than lists.example. For addresses in
that domain, it looks for a file that matches the local part. If there is no
such file, the router declines, but because no_more is set, no subsequent
routers are tried, and so the whole delivery fails.

The forbid_pipe and forbid_file options prevent a local part from being
expanded into a filename or a pipe delivery, which is usually inappropriate in
a mailing list.

The errors_to option specifies that any delivery errors caused by addresses
taken from a mailing list are to be sent to the given address rather than the
original sender of the message. However, before acting on this, Exim verifies
the error address, and ignores it if verification fails.

For example, using the configuration above, mail sent to dicts@lists.example is
passed on to those addresses contained in /usr/lists/dicts, with error reports
directed to dicts-request@lists.example, provided that this address can be
verified. There could be a file called /usr/lists/dicts-request containing the
address(es) of this particular list's manager(s), but other approaches, such as
setting up an earlier router (possibly using the local_part_prefix or 
local_part_suffix options) to handle addresses of the form owner-xxx or xxx-
request, are also possible.


51.3 Syntax errors in mailing lists
-----------------------------------

If an entry in redirection data contains a syntax error, Exim normally defers
delivery of the original address. That means that a syntax error in a mailing
list holds up all deliveries to the list. This may not be appropriate when a
list is being maintained automatically from data supplied by users, and the
addresses are not rigorously checked.

If the skip_syntax_errors option is set, the redirect router just skips entries
that fail to parse, noting the incident in the log. If in addition 
syntax_errors_to is set to a verifiable address, a message is sent to it
whenever a broken address is skipped. It is usually appropriate to set 
syntax_errors_to to the same address as errors_to.


51.4 Re-expansion of mailing lists
----------------------------------

Exim remembers every individual address to which a message has been delivered,
in order to avoid duplication, but it normally stores only the original
recipient addresses with a message. If all the deliveries to a mailing list
cannot be done at the first attempt, the mailing list is re-expanded when the
delivery is next tried. This means that alterations to the list are taken into
account at each delivery attempt, so addresses that have been added to the list
since the message arrived will therefore receive a copy of the message, even
though it pre-dates their subscription.

If this behaviour is felt to be undesirable, the one_time option can be set on
the redirect router. If this is done, any addresses generated by the router
that fail to deliver at the first attempt are added to the message as "top
level" addresses, and the parent address that generated them is marked
"delivered". Thus, expansion of the mailing list does not happen again at the
subsequent delivery attempts. The disadvantage of this is that if any of the
failing addresses are incorrect, correcting them in the file has no effect on
pre-existing messages.

The original top-level address is remembered with each of the generated
addresses, and is output in any log messages. However, any intermediate parent
addresses are not recorded. This makes a difference to the log only if the 
all_parents selector is set, but for mailing lists there is normally only one
level of expansion anyway.


51.5 Closed mailing lists
-------------------------

The examples so far have assumed open mailing lists, to which anybody may send
mail. It is also possible to set up closed lists, where mail is accepted from
specified senders only. This is done by making use of the generic senders
option to restrict the router that handles the list.

The following example uses the same file as a list of recipients and as a list
of permitted senders. It requires three routers:

lists_request:
  driver = redirect
  domains = lists.example
  local_part_suffix = -request
  local_parts = ${lookup {$local_part} dsearch,filter=file {/usr/lists}}
  file = /usr/lists/${local_part_data}-request
  no_more

lists_post:
  driver = redirect
  domains = lists.example
  local_parts = ${lookup {$local_part} dsearch,filter=file,ret=full {/usr/lists}}
  senders = ${if exists {$local_part_data} {lsearch;$local_part_data}{*}}
  file = ${lookup {$local_part} dsearch,ret=full {/usr/lists}}
  forbid_pipe
  forbid_file
  errors_to = ${quote_local_part:$local_part-request}@lists.example
  no_more

lists_closed:
  driver = redirect
  domains = lists.example
  allow_fail
  data = :fail: $local_part@lists.example is a closed mailing list

All three routers have the same domains setting, so for any other domains, they
are all skipped. The first router runs only if the local part ends in -request.
It handles messages to the list manager(s) by means of an open mailing list.

The second router runs only if the senders precondition is satisfied. It checks
for the existence of a list that corresponds to the local part, and then checks
that the sender is on the list by means of a linear search. It is necessary to
check for the existence of the file before trying to search it, because
otherwise Exim thinks there is a configuration error. If the file does not
exist, the expansion of senders is *, which matches all senders. This means
that the router runs, but because there is no list, declines, and no_more
ensures that no further routers are run. The address fails with an "unrouteable
address" error.

The third router runs only if the second router is skipped, which happens when
a mailing list exists, but the sender is not on it. This router forcibly fails
the address, giving a suitable error message.


51.6 Variable Envelope Return Paths (VERP)
------------------------------------------

Variable Envelope Return Paths - see https://cr.yp.to/proto/verp.txt - are a
way of helping mailing list administrators discover which subscription address
is the cause of a particular delivery failure. The idea is to encode the
original recipient address in the outgoing envelope sender address, so that if
the message is forwarded by another host and then subsequently bounces, the
original recipient can be extracted from the recipient address of the bounce.

Envelope sender addresses can be modified by Exim using two different
facilities: the errors_to option on a router (as shown in previous mailing list
examples), or the return_path option on a transport. The second of these is
effective only if the message is successfully delivered to another host; it is
not used for errors detected on the local host (see the description of 
return_path in chapter 24). Here is an example of the use of return_path to
implement VERP on an smtp transport:

verp_smtp:
  driver = smtp
  max_rcpt = 1
  return_path = \
    ${if match {$return_path}{^(.+?)-request@your.dom.example\$}\
      {${quote_local_part:$1-request+$local_part=$domain}@your.dom.example}fail}

This has the effect of rewriting the return path (envelope sender) on outgoing
SMTP messages, if the local part of the original return path ends in
"-request", and the domain is your.dom.example. The rewriting inserts the local
part and domain of the recipient into the return path. Suppose, for example,
that a message whose return path has been set to 
somelist-request@your.dom.example is sent to subscriber@other.dom.example. In
the transport, the return path is rewritten as

somelist-request+subscriber=other.dom.example@your.dom.example

For this to work, you must tell Exim to send multiple copies of messages that
have more than one recipient, so that each copy has just one recipient. This is
achieved by setting max_rcpt to 1. Without this, a single copy of a message
might be sent to several different recipients in the same domain, in which case
$local_part is not available in the transport, because it is not unique.

Unless your host is doing nothing but mailing list deliveries, you should
probably use a separate transport for the VERP deliveries, so as not to use
extra resources in making one-per-recipient copies for other deliveries. This
can easily be done by expanding the transport option in the router:

dnslookup:
  driver = dnslookup
  domains = ! +local_domains
  transport = \
    ${if match {$return_path}{^(.+?)-request@your.dom.example\$}\
      {verp_smtp}{remote_smtp}}
  no_more

If you want to change the return path using errors_to in a router instead of
using return_path in the transport, you need to set errors_to on all routers
that handle mailing list addresses. This will ensure that all delivery errors,
including those detected on the local host, are sent to the VERP address.

On a host that does no local deliveries and has no manual routing, only the 
dnslookup router needs to be changed. A special transport is not needed for
SMTP deliveries. Every mailing list recipient has its own return path value,
and so Exim must hand them to the transport one at a time. Here is an example
of a dnslookup router that implements VERP:

verp_dnslookup:
  driver = dnslookup
  domains = ! +local_domains
  transport = remote_smtp
  errors_to = \
    ${if match {$return_path}{^(.+?)-request@your.dom.example\$}}
     {${quote_local_part:$1-request+$local_part=$domain}@your.dom.example}fail}
  no_more

Before you start sending out messages with VERPed return paths, you must also
configure Exim to accept the bounce messages that come back to those paths.
Typically this is done by setting a local_part_suffix option for a router, and
using this to route the messages to wherever you want to handle them.

The overhead incurred in using VERP depends very much on the size of the
message, the number of recipient addresses that resolve to the same remote
host, and the speed of the connection over which the message is being sent. If
a lot of addresses resolve to the same host and the connection is slow, sending
a separate copy of the message for each address may take substantially longer
than sending a single copy with many recipients (for which VERP cannot be
used).


51.7 Virtual domains
--------------------

The phrase virtual domain is unfortunately used with two rather different
meanings:

  * A domain for which there are no real mailboxes; all valid local parts are
    aliases for other email addresses. Common examples are organizational
    top-level domains and "vanity" domains.

  * One of a number of independent domains that are all handled by the same
    host, with mailboxes on that host, but where the mailbox owners do not
    necessarily have login accounts on that host.

The first usage is probably more common, and does seem more "virtual" than the
second. This kind of domain can be handled in Exim with a straightforward
aliasing router. One approach is to create a separate alias file for each
virtual domain. Exim can test for the existence of the alias file to determine
whether the domain exists. The dsearch lookup type is useful here, leading to a
router of this form:

virtual:
  driver = redirect
  domains = dsearch;/etc/mail/virtual
  data = ${lookup{$local_part}lsearch{/etc/mail/virtual/$domain_data}}
  no_more

The domains option specifies that the router is to be skipped, unless there is
a file in the /etc/mail/virtual directory whose name is the same as the domain
that is being processed. The dsearch lookup used results in an untainted
version of $domain being placed into the $domain_data variable.

When the router runs, it looks up the local part in the file to find a new
address (or list of addresses). The no_more setting ensures that if the lookup
fails (leading to data being an empty string), Exim gives up on the address
without trying any subsequent routers.

This one router can handle all the virtual domains because the alias filenames
follow a fixed pattern. Permissions can be arranged so that appropriate people
can edit the different alias files. A successful aliasing operation results in
a new envelope recipient address, which is then routed from scratch.

The other kind of "virtual" domain can also be handled in a straightforward
way. One approach is to create a file for each domain containing a list of
valid local parts, and use it in a router like this:

my_domains:
  driver = accept
  domains = dsearch;/etc/mail/domains
  local_parts = lsearch;/etc/mail/domains/$domain
  transport = my_mailboxes

The address is accepted if there is a file for the domain, and the local part
can be found in the file. The domains option is used to check for the file's
existence because domains is tested before the local_parts option (see section
3.12). You cannot use require_files, because that option is tested after 
local_parts. The transport is as follows:

my_mailboxes:
  driver = appendfile
  file = /var/mail/$domain_data/$local_part_data
  user = mail

This uses a directory of mailboxes for each domain. The user setting is
required, to specify which uid is to be used for writing to the mailboxes.

The configuration shown here is just one example of how you might support this
requirement. There are many other ways this kind of configuration can be set
up, for example, by using a database instead of separate files to hold all the
information about the domains.


51.8 Multiple user mailboxes
----------------------------

Heavy email users often want to operate with multiple mailboxes, into which
incoming mail is automatically sorted. A popular way of handling this is to
allow users to use multiple sender addresses, so that replies can easily be
identified. Users are permitted to add prefixes or suffixes to their local
parts for this purpose. The wildcard facility of the generic router options 
local_part_prefix and local_part_suffix can be used for this. For example,
consider this router:

userforward:
  driver = redirect
  check_local_user
  file = $home/.forward
  local_part_suffix = -*
  local_part_suffix_optional
  allow_filter

It runs a user's .forward file for all local parts of the form username-*.
Within the filter file the user can distinguish different cases by testing the
variable $local_part_suffix. For example:

if $local_part_suffix contains -special then
save /home/$local_part_data/Mail/special
endif

If the filter file does not exist, or does not deal with such addresses, they
fall through to subsequent routers, and, assuming no subsequent use of the 
local_part_suffix option is made, they presumably fail. Thus, users have
control over which suffixes are valid.

Alternatively, a suffix can be used to trigger the use of a different .forward
file - which is the way a similar facility is implemented in another MTA:

userforward:
  driver = redirect
  check_local_user
  local_part_suffix = -*
  local_part_suffix_optional
  file = ${lookup {.forward$local_part_suffix} dsearch,ret=full {$home} {$value}fail}
  allow_filter

If there is no suffix, .forward is used; if the suffix is -special, for
example, .forward-special is used. Once again, if the appropriate file does not
exist, or does not deal with the address, it is passed on to subsequent
routers, which could, if required, look for an unqualified .forward file to use
as a default.


51.9 Simplified vacation processing
-----------------------------------

The traditional way of running the vacation program is for a user to set up a
pipe command in a .forward file (see section 22.6 for syntax details). This is
prone to error by inexperienced users. There are two features of Exim that can
be used to make this process simpler for users:

  * A local part prefix such as "vacation-" can be specified on a router which
    can cause the message to be delivered directly to the vacation program, or
    alternatively can use Exim's autoreply transport. The contents of a user's
    .forward file are then much simpler. For example:

    spqr, vacation-spqr

  * The require_files generic router option can be used to trigger a vacation
    delivery by checking for the existence of a certain file in the user's home
    directory. The unseen generic option should also be used, to ensure that
    the original delivery also proceeds. In this case, all the user has to do
    is to create a file called, say, .vacation, containing a vacation message.

Another advantage of both these methods is that they both work even when the
use of arbitrary pipes by users is locked out.


51.10 Taking copies of mail
---------------------------

Some installations have policies that require archive copies of all messages to
be made. A single copy of each message can easily be taken by an appropriate
command in a system filter, which could, for example, use a different file for
each day's messages.

There is also a shadow transport mechanism that can be used to take copies of
messages that are successfully delivered by local transports, one copy per
delivery. This could be used, inter alia, to implement automatic notification
of delivery by sites that insist on doing such things.


51.11 Intermittently connected hosts
------------------------------------

It has become quite common (because it is cheaper) for hosts to connect to the
Internet periodically rather than remain connected all the time. The normal
arrangement is that mail for such hosts accumulates on a system that is
permanently connected.

Exim was designed for use on permanently connected hosts, and so it is not
particularly well-suited to use in an intermittently connected environment.
Nevertheless there are some features that can be used.


51.12 Exim on the upstream server host
--------------------------------------

It is tempting to arrange for incoming mail for the intermittently connected
host to remain in Exim's queue until the client connects. However, this
approach does not scale very well. Two different kinds of waiting message are
being mixed up in the same queue - those that cannot be delivered because of
some temporary problem, and those that are waiting for their destination host
to connect. This makes it hard to manage the queue, as well as wasting
resources, because each queue runner scans the entire queue.

A better approach is to separate off those messages that are waiting for an
intermittently connected host. This can be done by delivering these messages
into local files in batch SMTP, "mailstore", or other envelope-preserving
format, from where they are transmitted by other software when their
destination connects. This makes it easy to collect all the mail for one host
in a single directory, and to apply local timeout rules on a per-message basis
if required.

On a very small scale, leaving the mail on Exim's queue can be made to work. If
you are doing this, you should configure Exim with a long retry period for the
intermittent host. For example:

cheshire.wonderland.fict.example    *   F,5d,24h

This stops a lot of failed delivery attempts from occurring, but Exim remembers
which messages it has queued up for that host. Once the intermittent host comes
online, forcing delivery of one message (either by using the -M or -R options,
or by using the ETRN SMTP command (see section 49.2.5) causes all the queued up
messages to be delivered, often down a single SMTP connection. While the host
remains connected, any new messages get delivered immediately.

If the connecting hosts do not have fixed IP addresses, that is, if a host is
issued with a different IP address each time it connects, Exim's retry
mechanisms on the holding host get confused, because the IP address is normally
used as part of the key string for holding retry information. This can be
avoided by unsetting retry_include_ip_address on the smtp transport. Since this
has disadvantages for permanently connected hosts, it is best to arrange a
separate transport for the intermittently connected ones.


51.13 Exim on the intermittently connected client host
------------------------------------------------------

The value of smtp_accept_queue_per_connection should probably be increased, or
even set to zero (that is, disabled) on the intermittently connected host, so
that all incoming messages down a single connection get delivered immediately.

Mail waiting to be sent from an intermittently connected host will probably not
have been routed, because without a connection DNS lookups are not possible.
This means that if a normal queue run is done at connection time, each message
is likely to be sent in a separate SMTP session. This can be avoided by
starting the queue run with a command line option beginning with -qq instead of
-q. In this case, the queue is scanned twice. In the first pass, routing is
done but no deliveries take place. The second pass is a normal queue run; since
all the messages have been previously routed, those destined for the same host
are likely to get sent as multiple deliveries in a single SMTP connection.



===============================================================================
52. USING EXIM AS A NON-QUEUEING CLIENT

On a personal computer, it is a common requirement for all email to be sent to
a "smart host". There are plenty of MUAs that can be configured to operate that
way, for all the popular operating systems. However, there are some MUAs for
Unix-like systems that cannot be so configured: they submit messages using the
command line interface of /usr/sbin/sendmail. Furthermore, utility programs
such as cron submit messages this way.

If the personal computer runs continuously, there is no problem, because it can
run a conventional MTA that handles delivery to the smart host, and deal with
any delays via its queueing mechanism. However, if the computer does not run
continuously or runs different operating systems at different times, queueing
email is not desirable.

There is therefore a requirement for something that can provide the /usr/sbin/
sendmail interface but deliver messages to a smart host without any queueing or
retrying facilities. Furthermore, the delivery to the smart host should be
synchronous, so that if it fails, the sending MUA is immediately informed. In
other words, we want something that extends an MUA that submits to a local MTA
via the command line so that it behaves like one that submits to a remote smart
host using TCP/SMTP.

There are a number of applications (for example, there is one called ssmtp)
that do this job. However, people have found them to be lacking in various
ways. For instance, you might want to allow aliasing and forwarding to be done
before sending a message to the smart host.

Exim already had the necessary infrastructure for doing this job. Just a few
tweaks were needed to make it behave as required, though it is somewhat of an
overkill to use a fully-featured MTA for this purpose.

There is a Boolean global option called mua_wrapper, defaulting false. Setting 
mua_wrapper true causes Exim to run in a special mode where it assumes that it
is being used to "wrap" a command-line MUA in the manner just described. As
well as setting mua_wrapper, you also need to provide a compatible router and
transport configuration. Typically there will be just one router and one
transport, sending everything to a smart host.

When run in MUA wrapping mode, the behaviour of Exim changes in the following
ways:

  * A daemon cannot be run, nor will Exim accept incoming messages from inetd.
    In other words, the only way to submit messages is via the command line.

  * Each message is synchronously delivered as soon as it is received (-odi is
    assumed). All queueing options (queue_only, queue_smtp_domains, control in
    an ACL, etc.) are quietly ignored. The Exim reception process does not
    finish until the delivery attempt is complete. If the delivery is
    successful, a zero return code is given.

  * Address redirection is permitted, but the final routing for all addresses
    must be to the same remote transport, and to the same list of hosts.
    Furthermore, the return address (envelope sender) must be the same for all
    recipients, as must any added or deleted header lines. In other words, it
    must be possible to deliver the message in a single SMTP transaction,
    however many recipients there are.

  * If these conditions are not met, or if routing any address results in a
    failure or defer status, or if Exim is unable to deliver all the recipients
    successfully to one of the smart hosts, delivery of the entire message
    fails.

  * Because no queueing is allowed, all failures are treated as permanent;
    there is no distinction between 4xx and 5xx SMTP response codes from the
    smart host. Furthermore, because only a single yes/no response can be given
    to the caller, it is not possible to deliver to some recipients and not
    others. If there is an error (temporary or permanent) for any recipient,
    all are failed.

  * If more than one smart host is listed, Exim will try another host after a
    connection failure or a timeout, in the normal way. However, if this kind
    of failure happens for all the hosts, the delivery fails.

  * When delivery fails, an error message is written to the standard error
    stream (as well as to Exim's log), and Exim exits to the caller with a
    return code value 1. The message is expunged from Exim's spool files. No
    bounce messages are ever generated.

  * No retry data is maintained, and any retry rules are ignored.

  * A number of Exim options are overridden: deliver_drop_privilege is forced
    true, max_rcpt in the smtp transport is forced to "unlimited", 
    remote_max_parallel is forced to one, and fallback hosts are ignored.

The overall effect is that Exim makes a single synchronous attempt to deliver
the message, failing if there is any kind of problem. Because no local
deliveries are done and no daemon can be run, Exim does not need root
privilege. It should be possible to run it setuid to exim instead of setuid to 
root. See section 56.3 for a general discussion about the advantages and
disadvantages of running without root privilege.



===============================================================================
53. LOG FILES

Exim writes three different logs, referred to as the main log, the reject log,
and the panic log:

  * The main log records the arrival of each message and each delivery in a
    single line in each case. The format is as compact as possible, in an
    attempt to keep down the size of log files. Two-character flag sequences
    make it easy to pick out these lines. A number of other events are recorded
    in the main log. Some of them are optional, in which case the log_selector
    option controls whether they are included or not. A Perl script called 
    eximstats, which does simple analysis of main log files, is provided in the
    Exim distribution (see section 54.7).

  * The reject log records information from messages that are rejected as a
    result of a configuration option (that is, for policy reasons). The first
    line of each rejection is a copy of the line that is also written to the
    main log. Then, if the message's header has been read at the time the log
    is written, its contents are written to this log. Only the original header
    lines are available; header lines added by ACLs are not logged. You can use
    the reject log to check that your policy controls are working correctly; on
    a busy host this may be easier than scanning the main log for rejection
    messages. You can suppress the writing of the reject log by setting 
    write_rejectlog false.

  * When certain serious errors occur, Exim writes entries to its panic log. If
    the error is sufficiently disastrous, Exim bombs out afterwards. Panic log
    entries are usually written to the main log as well, but can get lost amid
    the mass of other entries. The panic log should be empty under normal
    circumstances. It is therefore a good idea to check it (or to have a cron
    script check it) regularly, in order to become aware of any problems. When
    Exim cannot open its panic log, it tries as a last resort to write to the
    system log (syslog). This is opened with LOG_PID+LOG_CONS and the facility
    code of LOG_MAIL. The message itself is written at priority LOG_CRIT.

Every log line starts with a timestamp, in the format shown in the following
example. Note that many of the examples shown in this chapter are line-wrapped.
In the log file, this would be all on one line:

2001-09-16 16:09:47 SMTP connection from [127.0.0.1] closed
  by QUIT

By default, the timestamps are in the local timezone. There are two ways of
changing this:

  * You can set the timezone option to a different time zone; in particular, if
    you set

    timezone = UTC

    the timestamps will be in UTC (aka GMT).

  * If you set log_timezone true, the time zone is added to the timestamp, for
    example:

    2003-04-25 11:17:07 +0100 Start queue run: pid=12762

Exim does not include its process id in log lines by default, but you can
request that it does so by specifying the "pid" log selector (see section 53.15
). When this is set, the process id is output, in square brackets, immediately
after the time and date.


53.1 Where the logs are written
-------------------------------

The logs may be written to local files, or to syslog, or both. However, it
should be noted that many syslog implementations use UDP as a transport, and
are therefore unreliable in the sense that messages are not guaranteed to
arrive at the loghost, nor is the ordering of messages necessarily maintained.
It has also been reported that on large log files (tens of megabytes) you may
need to tweak syslog to prevent it syncing the file with each write - on Linux
this has been seen to make syslog take 90% plus of CPU time.

The destination for Exim's logs is configured by setting LOG_FILE_PATH in Local
/Makefile or by setting log_file_path in the runtime configuration. This latter
string is expanded, so it can contain, for example, references to the host
name:

log_file_path = /var/log/$primary_hostname/exim_%slog

It is generally advisable, however, to set the string in Local/Makefile rather
than at runtime, because then the setting is available right from the start of
Exim's execution. Otherwise, if there's something it wants to log before it has
read the configuration file (for example, an error in the configuration file)
it will not use the path you want, and may not be able to log at all.

The value of LOG_FILE_PATH or log_file_path is a colon-separated list,
currently limited to at most two items. This is one option where the facility
for changing a list separator may not be used. The list must always be
colon-separated. If an item in the list is "syslog" then syslog is used;
otherwise the item must either be an absolute path, containing "%s" at the
point where "main", "reject", or "panic" is to be inserted, or be empty,
implying the use of a default path.

When Exim encounters an empty item in the list, it searches the list defined by
LOG_FILE_PATH, and uses the first item it finds that is neither empty nor
"syslog". This means that an empty item in log_file_path can be used to mean
"use the path specified at build time". If no such item exists, log files are
written in the log subdirectory of the spool directory. This is equivalent to
the configuration file setting:

log_file_path = $spool_directory/log/%slog

If you do not specify anything at build time or runtime, or if you unset the
option at runtime (i.e. "log_file_path = "), that is where the logs are
written.

A log file path may also contain "%D" or "%M" if datestamped log filenames are
in use - see section 53.3 below.

Here are some examples of possible Makefile settings:

LOG_FILE_PATH=syslog                     syslog only
LOG_FILE_PATH=:syslog                    syslog and default path
LOG_FILE_PATH=syslog : /usr/log/exim_%s  syslog and specified path
LOG_FILE_PATH=/usr/log/exim_%s           specified path only

If there are more than two paths in the list, the first is used and a panic
error is logged.


53.2 Logging to local files that are periodically "cycled"
----------------------------------------------------------

Some operating systems provide centralized and standardized methods for cycling
log files. For those that do not, a utility script called exicyclog is provided
(see section 54.6). This renames and compresses the main and reject logs each
time it is called. The maximum number of old logs to keep can be set. It is
suggested this script is run as a daily cron job.

An Exim delivery process opens the main log when it first needs to write to it,
and it keeps the file open in case subsequent entries are required - for
example, if a number of different deliveries are being done for the same
message. However, remote SMTP deliveries can take a long time, and this means
that the file may be kept open long after it is renamed if exicyclog or
something similar is being used to rename log files on a regular basis. To
ensure that a switch of log files is noticed as soon as possible, Exim calls
stat() on the main log's name before reusing an open file, and if the file does
not exist, or its inode has changed, the old file is closed and Exim tries to
open the main log from scratch. Thus, an old log file may remain open for quite
some time, but no Exim processes should write to it once it has been renamed.


53.3 Datestamped log files
--------------------------

Instead of cycling the main and reject log files by renaming them periodically,
some sites like to use files whose names contain a datestamp, for example,
mainlog-20031225. The datestamp is in the form yyyymmdd or yyyymm. Exim has
support for this way of working. It is enabled by setting the log_file_path
option to a path that includes "%D" or "%M" at the point where the datestamp is
required. For example:

log_file_path = /var/spool/exim/log/%slog-%D
log_file_path = /var/log/exim-%s-%D.log
log_file_path = /var/spool/exim/log/%D-%slog
log_file_path = /var/log/exim/%s.%M

As before, "%s" is replaced by "main" or "reject"; the following are examples
of names generated by the above examples:

/var/spool/exim/log/mainlog-20021225
/var/log/exim-reject-20021225.log
/var/spool/exim/log/20021225-mainlog
/var/log/exim/main.200212

When this form of log file is specified, Exim automatically switches to new
files at midnight. It does not make any attempt to compress old logs; you will
need to write your own script if you require this. You should not run exicyclog
with this form of logging.

The location of the panic log is also determined by log_file_path, but it is
not datestamped, because rotation of the panic log does not make sense. When
generating the name of the panic log, "%D" or "%M" are removed from the string.
In addition, if it immediately follows a slash, a following non-alphanumeric
character is removed; otherwise a preceding non-alphanumeric character is
removed. Thus, the four examples above would give these panic log names:

/var/spool/exim/log/paniclog
/var/log/exim-panic.log
/var/spool/exim/log/paniclog
/var/log/exim/panic


53.4 Logging to syslog
----------------------

The use of syslog does not change what Exim logs or the format of its messages,
except in one respect. If syslog_timestamp is set false, the timestamps on
Exim's log lines are omitted when these lines are sent to syslog. Apart from
that, the same strings are written to syslog as to log files. The syslog
"facility" is set to LOG_MAIL, and the program name to "exim" by default, but
you can change these by setting the syslog_facility and syslog_processname
options, respectively. If Exim was compiled with SYSLOG_LOG_PID set in Local/
Makefile (this is the default in src/EDITME), then, on systems that permit it
(all except ULTRIX), the LOG_PID flag is set so that the syslog() call adds the
pid as well as the time and host name to each line. The three log streams are
mapped onto syslog priorities as follows:

  * mainlog is mapped to LOG_INFO

  * rejectlog is mapped to LOG_NOTICE

  * paniclog is mapped to LOG_ALERT

Many log lines are written to both mainlog and rejectlog, and some are written
to both mainlog and paniclog, so there will be duplicates if these are routed
by syslog to the same place. You can suppress this duplication by setting 
syslog_duplication false.

Exim's log lines can sometimes be very long, and some of its rejectlog entries
contain multiple lines when headers are included. To cope with both these
cases, entries written to syslog are split into separate syslog() calls at each
internal newline, and also after a maximum of 870 data characters. (This allows
for a total syslog line length of 1024, when additions such as timestamps are
added.) If you are running a syslog replacement that can handle lines longer
than the 1024 characters allowed by RFC 3164, you should set

SYSLOG_LONG_LINES=yes

in Local/Makefile before building Exim. That stops Exim from splitting long
lines, but it still splits at internal newlines in reject log entries.

To make it easy to re-assemble split lines later, each component of a split
entry starts with a string of the form [<n>/<m>] or [<n>\<m>] where <n> is the
component number and <m> is the total number of components in the entry. The /
delimiter is used when the line was split because it was too long; if it was
split because of an internal newline, the \ delimiter is used. For example,
supposing the length limit to be 50 instead of 870, the following would be the
result of a typical rejection message to mainlog (LOG_INFO), each line in
addition being preceded by the time, host name, and pid as added by syslog:

[1/5] 2002-09-16 16:09:43 16RdAL-0006pc-00 rejected from
[2/5]  [127.0.0.1] (ph10): syntax error in 'From' header
[3/5]  when scanning for sender: missing or malformed lo
[4/5] cal part in "<>" (envelope sender is <ph10@cam.exa
[5/5] mple>)

The same error might cause the following lines to be written to "rejectlog"
(LOG_NOTICE):

[1/18] 2002-09-16 16:09:43 16RdAL-0006pc-00 rejected fro
[2/18] m [127.0.0.1] (ph10): syntax error in 'From' head
[3/18] er when scanning for sender: missing or malformed
[4/18]  local part in "<>" (envelope sender is <ph10@cam
[5\18] .example>)
[6\18] Recipients: ph10@some.domain.cam.example
[7\18] P Received: from [127.0.0.1] (ident=ph10)
[8\18]        by xxxxx.cam.example with smtp (Exim 4.00)
[9\18]        id 16RdAL-0006pc-00
[10/18]        for ph10@cam.example; Mon, 16 Sep 2002 16:
[11\18] 09:43 +0100
[12\18] F From: <>
[13\18]   Subject: this is a test header
[18\18]   X-something: this is another header
[15/18] I Message-Id: <E16RdAL-0006pc-00@xxxxx.cam.examp
[16\18] le>
[17\18] B Bcc:
[18/18]   Date: Mon, 16 Sep 2002 16:09:43 +0100

Log lines that are neither too long nor contain newlines are written to syslog
without modification.

If only syslog is being used, the Exim monitor is unable to provide a log tail
display, unless syslog is routing mainlog to a file on the local host and the
environment variable EXIMON_LOG_FILE_PATH is set to tell the monitor where it
is.


53.5 Log line flags
-------------------

One line is written to the main log for each message received, and for each
successful, unsuccessful, and delayed delivery. These lines can readily be
picked out by the distinctive two-character flags that immediately follow the
timestamp. The flags are:

    <= message arrival
    (= message fakereject
    => normal message delivery
    -> additional address in same delivery
    >> cutthrough message delivery
    *> delivery suppressed by -N
    ** delivery failed; address bounced
    == delivery deferred; temporary problem


53.6 Logging message reception
------------------------------

The format of the single-line entry in the main log that is written for every
message received is shown in the basic example below, which is split over
several lines in order to fit it on the page:

2002-10-31 08:57:53 16ZCW1-0005MB-00 <= kryten@dwarf.fict.example
  H=mailer.fict.example [192.168.123.123] U=exim
  P=smtp S=5678 id=<incoming message id>

The address immediately following "<=" is the envelope sender address. A bounce
message is shown with the sender address "<>", and if it is locally generated,
this is followed by an item of the form

R=<message id>

which is a reference to the message that caused the bounce to be sent.

For messages from other hosts, the H and U fields identify the remote host and
record the RFC 1413 identity of the user that sent the message, if one was
received. The number given in square brackets is the IP address of the sending
host. If there is a single, unparenthesized host name in the H field, as above,
it has been verified to correspond to the IP address (see the host_lookup
option). If the name is in parentheses, it was the name quoted by the remote
host in the SMTP HELO or EHLO command, and has not been verified. If
verification yields a different name to that given for HELO or EHLO, the
verified name appears first, followed by the HELO or EHLO name in parentheses.

Misconfigured hosts (and mail forgers) sometimes put an IP address, with or
without brackets, in the HELO or EHLO command, leading to entries in the log
containing text like these examples:

H=(10.21.32.43) [192.168.8.34]
H=([10.21.32.43]) [192.168.8.34]

This can be confusing. Only the final address in square brackets can be relied
on.

For locally generated messages (that is, messages not received over TCP/IP),
the H field is omitted, and the U field contains the login name of the caller
of Exim.

For all messages, the P field specifies the protocol used to receive the
message. This is the value that is stored in $received_protocol. In the case of
incoming SMTP messages, the value indicates whether or not any SMTP extensions
(ESMTP), encryption, or authentication were used. If the SMTP session was
encrypted, there is an additional X field that records the cipher suite that
was used.

The protocol is set to "esmtpsa" or "esmtpa" for messages received from hosts
that have authenticated themselves using the SMTP AUTH command. The first value
is used when the SMTP connection was encrypted ("secure"). In this case there
is an additional item A= followed by the name of the authenticator that was
used. If an authenticated identification was set up by the authenticator's 
server_set_id option, this is logged too, separated by a colon from the
authenticator name.

The id field records the existing message id, if present. The size of the
received message is given by the S field. When the message is delivered,
headers may be removed or added, so that the size of delivered copies of the
message may not correspond with this value (and indeed may be different to each
other).

The log_selector option can be used to request the logging of additional data
when a message is received. See section 53.15 below.


53.7 Logging deliveries
-----------------------

The format of the single-line entry in the main log that is written for every
delivery is shown in one of the examples below, for local and remote
deliveries, respectively. Each example has been split into multiple lines in
order to fit it on the page:

2002-10-31 08:59:13 16ZCW1-0005MB-00 => marv
  <marv@hitch.fict.example> R=localuser T=local_delivery
2002-10-31 09:00:10 16ZCW1-0005MB-00 =>
  monk@holistic.fict.example R=dnslookup T=remote_smtp
  H=holistic.fict.example [192.168.234.234]

For ordinary local deliveries, the original address is given in angle brackets
after the final delivery address, which might be a pipe or a file. If
intermediate address(es) exist between the original and the final address, the
last of these is given in parentheses after the final address. The R and T
fields record the router and transport that were used to process the address.

If SMTP AUTH was used for the delivery there is an additional item A= followed
by the name of the authenticator that was used. If an authenticated
identification was set up by the authenticator's client_set_id option, this is
logged too, as a second colon-separated list item. Optionally (see the 
smtp_mailauth log_selector) there may be a third list item.

If a shadow transport was run after a successful local delivery, the log line
for the successful delivery has an item added on the end, of the form

ST=<shadow transport name>

If the shadow transport did not succeed, the error message is put in
parentheses afterwards.

When more than one address is included in a single delivery (for example, two
SMTP RCPT commands in one transaction) the second and subsequent addresses are
flagged with "->" instead of "=>". When two or more messages are delivered down
a single SMTP connection, an asterisk follows the remote IP address (and port
if enabled) in the log lines for the second and subsequent messages. When two
or more messages are delivered down a single TLS connection, the DNS and some
TLS-related information logged for the first message delivered will not be
present in the log lines for the second and subsequent messages. TLS cipher
information is still available.

When delivery is done in cutthrough mode it is flagged with ">>" and the log
line precedes the reception line, since cutthrough waits for a possible
rejection from the destination in case it can reject the sourced item.

The generation of a reply message by a filter file gets logged as a "delivery"
to the addressee, preceded by ">".

The log_selector option can be used to request the logging of additional data
when a message is delivered. See section 53.15 below.


53.8 Discarded deliveries
-------------------------

When a message is discarded as a result of the command "seen finish" being
obeyed in a filter file which generates no deliveries, a log entry of the form

2002-12-10 00:50:49 16auJc-0001UB-00 => discarded
  <low.club@bridge.example> R=userforward

is written, to record why no deliveries are logged. When a message is discarded
because it is aliased to ":blackhole:" the log line is like this:

1999-03-02 09:44:33 10HmaX-0005vi-00 => :blackhole:
  <hole@nowhere.example> R=blackhole_router


53.9 Deferred deliveries
------------------------

When a delivery is deferred, a line of the following form is logged:

2002-12-19 16:20:23 16aiQz-0002Q5-00 == marvin@endrest.example
  R=dnslookup T=smtp defer (146): Connection refused

In the case of remote deliveries, the error is the one that was given for the
last IP address that was tried. Details of individual SMTP failures are also
written to the log, so the above line would be preceded by something like

2002-12-19 16:20:23 16aiQz-0002Q5-00 Failed to connect to
  mail1.endrest.example [192.168.239.239]: Connection refused

When a deferred address is skipped because its retry time has not been reached,
a message is written to the log, but this can be suppressed by setting an
appropriate value in log_selector.


53.10 Delivery failures
-----------------------

If a delivery fails because an address cannot be routed, a line of the
following form is logged:

1995-12-19 16:20:23 0tRiQz-0002Q5-00 ** jim@trek99.example
  <jim@trek99.example>: unknown mail domain

If a delivery fails at transport time, the router and transport are shown, and
the response from the remote host is included, as in this example:

2002-07-11 07:14:17 17SXDU-000189-00 ** ace400@pb.example
  R=dnslookup T=remote_smtp: SMTP error from remote mailer
  after pipelined RCPT TO:<ace400@pb.example>: host
  pbmail3.py.example [192.168.63.111]: 553 5.3.0
  <ace400@pb.example>...Addressee unknown

The word "pipelined" indicates that the SMTP PIPELINING extension was being
used. See hosts_avoid_esmtp in the smtp transport for a way of disabling
PIPELINING. The log lines for all forms of delivery failure are flagged with
"**".


53.11 Fake deliveries
---------------------

If a delivery does not actually take place because the -N option has been used
to suppress it, a normal delivery line is written to the log, except that "=>"
is replaced by "*>".


53.12 Completion
----------------

A line of the form

2002-10-31 09:00:11 16ZCW1-0005MB-00 Completed

is written to the main log when a message is about to be removed from the spool
at the end of its processing.


53.13 Summary of Fields in Log Lines
------------------------------------

A summary of the field identifiers that are used in log lines is shown in the
following table:

A           authenticator name (and optional id and sender)
C           SMTP confirmation on delivery
Ci          connection identifier
            command list for "no mail in SMTP session"
CV          certificate verification status
D           duration of "no mail in SMTP session"
DKIM        domain verified in incoming message
DN          distinguished name from peer certificate
DS          DNSSEC secured lookups
DT          on =>, == and ** lines: time taken for, or to attempt, a delivery
F           sender address (on delivery lines)
H           host name and IP address
I           local interface used
id          message id (from header) for incoming message
K           CHUNKING extension used
L           on <= and => lines: PIPELINING extension used
M8S         8BITMIME status for incoming message
P           on <= lines: protocol used
            on => and ** lines: return path
PRDR        PRDR extension used
PRX         on <= and => lines: proxy address
Q           alternate queue name
QT          on => lines: time spent on queue so far
            on "Completed" lines: time spent on queue
R           on <= lines: reference for local bounce
            on =>  >> ** and == lines: router name
RT          on <= lines: time taken for reception
S           size of message in bytes
SNI         server name indication from TLS client hello
ST          shadow transport name
T           on <= lines: message subject (topic)
TFO         connection took advantage of TCP Fast Open
            on => ** and == lines: transport name
U           local user or RFC 1413 identity
X           TLS cipher suite


53.14 Other log entries
-----------------------

Various other types of log entry are written from time to time. Most should be
self-explanatory. Among the more common are:

  * retry time not reached  An address previously suffered a temporary error
    during routing or local delivery, and the time to retry has not yet
    arrived. This message is not written to an individual message log file
    unless it happens during the first delivery attempt.

  * retry time not reached for any host  An address previously suffered
    temporary errors during remote delivery, and the retry time has not yet
    arrived for any of the hosts to which it is routed.

  * spool file locked  An attempt to deliver a message cannot proceed because
    some other Exim process is already working on the message. This can be
    quite common if queue running processes are started at frequent intervals.
    The exiwhat utility script can be used to find out what Exim processes are
    doing.

  * error ignored  There are several circumstances that give rise to this
    message:

     1. Exim failed to deliver a bounce message whose age was greater than 
        ignore_bounce_errors_after. The bounce was discarded.

     2. A filter file set up a delivery using the "noerror" option, and the
        delivery failed. The delivery was discarded.

     3. A delivery set up by a router configured with

            errors_to = <>

        failed. The delivery was discarded.

  * DKIM: d=  Verbose results of a DKIM verification attempt, if enabled for
    logging and the message has a DKIM signature header.


53.15 Reducing or increasing what is logged
-------------------------------------------

By setting the log_selector global option, you can disable some of Exim's
default logging to the main log, or you can request additional logging. The
value of log_selector is made up of names preceded by plus or minus characters.
For example:

log_selector = +arguments -retry_defer

The list of optional log items is in the following table, with the default
selection marked by asterisks:

    "8bitmime"                      received 8BITMIME status
    "acl_warn_skipped"            * skipped warn statement in ACL
    "address_rewrite"               address rewriting
    "all_parents"                   all parents in => lines
    "arguments"                     command line arguments
    "connection_id"                 connection identifier
    "connection_reject"           * connection rejections
    "delay_delivery"              * immediate delivery delayed
    "deliver_time"                  time taken to attempt delivery
    "delivery_size"                 add "S="nnn to => lines
    "dkim"                        * DKIM verified domain on <= lines
    "dkim_verbose"                  separate full DKIM verification result
                                    line, per signature
    "dnslist_defer"               * defers of DNS list (aka RBL) lookups
    "dnssec"                        DNSSEC secured lookups
    "etrn"                        * ETRN commands
    "host_lookup_failed"          * as it says
    "ident_timeout"                 timeout for ident connection
    "incoming_interface"            local interface & port on <= and => lines
    "incoming_port"                 remote port on <= lines
    "lost_incoming_connection"    * as it says (includes timeouts)
    "millisec"                      millisecond timestamps and RT,QT,DT,D times
    "msg_id"                      * on <= lines, Message-ID: header value
    "msg_id_created"                on <= lines, Message-ID: header value when
                                    one had to be added
    "outgoing_interface"            local interface on => lines
    "outgoing_port"                 add remote port to => lines
    "queue_run"                   * start and end queue runs
    "queue_time"                    time on queue for one recipient
    "queue_time_exclusive"          exclude recieve time from QT times
    "queue_time_overall"            time on queue for whole message
    "pid"                           Exim process id
    "pipelining"                    PIPELINING use, on <= and => lines
    "proxy"                         proxy address on <= and => lines
    "receive_time"                  time taken to receive message
    "received_recipients"           recipients on <= lines
    "received_sender"               sender on <= lines
    "rejected_header"             * header contents on reject log
    "retry_defer"                 * "retry time not reached"
    "return_path_on_delivery"       put return path on => and ** lines
    "sender_on_delivery"            add sender to => lines
    "sender_verify_fail"          * sender verification failures
    "size_reject"                 * rejection because too big
    "skip_delivery"               * delivery skipped in a queue run
    "smtp_confirmation"           * SMTP confirmation on => lines
    "smtp_connection"               incoming SMTP connections
    "smtp_incomplete_transaction"   incomplete SMTP transactions
    "smtp_mailauth"                 AUTH argument to MAIL commands
    "smtp_no_mail"                  session with no MAIL commands
    "smtp_protocol_error"           SMTP protocol errors
    "smtp_syntax_error"             SMTP syntax errors
    "subject"                       contents of Subject: on <= lines
    "tls_certificate_verified"    * certificate verification status
    "tls_cipher"                  * TLS cipher suite on <= and => lines
    "tls_peerdn"                    TLS peer DN on <= and => lines
    "tls_resumption"                append * to cipher field
    "tls_sni"                       TLS SNI on <= lines
    "unknown_in_list"               DNS lookup failed in list match
    "all"                           all of the above

See also the slow_lookup_log main configuration option, section 14.4

More details on each of these items follows:

  * 8bitmime: This causes Exim to log any 8BITMIME status of received messages,
    which may help in tracking down interoperability issues with ancient MTAs
    that are not 8bit clean. This is added to the "<=" line, tagged with "M8S="
    and a value of "0", "7" or "8", corresponding to "not given", "7BIT" and
    "8BITMIME" respectively.

  * acl_warn_skipped: When an ACL warn statement is skipped because one of its
    conditions cannot be evaluated, a log line to this effect is written if
    this log selector is set.

  * address_rewrite: This applies both to global rewrites and per-transport
    rewrites, but not to rewrites in filters run as an unprivileged user
    (because such users cannot access the log).

  * all_parents: Normally only the original and final addresses are logged on
    delivery lines; with this selector, intermediate parents are given in
    parentheses between them.

  * arguments: This causes Exim to write the arguments with which it was called
    to the main log, preceded by the current working directory. This is a
    debugging feature, added to make it easier to find out how certain MUAs
    call /usr/sbin/sendmail. The logging does not happen if Exim has given up
    root privilege because it was called with the -C or -D options. Arguments
    that are empty or that contain white space are quoted. Non-printing
    characters are shown as escape sequences. This facility cannot log
    unrecognized arguments, because the arguments are checked before the
    configuration file is read. The only way to log such cases is to interpose
    a script such as util/logargs.sh between the caller and Exim.

  * 
    connection_identifier: An identifier for the accepted connection is added
    to connection start and end lines and to message accept lines. The
    identifier is tagged by Ci=. The value is PID-based, so will reset on
    reboot and will wrap.

  * connection_reject: A log entry is written whenever an incoming SMTP
    connection is rejected, for whatever reason.

  * delay_delivery: A log entry is written whenever a delivery process is not
    started for an incoming message because the load is too high or too many
    messages were received on one connection. Logging does not occur if no
    delivery process is started because queue_only is set or -odq was used.

  * deliver_time: For each delivery, the amount of real time it has taken to
    perform the actual delivery is logged as DT=<time>, for example, "DT=1s".
    If millisecond logging is enabled, short times will be shown with greater
    precision, eg. "DT=0.304s".

  * delivery_size: For each delivery, the size of message delivered is added to
    the "=>" line, tagged with S=.

  * dkim: For message acceptance log lines, when an DKIM signature in the
    header verifies successfully a tag of DKIM is added, with one of the
    verified domains.

  * dkim_verbose: A log entry is written for each attempted DKIM verification.

  * dnslist_defer: A log entry is written if an attempt to look up a host in a
    DNS black list suffers a temporary error.

  * dnssec: For message acceptance and (attempted) delivery log lines, when dns
    lookups gave secure results a tag of DS is added. For acceptance this
    covers the reverse and forward lookups for host name verification. It does
    not cover helo-name verification. For delivery this covers the SRV, MX, A
    and/or AAAA lookups.

  * etrn: Every valid ETRN command that is received is logged, before the ACL
    is run to determine whether or not it is actually accepted. An invalid ETRN
    command, or one received within a message transaction is not logged by this
    selector (see smtp_syntax_error and smtp_protocol_error).

  * host_lookup_failed: When a lookup of a host's IP addresses fails to find
    any addresses, or when a lookup of an IP address fails to find a host name,
    a log line is written. This logging does not apply to direct DNS lookups
    when routing email addresses, but it does apply to "byname" lookups.

  * ident_timeout: A log line is written whenever an attempt to connect to a
    client's ident port times out.

  * incoming_interface: The interface on which a message was received is added
    to the "<=" line as an IP address in square brackets, tagged by I= and
    followed by a colon and the port number. The local interface and port are
    also added to other SMTP log lines, for example, "SMTP connection from", to
    rejection lines, and (despite the name) to outgoing "=>", "->", "==" and
    "**" lines. The latter can be disabled by turning off the 
    outgoing_interface option.

  * proxy: The internal (closest to the system running Exim) IP address of the
    proxy, tagged by PRX=, on the "<=" line for a message accepted on a proxied
    connection or the "=>" line for a message delivered on a proxied
    connection. See 59.1 for more information.

  * incoming_port: The remote port number from which a message was received is
    added to log entries and Received: header lines, following the IP address
    in square brackets, and separated from it by a colon. This is implemented
    by changing the value that is put in the $sender_fullhost and
    $sender_rcvhost variables. Recording the remote port number has become more
    important with the widening use of NAT (see RFC 2505).

  * lost_incoming_connection: A log line is written when an incoming SMTP
    connection is unexpectedly dropped.

  * millisec: Timestamps have a period and three decimal places of finer
    granularity appended to the seconds value.

  * msg_id: The value of the Message-ID: header.

  * msg_id_created: The value of the Message-ID: header, when one had to be
    created. This will be either because the message is a bounce, or was
    submitted locally (submission mode) without one. The field identifier will
    have an asterix appended: "id*=".

  * outgoing_interface: If incoming_interface is turned on, then the interface
    on which a message was sent is added to delivery lines as an I= tag
    followed by IP address in square brackets. You can disable this by turning
    off the outgoing_interface option.

  * outgoing_port: The remote port number is added to delivery log lines (those
    containing => tags) following the IP address. The local port is also added
    if incoming_interface and outgoing_interface are both enabled. This option
    is not included in the default setting, because for most ordinary
    configurations, the remote port number is always 25 (the SMTP port), and
    the local port is a random ephemeral port.

  * pid: The current process id is added to every log line, in square brackets,
    immediately after the time and date.

  * pipelining: A field is added to delivery and accept log lines when the
    ESMTP PIPELINING extension was used. The field is a single "L".

    On accept lines, where PIPELINING was offered but not used by the client,
    the field has a minus appended.

    If Exim is built without the DISABLE_PIPE_CONNECT build option accept "L"
    fields have a period appended if the feature was offered but not used, or
    an asterisk appended if used. Delivery "L" fields have an asterisk appended
    if used.

  * queue_run: The start and end of every queue run are logged.

  * queue_time: The amount of time the message has been in the queue on the
    local host is logged as QT=<time> on delivery ("=>") lines, for example,
    "QT=3m45s". If millisecond logging is enabled, short times will be shown
    with greater precision, eg. "QT=1.578s".

  * queue_time_overall: The amount of time the message has been in the queue on
    the local host is logged as QT=<time> on "Completed" lines, for example,
    "QT=3m45s".

  * receive_time: For each message, the amount of real time it has taken to
    perform the reception is logged as RT=<time>, for example, "RT=1s". If
    millisecond logging is enabled, short times will be shown with greater
    precision, eg. "RT=0.204s".

  * received_recipients: The recipients of a message are listed in the main log
    as soon as the message is received. The list appears at the end of the log
    line that is written when a message is received, preceded by the word
    "for". The addresses are listed after they have been qualified, but before
    any rewriting has taken place. Recipients that were discarded by an ACL for
    MAIL or RCPT do not appear in the list.

  * received_sender: The unrewritten original sender of a message is added to
    the end of the log line that records the message's arrival, after the word
    "from" (before the recipients if received_recipients is also set).

  * rejected_header: If a message's header has been received at the time a
    rejection is written to the reject log, the complete header is added to the
    log. Header logging can be turned off individually for messages that are
    rejected by the local_scan() function (see section 46.2).

  * retry_defer: A log line is written if a delivery is deferred because a
    retry time has not yet been reached. However, this "retry time not reached"
    message is always omitted from individual message logs after the first
    delivery attempt.

  * return_path_on_delivery: The return path that is being transmitted with the
    message is included in delivery and bounce lines, using the tag P=. This is
    omitted if no delivery actually happens, for example, if routing fails, or
    if delivery is to /dev/null or to ":blackhole:".

  * sender_on_delivery: The message's sender address is added to every delivery
    and bounce line, tagged by F= (for "from"). This is the original sender
    that was received with the message; it is not necessarily the same as the
    outgoing return path.

  * sender_verify_fail: If this selector is unset, the separate log line that
    gives details of a sender verification failure is not written. Log lines
    for the rejection of SMTP commands contain just "sender verify failed", so
    some detail is lost.

  * size_reject: A log line is written whenever a message is rejected because
    it is too big.

  * skip_delivery: A log line is written whenever a message is skipped during a
    queue run because it another process is already delivering it or because it
    is frozen. The message that is written is either "spool file is locked" or
    "message is frozen".

  * smtp_confirmation: The response to the final "." in the SMTP or LMTP
    dialogue for outgoing messages is added to delivery log lines in the form
    "C="<text>. A number of MTAs (including Exim) return an identifying string
    in this response.

  * smtp_connection: A log line is written whenever an incoming SMTP connection
    is established or closed, unless the connection is from a host that matches
    hosts_connection_nolog. (In contrast, lost_incoming_connection applies only
    when the closure is unexpected.) This applies to connections from local
    processes that use -bs as well as to TCP/IP connections. If a connection is
    dropped in the middle of a message, a log line is always written, whether
    or not this selector is set, but otherwise nothing is written at the start
    and end of connections unless this selector is enabled.

    For TCP/IP connections to an Exim daemon, the current number of connections
    is included in the log message for each new connection, but note that the
    count is reset if the daemon is restarted. Also, because connections are
    closed (and the closure is logged) in subprocesses, the count may not
    include connections that have been closed but whose termination the daemon
    has not yet noticed. Thus, while it is possible to match up the opening and
    closing of connections in the log, the value of the logged counts may not
    be entirely accurate.

  * smtp_incomplete_transaction: When a mail transaction is aborted by RSET,
    QUIT, loss of connection, or otherwise, the incident is logged, and the
    message sender plus any accepted recipients are included in the log line.
    This can provide evidence of dictionary attacks.

  * smtp_no_mail: A line is written to the main log whenever an accepted SMTP
    connection terminates without having issued a MAIL command. This includes
    both the case when the connection is dropped, and the case when QUIT is
    used. It does not include cases where the connection is rejected right at
    the start (by an ACL, or because there are too many connections, or
    whatever). These cases already have their own log lines.

    The log line that is written contains the identity of the client in the
    usual way, followed by D= and a time, which records the duration of the
    connection. If the connection was authenticated, this fact is logged
    exactly as it is for an incoming message, with an A= item. If the
    connection was encrypted, CV=, DN=, and X= items may appear as they do for
    an incoming message, controlled by the same logging options.

    Finally, if any SMTP commands were issued during the connection, a C= item
    is added to the line, listing the commands that were used. For example,

    C=EHLO,QUIT

    shows that the client issued QUIT straight after EHLO. If there were fewer
    than 20 commands, they are all listed. If there were more than 20 commands,
    the last 20 are listed, preceded by "...". However, with the default
    setting of 10 for smtp_accept_max_nonmail, the connection will in any case
    have been aborted before 20 non-mail commands are processed.

  * smtp_mailauth: A third subfield with the authenticated sender,
    colon-separated, is appended to the A= item for a message arrival or
    delivery log line, if an AUTH argument to the SMTP MAIL command (see 33.2)
    was accepted or used.

  * smtp_protocol_error: A log line is written for every SMTP protocol error
    encountered. Exim does not have perfect detection of all protocol errors
    because of transmission delays and the use of pipelining. If PIPELINING has
    been advertised to a client, an Exim server assumes that the client will
    use it, and therefore it does not count "expected" errors (for example,
    RCPT received after rejecting MAIL) as protocol errors.

  * smtp_syntax_error: A log line is written for every SMTP syntax error
    encountered. An unrecognized command is treated as a syntax error. For an
    external connection, the host identity is given; for an internal connection
    using -bs the sender identification (normally the calling user) is given.

  * subject: The subject of the message is added to the arrival log line,
    preceded by "T=" (T for "topic", since S is already used for "size"). Any
    MIME "words" in the subject are decoded. The print_topbitchars option
    specifies whether characters with values greater than 127 should be logged
    unchanged, or whether they should be rendered as escape sequences.

  * tls_certificate_verified: An extra item is added to <= and => log lines
    when TLS is in use. The item is "CV=yes" if the peer's certificate was
    verified using a CA trust anchor, "CV=dane" if using a DNS trust anchor,
    and "CV=no" if not.

  * tls_cipher: When a message is sent or received over an encrypted
    connection, the cipher suite used is added to the log line, preceded by X=.

  * tls_peerdn: When a message is sent or received over an encrypted
    connection, and a certificate is supplied by the remote host, the peer DN
    is added to the log line, preceded by DN=.

  * tls_resumption: When a message is sent or received over an encrypted
    connection and the TLS session resumed one used on a previous TCP
    connection, an asterisk is appended to the X= cipher field in the log line.

  * tls_sni: When a message is received over an encrypted connection, and the
    remote host provided the Server Name Indication extension, the SNI is added
    to the log line, preceded by SNI=.

  * unknown_in_list: This setting causes a log entry to be written when the
    result of a list match is failure because a DNS lookup failed.


53.16 Message log
-----------------

In addition to the general log files, Exim writes a log file for each message
that it handles. The names of these per-message logs are the message ids, and
they are kept in the msglog sub-directory of the spool directory. Each message
log contains copies of the log lines that apply to the message. This makes it
easier to inspect the status of an individual message without having to search
the main log. A message log is deleted when processing of the message is
complete, unless preserve_message_logs is set, but this should be used only
with great care because they can fill up your disk very quickly.

On a heavily loaded system, it may be desirable to disable the use of
per-message logs, in order to reduce disk I/O. This can be done by setting the 
message_logs option false.



===============================================================================
54. EXIM UTILITIES

A number of utility scripts and programs are supplied with Exim and are
described in this chapter. There is also the Exim Monitor, which is covered in
the next chapter. The utilities described here are:

    54.1  exiwhat          list what Exim processes are doing
    54.2  exiqgrep         grep the queue
    54.3  exiqsumm         summarize the queue
    54.4  exigrep          search the main log
    54.5  exipick          select messages on various criteria
    54.6  exicyclog        cycle (rotate) log files
    54.7  eximstats        extract statistics from the log
    54.8  exim_checkaccess check address acceptance from given IP
    54.9  exim_dbmbuild    build a DBM file
    54.10 exinext          extract retry information
    54.12 exim_dumpdb      dump a hints database
    54.13 exim_tidydb      clean up a hints database
    54.14 exim_fixdb       patch a hints database
    54.15 exim_lock        lock a mailbox file
    54.16 exim_msgdate     Message Ids for humans (exim_msgdate)

Another utility that might be of use to sites with many MTAs is Tom Kistner's 
exilog. It provides log visualizations across multiple Exim servers. See https:
//duncanthrax.net/exilog/ for details.


54.1 Finding out what Exim processes are doing (exiwhat)
--------------------------------------------------------

On operating systems that can restart a system call after receiving a signal
(most modern OS), an Exim process responds to the SIGUSR1 signal by writing a
line describing what it is doing to the file exim-process.info in the Exim
spool directory. The exiwhat script sends the signal to all Exim processes it
can find, having first emptied the file. It then waits for one second to allow
the Exim processes to react before displaying the results. In order to run 
exiwhat successfully you have to have sufficient privilege to send the signal
to the Exim processes, so it is normally run as root.

Warning: This is not an efficient process. It is intended for occasional use by
system administrators. It is not sensible, for example, to set up a script that
sends SIGUSR1 signals to Exim processes at short intervals.

Unfortunately, the ps command that exiwhat uses to find Exim processes varies
in different operating systems. Not only are different options used, but the
format of the output is different. For this reason, there are some system
configuration options that configure exactly how exiwhat works. If it doesn't
seem to be working for you, check the following compile-time options:

    "EXIWHAT_PS_CMD"    the command for running ps
    "EXIWHAT_PS_ARG"    the argument for ps
    "EXIWHAT_EGREP_ARG" the argument for egrep to select from ps output
    "EXIWHAT_KILL_ARG"  the argument for the kill command

An example of typical output from exiwhat is

164 daemon: -q1h, listening on port 25
10483 running queue: waiting for 0tAycK-0002ij-00 (10492)
10492 delivering 0tAycK-0002ij-00 to mail.ref.example
  [10.19.42.42] (editor@ref.example)
10592 handling incoming call from [192.168.243.242]
10628 accepting a local non-SMTP message

The first number in the output line is the process number. The third line has
been split here, in order to fit it on the page.


54.2 Selective queue listing (exiqgrep)
---------------------------------------

This utility is a Perl script contributed by Matt Hubbard. It runs

exim -bpu

or (in case -a switch is specified)

exim -bp

to obtain a queue listing, and then greps the output to select messages that
match given criteria. The following selection options are available:

-f <regex>

    Match the sender address using a case-insensitive search. The field that is
    tested is enclosed in angle brackets, so you can test for bounce messages
    with

    exiqgrep -f '^<>$'

-r <regex>

    Match a recipient address using a case-insensitive search. The field that
    is tested is not enclosed in angle brackets.

-s <regex>

    Match against the size field.

-y <seconds>

    Match messages that are younger than the given time.

-o <seconds>

    Match messages that are older than the given time.

-z

    Match only frozen messages.

-x

    Match only non-frozen messages.

-G <queuename>

    Match only messages in the given queue. Without this, the default queue is
    searched.

The following options control the format of the output:

-c

    Display only the count of matching messages.

-l

    Long format - display the full message information as output by Exim. This
    is the default.

-i

    Display message ids only.

-b

    Brief format - one line per message.

-R

    Display messages in reverse order.

-a

    Include delivered recipients in queue listing.

The following options give alternates for configuration:

-C <config file>

    is used to specify an alternate exim.conf which might contain alternate
    exim configuration the queue management might be using.

-E <path>

    can be used to specify a path for the exim binary, overriding the built-in
    one.

There is one more option, -h, which outputs a list of options. At least one
selection option, or either the -c or -h option, must be given.


54.3 Summarizing the queue (exiqsumm)
-------------------------------------

The exiqsumm utility is a Perl script which reads the output of "exim -bp" and
produces a summary of the messages in the queue. Thus, you use it by running a
command such as

exim -bp | exiqsumm

The output consists of one line for each domain that has messages waiting for
it, as in the following example:

3   2322   74m   66m  msn.com.example

Each line lists the number of pending deliveries for a domain, their total
volume, and the length of time that the oldest and the newest messages have
been waiting. Note that the number of pending deliveries is greater than the
number of messages when messages have more than one recipient.

A summary line is output at the end. By default the output is sorted on the
domain name, but exiqsumm has the options -a and -c, which cause the output to
be sorted by oldest message and by count of messages, respectively. There are
also three options that split the messages for each domain into two or more
subcounts: -b separates bounce messages, -f separates frozen messages, and -s
separates messages according to their sender.

The output of exim -bp contains the original addresses in the message, so this
also applies to the output from exiqsumm. No domains from addresses generated
by aliasing or forwarding are included (unless the one_time option of the 
redirect router has been used to convert them into "top level" addresses).


54.4 Extracting specific information from the log (exigrep)
-----------------------------------------------------------

The exigrep utility is a Perl script that searches one or more main log files
for entries that match a given pattern. When it finds a match, it extracts all
the log entries for the relevant message, not just those that match the
pattern. Thus, exigrep can extract complete log entries for a given message, or
all mail for a given user, or for a given host, for example. The input files
can be in Exim log format or syslog format. If a matching log line is not
associated with a specific message, it is included in exigrep's output without
any additional lines. The usage is:

exigrep [-t<n>] [-I] [-l] [-M] [-v] <pattern> [<log file>] ...

If no log filenames are given on the command line, the standard input is read.

The -t argument specifies a number of seconds. It adds an additional condition
for message selection. Messages that are complete are shown only if they spent
more than <n> seconds in the queue.

By default, exigrep does case-insensitive matching. The -I option makes it
case-sensitive. This may give a performance improvement when searching large
log files. Without -I, the Perl pattern matches use Perl's "/i" option; with -I
they do not. In both cases it is possible to change the case sensitivity within
the pattern by using "(?i)" or "(?-i)".

The -l option means "literal", that is, treat all characters in the pattern as
standing for themselves. Otherwise the pattern must be a Perl regular
expression.

The -v option inverts the matching condition. That is, a line is selected if it
does not match the pattern.

The -M options means "related messages". exigrep will show messages that are
generated as a result/response to a message that exigrep matched normally.

Example of -M: user_a sends a message to user_b, which generates a bounce back
to user_b. If exigrep is used to search for "user_a", only the first message
will be displayed. But if exigrep is used to search for "user_b", the first and
the second (bounce) message will be displayed. Using -M with exigrep when
searching for "user_a" will show both messages since the bounce is "related" to
or a "result" of the first message that was found by the search term.

If the location of a zcat command is known from the definition of ZCAT_COMMAND
in Local/Makefile, exigrep automatically passes any file whose name ends in
COMPRESS_SUFFIX through zcat as it searches it. If the ZCAT_COMMAND is not
executable, exigrep tries to use autodetection of some well known compression
extensions.


54.5 Selecting messages by various criteria (exipick)
-----------------------------------------------------

John Jetmore's exipick utility is included in the Exim distribution. It lists
messages from the queue according to a variety of criteria. For details of 
exipick's facilities, run exipick with the --help option.


54.6 Cycling log files (exicyclog)
----------------------------------

The exicyclog script can be used to cycle (rotate) mainlog and rejectlog files.
This is not necessary if only syslog is being used, or if you are using log
files with datestamps in their names (see section 53.3). Some operating systems
have their own standard mechanisms for log cycling, and these can be used
instead of exicyclog if preferred. There are two command line options for 
exicyclog:

  * -k <count> specifies the number of log files to keep, overriding the
    default that is set when Exim is built. The default default is 10.

  * -l <path> specifies the log file path, in the same format as Exim's 
    log_file_path option (for example, "/var/log/exim_%slog"), again overriding
    the script's default, which is to find the setting from Exim's
    configuration.

Each time exicyclog is run the filenames get "shuffled down" by one. If the
main log filename is mainlog (the default) then when exicyclog is run mainlog
becomes mainlog.01, the previous mainlog.01 becomes mainlog.02 and so on, up to
the limit that is set in the script or by the -k option. Log files whose
numbers exceed the limit are discarded. Reject logs are handled similarly.

If the limit is greater than 99, the script uses 3-digit numbers such as
mainlog.001, mainlog.002, etc. If you change from a number less than 99 to one
that is greater, or vice versa, you will have to fix the names of any existing
log files.

If no mainlog file exists, the script does nothing. Files that "drop off" the
end are deleted. All files with numbers greater than 01 are compressed, using a
compression command which is configured by the COMPRESS_COMMAND setting in
Local/Makefile. It is usual to run exicyclog daily from a root crontab entry of
the form

1 0 * * * su exim -c /usr/exim/bin/exicyclog

assuming you have used the name "exim" for the Exim user. You can run exicyclog
as root if you wish, but there is no need.


54.7 Mail statistics (eximstats)
--------------------------------

A Perl script called eximstats is provided for extracting statistical
information from log files. The output is either plain text, or HTML.

The eximstats script has been hacked about quite a bit over time. The latest
version is the result of some extensive revision by Steve Campbell. A lot of
information is given by default, but there are options for suppressing various
parts of it. Following any options, the arguments to the script are a list of
files, which should be main log files. For example:

eximstats -nr /var/spool/exim/log/mainlog.01

By default, eximstats extracts information about the number and volume of
messages received from or delivered to various hosts. The information is sorted
both by message count and by volume, and the top fifty hosts in each category
are listed on the standard output. Similar information, based on email
addresses or domains instead of hosts can be requested by means of various
options. For messages delivered and received locally, similar statistics are
also produced per user.

The output also includes total counts and statistics about delivery errors, and
histograms showing the number of messages received and deliveries made in each
hour of the day. A delivery with more than one address in its envelope (for
example, an SMTP transaction with more than one RCPT command) is counted as a
single delivery by eximstats.

Though normally more deliveries than receipts are reported (as messages may
have multiple recipients), it is possible for eximstats to report more messages
received than delivered, even though the queue is empty at the start and end of
the period in question. If an incoming message contains no valid recipients, no
deliveries are recorded for it. A bounce message is handled as an entirely
separate message.

eximstats always outputs a grand total summary giving the volume and number of
messages received and deliveries made, and the number of hosts involved in each
case. It also outputs the number of messages that were delayed (that is, not
completely delivered at the first attempt), and the number that had at least
one address that failed.

The remainder of the output is in sections that can be independently disabled
or modified by various options. It consists of a summary of deliveries by
transport, histograms of messages received and delivered per time interval
(default per hour), information about the time messages spent in the queue, a
list of relayed messages, lists of the top fifty sending hosts, local senders,
destination hosts, and destination local users by count and by volume, and a
list of delivery errors that occurred.

The relay information lists messages that were actually relayed, that is, they
came from a remote host and were directly delivered to some other remote host,
without being processed (for example, for aliasing or forwarding) locally.

There are quite a few options for eximstats to control exactly what it outputs.
These are documented in the Perl script itself, and can be extracted by running
the command perldoc on the script. For example:

perldoc /usr/exim/bin/eximstats


54.8 Checking access policy (exim_checkaccess)
----------------------------------------------

The -bh command line argument allows you to run a fake SMTP session with
debugging output, in order to check what Exim is doing when it is applying
policy controls to incoming SMTP mail. However, not everybody is sufficiently
familiar with the SMTP protocol to be able to make full use of -bh, and
sometimes you just want to answer the question "Does this address have access?"
without bothering with any further details.

The exim_checkaccess utility is a "packaged" version of -bh. It takes two
arguments, an IP address and an email address:

exim_checkaccess 10.9.8.7 A.User@a.domain.example

The utility runs a call to Exim with the -bh option, to test whether the given
email address would be accepted in a RCPT command in a TCP/IP connection from
the host with the given IP address. The output of the utility is either the
word "accepted", or the SMTP error response, for example:

Rejected:
550 Relay not permitted

When running this test, the utility uses "<>" as the envelope sender address
for the MAIL command, but you can change this by providing additional options.
These are passed directly to the Exim command. For example, to specify that the
test is to be run with the sender address himself@there.example you can use:

exim_checkaccess 10.9.8.7 A.User@a.domain.example \
                 -f himself@there.example

Note that these additional Exim command line items must be given after the two
mandatory arguments.

Because the exim_checkaccess uses -bh, it does not perform callouts while
running its checks. You can run checks that include callouts by using -bhc, but
this is not yet available in a "packaged" form.


54.9 Making DBM files (exim_dbmbuild)
-------------------------------------

The exim_dbmbuild program reads an input file containing keys and data in the
format used by the lsearch lookup (see section 9.3). It writes a DBM file using
the lower-cased alias names as keys and the remainder of the information as
data. The lower-casing can be prevented by calling the program with the -nolc
option.

A terminating zero is included as part of the key string. This is expected by
the dbm lookup type. However, if the option -nozero is given, exim_dbmbuild
creates files without terminating zeroes in either the key strings or the data
strings. The dbmnz lookup type can be used with such files.

The program requires two arguments: the name of the input file (which can be a
single hyphen to indicate the standard input), and the name of the output file.
It creates the output under a temporary name, and then renames it if all went
well.

If the native DB interface is in use (USE_DB is set in a compile-time
configuration file - this is common in free versions of Unix) the two filenames
must be different, because in this mode the Berkeley DB functions create a
single output file using exactly the name given. For example,

exim_dbmbuild /etc/aliases /etc/aliases.db

reads the system alias file and creates a DBM version of it in /etc/aliases.db.

In systems that use the ndbm routines (mostly proprietary versions of Unix),
two files are used, with the suffixes .dir and .pag. In this environment, the
suffixes are added to the second argument of exim_dbmbuild, so it can be the
same as the first. This is also the case when the Berkeley functions are used
in compatibility mode (though this is not recommended), because in that case it
adds a .db suffix to the filename.

If a duplicate key is encountered, the program outputs a warning, and when it
finishes, its return code is 1 rather than zero, unless the -noduperr option is
used. By default, only the first of a set of duplicates is used - this makes it
compatible with lsearch lookups. There is an option -lastdup which causes it to
use the data for the last duplicate instead. There is also an option -nowarn,
which stops it listing duplicate keys to stderr. For other errors, where it
doesn't actually make a new file, the return code is 2.


54.10 Finding individual retry times (exinext)
----------------------------------------------

A utility called exinext (mostly a Perl script) provides the ability to fish
specific information out of the retry database. Given a mail domain (or a
complete address), it looks up the hosts for that domain, and outputs any retry
information for the hosts or for the domain. At present, the retry information
is obtained by running exim_dumpdb (see below) and post-processing the output.
For example:

$ exinext piglet@milne.fict.example
kanga.milne.example:192.168.8.1 error 146: Connection refused
  first failed: 21-Feb-1996 14:57:34
  last tried:   21-Feb-1996 14:57:34
  next try at:  21-Feb-1996 15:02:34
roo.milne.example:192.168.8.3 error 146: Connection refused
  first failed: 20-Jan-1996 13:12:08
  last tried:   21-Feb-1996 11:42:03
  next try at:  21-Feb-1996 19:42:03
  past final cutoff time

You can also give exinext a local part, without a domain, and it will give any
retry information for that local part in your default domain. A message id can
be used to obtain retry information pertaining to a specific message. This
exists only when an attempt to deliver a message to a remote host suffers a
message-specific error (see section 49.1.1). exinext is not particularly
efficient, but then it is not expected to be run very often.

The exinext utility calls Exim to find out information such as the location of
the spool directory. The utility has -C and -D options, which are passed on to
the exim commands. The first specifies an alternate Exim configuration file,
and the second sets macros for use within the configuration file. These
features are mainly to help in testing, but might also be useful in
environments where more than one configuration file is in use.


54.11 Hints database maintenance
--------------------------------

Three utility programs are provided for maintaining the DBM files that Exim
uses to contain its delivery hint information. Each program requires two
arguments. The first specifies the name of Exim's spool directory, and the
second is the name of the database it is to operate on. These are as follows:

  * retry: the database of retry information

  * wait-<transport name>: databases of information about messages waiting for
    remote hosts

  * callout: the callout cache

  * ratelimit: the data for implementing the ratelimit ACL condition

  * tls: TLS session resumption data

  * misc: other hints data

The misc database is used for

  * Serializing ETRN runs (when smtp_etrn_serialize is set)

  * Serializing delivery to a specific host (when serialize_hosts is set in an 
    smtp transport)

  * Limiting the concurrency of specific transports (when max_parallel is set
    in a transport)

  * Recording EHLO-time facilities advertised by hosts


54.12 exim_dumpdb
-----------------

The entire contents of a database are written to the standard output by the 
exim_dumpdb program, taking as arguments the spool and database names. An
option -z may be given to request times in UTC; otherwise times are in the
local timezone. An option -k may be given to dump only the record keys. For
example, to dump the retry database:

exim_dumpdb /var/spool/exim retry

For the retry database two lines of output are produced for each entry:

T:mail.ref.example:192.168.242.242 146 77 Connection refused
31-Oct-1995 12:00:12 02-Nov-1995 12:21:39 02-Nov-1995 20:21:39 *

The first item on the first line is the key of the record. It starts with one
of the letters R, or T, depending on whether it refers to a routing or
transport retry. For a local delivery, the next part is the local address; for
a remote delivery it is the name of the remote host, followed by its failing IP
address (unless retry_include_ip_address is set false on the smtp transport).
If the remote port is not the standard one (port 25), it is added to the IP
address. Then there follows an error code, an additional error code, and a
textual description of the error.

The three times on the second line are the time of first failure, the time of
the last delivery attempt, and the computed time for the next attempt. The line
ends with an asterisk if the cutoff time for the last retry rule has been
exceeded.

Each output line from exim_dumpdb for the wait-xxx databases consists of a host
name followed by a list of ids for messages that are or were waiting to be
delivered to that host. If there are a very large number for any one host,
continuation records, with a sequence number added to the host name, may be
seen. The data in these records is often out of date, because a message may be
routed to several alternative hosts, and Exim makes no effort to keep
cross-references.


54.13 exim_tidydb
-----------------

The exim_tidydb utility program is used to tidy up the contents of a hints
database. If run with no options, it removes all records that are more than 30
days old. The age is calculated from the date and time that the record was last
updated. Note that, in the case of the retry database, it is not the time since
the first delivery failure. Information about a host that has been down for
more than 30 days will remain in the database, provided that the record is
updated sufficiently often.

The cutoff date can be altered by means of the -t option, which must be
followed by a time. For example, to remove all records older than a week from
the retry database:

exim_tidydb -t 7d /var/spool/exim retry

Both the wait-xxx and retry databases contain items that involve message ids.
In the former these appear as data in records keyed by host - they were
messages that were waiting for that host - and in the latter they are the keys
for retry information for messages that have suffered certain types of error.
When exim_tidydb is run, a check is made to ensure that message ids in database
records are those of messages that are still on the queue. Message ids for
messages that no longer exist are removed from wait-xxx records, and if this
leaves any records empty, they are deleted. For the retry database, records
whose keys are non-existent message ids are removed. The exim_tidydb utility
outputs comments on the standard output whenever it removes information from
the database.

Certain records are automatically removed by Exim when they are no longer
needed, but others are not. For example, if all the MX hosts for a domain are
down, a retry record is created for each one. If the primary MX host comes back
first, its record is removed when Exim successfully delivers to it, but the
records for the others remain because Exim has not tried to use those hosts.

It is important, therefore, to run exim_tidydb periodically on all the hints
databases. You should do this at a quiet time of day, because it requires a
database to be locked (and therefore inaccessible to Exim) while it does its
work. Removing records from a DBM file does not normally make the file smaller,
but all the common DBM libraries are able to re-use the space that is released.
After an initial phase of increasing in size, the databases normally reach a
point at which they no longer get any bigger, as long as they are regularly
tidied.

Warning: If you never run exim_tidydb, the space used by the hints databases is
likely to keep on increasing.


54.14 exim_fixdb
----------------

The exim_fixdb program is a utility for interactively modifying databases. Its
main use is for testing Exim, but it might also be occasionally useful for
getting round problems in a live system. Its interface is somewhat crude. On
entry, it prompts for input with a right angle-bracket. A key of a database
record can then be entered, and the data for that record is displayed.

If "d" is typed at the next prompt, the entire record is deleted. For all
except the retry database, that is the only operation that can be carried out.
For the retry database, each field is output preceded by a number, and data for
individual fields can be changed by typing the field number followed by new
data, for example:

> 4 951102:1000

resets the time of the next delivery attempt. Time values are given as a
sequence of digit pairs for year, month, day, hour, and minute. Colons can be
used as optional separators.

Both displayed and input times are in the local timezone by default. If an
option -z is used on the command line, displayed times are in UTC.


54.15 Mailbox maintenance (exim_lock)
-------------------------------------

The exim_lock utility locks a mailbox file using the same algorithm as Exim.
For a discussion of locking issues, see section 26.3. Exim_lock can be used to
prevent any modification of a mailbox by Exim or a user agent while
investigating a problem. The utility requires the name of the file as its first
argument. If the locking is successful, the second argument is run as a command
(using C's system() function); if there is no second argument, the value of the
SHELL environment variable is used; if this is unset or empty, /bin/sh is run.
When the command finishes, the mailbox is unlocked and the utility ends. The
following options are available:

-fcntl

    Use fcntl() locking on the open mailbox.

-flock

    Use flock() locking on the open mailbox, provided the operating system
    supports it.

-interval

    This must be followed by a number, which is a number of seconds; it sets
    the interval to sleep between retries (default 3).

-lockfile

    Create a lock file before opening the mailbox.

-mbx

    Lock the mailbox using MBX rules.

-q

    Suppress verification output.

-retries

    This must be followed by a number; it sets the number of times to try to
    get the lock (default 10).

-restore_time

    This option causes exim_lock to restore the modified and read times to the
    locked file before exiting. This allows you to access a locked mailbox (for
    example, to take a backup copy) without disturbing the times that the user
    subsequently sees.

-timeout

    This must be followed by a number, which is a number of seconds; it sets a
    timeout to be used with a blocking fcntl() lock. If it is not set (the
    default), a non-blocking call is used.

-v

    Generate verbose output.

If none of -fcntl, -flock, -lockfile or -mbx are given, the default is to
create a lock file and also to use fcntl() locking on the mailbox, which is the
same as Exim's default. The use of -flock or -fcntl requires that the file be
writeable; the use of -lockfile requires that the directory containing the file
be writeable. Locking by lock file does not last forever; Exim assumes that a
lock file is expired if it is more than 30 minutes old.

The -mbx option can be used with either or both of -fcntl or -flock. It assumes
-fcntl by default. MBX locking causes a shared lock to be taken out on the open
mailbox, and an exclusive lock on the file /tmp/.n.m where n and m are the
device number and inode number of the mailbox file. When the locking is
released, if an exclusive lock can be obtained for the mailbox, the file in /
tmp is deleted.

The default output contains verification of the locking that takes place. The 
-v option causes some additional information to be given. The -q option
suppresses all output except error messages.

A command such as

exim_lock /var/spool/mail/spqr

runs an interactive shell while the file is locked, whereas

exim_lock -q /var/spool/mail/spqr <<End
<some commands>
End

runs a specific non-interactive sequence of commands while the file is locked,
suppressing all verification output. A single command can be run by a command
such as

exim_lock -q /var/spool/mail/spqr \
  "cp /var/spool/mail/spqr /some/where"

Note that if a command is supplied, it must be entirely contained within the
second argument - hence the quotes.


54.16 Message Ids for humans (exim_msgdate)
-------------------------------------------

The exim_msgdate utility is written by Andrew Aitchison and included in the
Exim distribution. This Perl script converts an Exim Mesage ID back into a
human readable form. For details of exim_msgdate's options, run exim_msgdate
with the --help option.

Section 3.4 (Message identification) describes Exim Mesage IDs.



===============================================================================
55. THE EXIM MONITOR

The Exim monitor is an application which displays in an X window information
about the state of Exim's queue and what Exim is doing. An admin user can
perform certain operations on messages from this GUI interface; however all
such facilities are also available from the command line, and indeed, the
monitor itself makes use of the command line to perform any actions requested.


55.1 Running the monitor
------------------------

The monitor is started by running the script called eximon. This is a shell
script that sets up a number of environment variables, and then runs the binary
called eximon.bin. The default appearance of the monitor window can be changed
by editing the Local/eximon.conf file created by editing exim_monitor/EDITME.
Comments in that file describe what the various parameters are for.

The parameters that get built into the eximon script can be overridden for a
particular invocation by setting up environment variables of the same names,
preceded by "EXIMON_". For example, a shell command such as

EXIMON_LOG_DEPTH=400 eximon

(in a Bourne-compatible shell) runs eximon with an overriding setting of the
LOG_DEPTH parameter. If EXIMON_LOG_FILE_PATH is set in the environment, it
overrides the Exim log file configuration. This makes it possible to have 
eximon tailing log data that is written to syslog, provided that MAIL.INFO
syslog messages are routed to a file on the local host.

X resources can be used to change the appearance of the window in the normal
way. For example, a resource setting of the form

Eximon*background: gray94

changes the colour of the background to light grey rather than white. The
stripcharts are drawn with both the data lines and the reference lines in
black. This means that the reference lines are not visible when on top of the
data. However, their colour can be changed by setting a resource called
"highlight" (an odd name, but that's what the Athena stripchart widget uses).
For example, if your X server is running Unix, you could set up lighter
reference lines in the stripcharts by obeying

xrdb -merge <<End
Eximon*highlight: gray
End

In order to see the contents of messages in the queue, and to operate on them, 
eximon must either be run as root or by an admin user.

The command-line parameters of eximon are passed to eximon.bin and may contain
X11 resource parameters interpreted by the X11 library. In addition, if the
first parameter starts with the string "gdb" then it is removed and the binary
is invoked under gdb (the parameter is used as the gdb command-name, so
versioned variants of gdb can be invoked).

The monitor's window is divided into three parts. The first contains one or
more stripcharts and two action buttons, the second contains a "tail" of the
main log file, and the third is a display of the queue of messages awaiting
delivery, with two more action buttons. The following sections describe these
different parts of the display.


55.2 The stripcharts
--------------------

The first stripchart is always a count of messages in the queue. Its name can
be configured by setting QUEUE_STRIPCHART_NAME in the Local/eximon.conf file.
The remaining stripcharts are defined in the configuration script by regular
expression matches on log file entries, making it possible to display, for
example, counts of messages delivered to certain hosts or using certain
transports. The supplied defaults display counts of received and delivered
messages, and of local and SMTP deliveries. The default period between
stripchart updates is one minute; this can be adjusted by a parameter in the
Local/eximon.conf file.

The stripchart displays rescale themselves automatically as the value they are
displaying changes. There are always 10 horizontal lines in each chart; the
title string indicates the value of each division when it is greater than one.
For example, "x2" means that each division represents a value of 2.

It is also possible to have a stripchart which shows the percentage fullness of
a particular disk partition, which is useful when local deliveries are confined
to a single partition.

This relies on the availability of the statvfs() function or equivalent in the
operating system. Most, but not all versions of Unix that support Exim have
this. For this particular stripchart, the top of the chart always represents
100%, and the scale is given as "x10%". This chart is configured by setting
SIZE_STRIPCHART and (optionally) SIZE_STRIPCHART_NAME in the Local/eximon.conf
file.


55.3 Main action buttons
------------------------

Below the stripcharts there is an action button for quitting the monitor. Next
to this is another button marked "Size". They are placed here so that shrinking
the window to its default minimum size leaves just the queue count stripchart
and these two buttons visible. Pressing the "Size" button causes the window to
expand to its maximum size, unless it is already at the maximum, in which case
it is reduced to its minimum.

When expanding to the maximum, if the window cannot be fully seen where it
currently is, it is moved back to where it was the last time it was at full
size. When it is expanding from its minimum size, the old position is
remembered, and next time it is reduced to the minimum it is moved back there.

The idea is that you can keep a reduced window just showing one or two
stripcharts at a convenient place on your screen, easily expand it to show the
full window when required, and just as easily put it back to what it was. The
idea is copied from what the twm window manager does for its f.fullzoom action.
The minimum size of the window can be changed by setting the MIN_HEIGHT and
MIN_WIDTH values in Local/eximon.conf.

Normally, the monitor starts up with the window at its full size, but it can be
built so that it starts up with the window at its smallest size, by setting
START_SMALL=yes in Local/eximon.conf.


55.4 The log display
--------------------

The second section of the window is an area in which a display of the tail of
the main log is maintained. To save space on the screen, the timestamp on each
log line is shortened by removing the date and, if log_timezone is set, the
timezone. The log tail is not available when the only destination for logging
data is syslog, unless the syslog lines are routed to a local file whose name
is passed to eximon via the EXIMON_LOG_FILE_PATH environment variable.

The log sub-window has a scroll bar at its lefthand side which can be used to
move back to look at earlier text, and the up and down arrow keys also have a
scrolling effect. The amount of log that is kept depends on the setting of
LOG_BUFFER in Local/eximon.conf, which specifies the amount of memory to use.
When this is full, the earlier 50% of data is discarded - this is much more
efficient than throwing it away line by line. The sub-window also has a
horizontal scroll bar for accessing the ends of long log lines. This is the
only means of horizontal scrolling; the right and left arrow keys are not
available. Text can be cut from this part of the window using the mouse in the
normal way. The size of this subwindow is controlled by parameters in the
configuration file Local/eximon.conf.

Searches of the text in the log window can be carried out by means of the ^R
and ^S keystrokes, which default to a reverse and a forward search,
respectively. The search covers only the text that is displayed in the window.
It cannot go further back up the log.

The point from which the search starts is indicated by a caret marker. This is
normally at the end of the text in the window, but can be positioned explicitly
by pointing and clicking with the left mouse button, and is moved automatically
by a successful search. If new text arrives in the window when it is scrolled
back, the caret remains where it is, but if the window is not scrolled back,
the caret is moved to the end of the new text.

Pressing ^R or ^S pops up a window into which the search text can be typed.
There are buttons for selecting forward or reverse searching, for carrying out
the search, and for cancelling. If the "Search" button is pressed, the search
happens and the window remains so that further searches can be done. If the
"Return" key is pressed, a single search is done and the window is closed. If ^
C is typed the search is cancelled.

The searching facility is implemented using the facilities of the Athena text
widget. By default this pops up a window containing both "search" and "replace"
options. In order to suppress the unwanted "replace" portion for eximon, a
modified version of the TextPop widget is distributed with Exim. However, the
linkers in BSDI and HP-UX seem unable to handle an externally provided version
of TextPop when the remaining parts of the text widget come from the standard
libraries. The compile-time option EXIMON_TEXTPOP can be unset to cut out the
modified TextPop, making it possible to build Eximon on these systems, at the
expense of having unwanted items in the search popup window.


55.5 The queue display
----------------------

The bottom section of the monitor window contains a list of all messages that
are in the queue, which includes those currently being received or delivered,
as well as those awaiting delivery. The size of this subwindow is controlled by
parameters in the configuration file Local/eximon.conf, and the frequency at
which it is updated is controlled by another parameter in the same file - the
default is 5 minutes, since queue scans can be quite expensive. However, there
is an "Update" action button just above the display which can be used to force
an update of the queue display at any time.

When a host is down for some time, a lot of pending mail can build up for it,
and this can make it hard to deal with other messages in the queue. To help
with this situation there is a button next to "Update" called "Hide". If
pressed, a dialogue box called "Hide addresses ending with" is put up. If you
type anything in here and press "Return", the text is added to a chain of such
texts, and if every undelivered address in a message matches at least one of
the texts, the message is not displayed.

If there is an address that does not match any of the texts, all the addresses
are displayed as normal. The matching happens on the ends of addresses so, for
example, cam.ac.uk specifies all addresses in Cambridge, while 
xxx@foo.com.example specifies just one specific address. When any hiding has
been set up, a button called "Unhide" is displayed. If pressed, it cancels all
hiding. Also, to ensure that hidden messages do not get forgotten, a hide
request is automatically cancelled after one hour.

While the dialogue box is displayed, you can't press any buttons or do anything
else to the monitor window. For this reason, if you want to cut text from the
queue display to use in the dialogue box, you have to do the cutting before
pressing the "Hide" button.

The queue display contains, for each unhidden queued message, the length of
time it has been in the queue, the size of the message, the message id, the
message sender, and the first undelivered recipient, all on one line. If it is
a bounce message, the sender is shown as "<>". If there is more than one
recipient to which the message has not yet been delivered, subsequent ones are
listed on additional lines, up to a maximum configured number, following which
an ellipsis is displayed. Recipients that have already received the message are
not shown.

If a message is frozen, an asterisk is displayed at the left-hand side.

The queue display has a vertical scroll bar, and can also be scrolled by means
of the arrow keys. Text can be cut from it using the mouse in the normal way.
The text searching facilities, as described above for the log window, are also
available, but the caret is always moved to the end of the text when the queue
display is updated.


55.6 The queue menu
-------------------

If the shift key is held down and the left button is clicked when the mouse
pointer is over the text for any message, an action menu pops up, and the first
line of the queue display for the message is highlighted. This does not affect
any selected text.

If you want to use some other event for popping up the menu, you can set the
MENU_EVENT parameter in Local/eximon.conf to change the default, or set
EXIMON_MENU_EVENT in the environment before starting the monitor. The value set
in this parameter is a standard X event description. For example, to run eximon
using ctrl rather than shift you could use

EXIMON_MENU_EVENT='Ctrl<Btn1Down>' eximon

The title of the menu is the message id, and it contains entries which act as
follows:

  * message log: The contents of the message log for the message are displayed
    in a new text window.

  * headers: Information from the spool file that contains the envelope
    information and headers is displayed in a new text window. See chapter 57
    for a description of the format of spool files.

  * body: The contents of the spool file containing the body of the message are
    displayed in a new text window. There is a default limit of 20,000 bytes to
    the amount of data displayed. This can be changed by setting the BODY_MAX
    option at compile time, or the EXIMON_BODY_MAX option at runtime.

  * deliver message: A call to Exim is made using the -M option to request
    delivery of the message. This causes an automatic thaw if the message is
    frozen. The -v option is also set, and the output from Exim is displayed in
    a new text window. The delivery is run in a separate process, to avoid
    holding up the monitor while the delivery proceeds.

  * freeze message: A call to Exim is made using the -Mf option to request that
    the message be frozen.

  * thaw message: A call to Exim is made using the -Mt option to request that
    the message be thawed.

  * give up on msg: A call to Exim is made using the -Mg option to request that
    Exim gives up trying to deliver the message. A bounce message is generated
    for any remaining undelivered addresses.

  * remove message: A call to Exim is made using the -Mrm option to request
    that the message be deleted from the system without generating a bounce
    message.

  * add recipient: A dialog box is displayed into which a recipient address can
    be typed. If the address is not qualified and the QUALIFY_DOMAIN parameter
    is set in Local/eximon.conf, the address is qualified with that domain.
    Otherwise it must be entered as a fully qualified address. Pressing RETURN
    causes a call to Exim to be made using the -Mar option to request that an
    additional recipient be added to the message, unless the entry box is
    empty, in which case no action is taken.

  * mark delivered: A dialog box is displayed into which a recipient address
    can be typed. If the address is not qualified and the QUALIFY_DOMAIN
    parameter is set in Local/eximon.conf, the address is qualified with that
    domain. Otherwise it must be entered as a fully qualified address. Pressing
    RETURN causes a call to Exim to be made using the -Mmd option to mark the
    given recipient address as already delivered, unless the entry box is
    empty, in which case no action is taken.

  * mark all delivered: A call to Exim is made using the -Mmad option to mark
    all recipient addresses as already delivered.

  * edit sender: A dialog box is displayed initialized with the current
    sender's address. Pressing RETURN causes a call to Exim to be made using
    the -Mes option to replace the sender address, unless the entry box is
    empty, in which case no action is taken. If you want to set an empty sender
    (as in bounce messages), you must specify it as "<>". Otherwise, if the
    address is not qualified and the QUALIFY_DOMAIN parameter is set in Local/
    eximon.conf, the address is qualified with that domain.

When a delivery is forced, a window showing the -v output is displayed. In
other cases when a call to Exim is made, if there is any output from Exim (in
particular, if the command fails) a window containing the command and the
output is displayed. Otherwise, the results of the action are normally apparent
from the log and queue displays. However, if you set ACTION_OUTPUT=yes in Local
/eximon.conf, a window showing the Exim command is always opened, even if no
output is generated.

The queue display is automatically updated for actions such as freezing and
thawing, unless ACTION_QUEUE_UPDATE=no has been set in Local/eximon.conf. In
this case the "Update" button has to be used to force an update of the display
after one of these actions.

In any text window that is displayed as result of a menu action, the normal
cut-and-paste facility is available, and searching can be carried out using ^R
and ^S, as described above for the log tail window.



===============================================================================
56. SECURITY CONSIDERATIONS

This chapter discusses a number of issues concerned with security, some of
which are also covered in other parts of this manual.

For reasons that this author does not understand, some people have promoted
Exim as a "particularly secure" mailer. Perhaps it is because of the existence
of this chapter in the documentation. However, the intent of the chapter is
simply to describe the way Exim works in relation to certain security concerns,
not to make any specific claims about the effectiveness of its security as
compared with other MTAs.

What follows is a description of the way Exim is supposed to be. Best efforts
have been made to try to ensure that the code agrees with the theory, but an
absence of bugs can never be guaranteed. Any that are reported will get fixed
as soon as possible.


56.1 Building a more "hardened" Exim
------------------------------------

There are a number of build-time options that can be set in Local/Makefile to
create Exim binaries that are "harder" to attack, in particular by a rogue Exim
administrator who does not have the root password, or by someone who has
penetrated the Exim (but not the root) account. These options are as follows:

  * ALT_CONFIG_PREFIX can be set to a string that is required to match the
    start of any filenames used with the -C option. When it is set, these
    filenames are also not allowed to contain the sequence "/../". (However, if
    the value of the -C option is identical to the value of CONFIGURE_FILE in
    Local/Makefile, Exim ignores -C and proceeds as usual.) There is no default
    setting for ALT_CONFIG_PREFIX.

    If the permitted configuration files are confined to a directory to which
    only root has access, this guards against someone who has broken into the
    Exim account from running a privileged Exim with an arbitrary configuration
    file, and using it to break into other accounts.

  * If a non-trusted configuration file (i.e. not the default configuration
    file or one which is trusted by virtue of being listed in the
    TRUSTED_CONFIG_LIST file) is specified with -C, or if macros are given with
    -D (but see the next item), then root privilege is retained only if the
    caller of Exim is root. This locks out the possibility of testing a
    configuration using -C right through message reception and delivery, even
    if the caller is root. The reception works, but by that time, Exim is
    running as the Exim user, so when it re-execs to regain privilege for the
    delivery, the use of -C causes privilege to be lost. However, root can test
    reception and delivery using two separate commands.

  * The WHITELIST_D_MACROS build option declares some macros to be safe to
    override with -D if the real uid is one of root, the Exim run-time user or
    the CONFIGURE_OWNER, if defined. The potential impact of this option is
    limited by requiring the run-time value supplied to -D to match a regex
    that errs on the restrictive side. Requiring build-time selection of safe
    macros is onerous but this option is intended solely as a transition
    mechanism to permit previously-working configurations to continue to work
    after release 4.73.

  * If DISABLE_D_OPTION is defined, the use of the -D command line option is
    disabled.

  * FIXED_NEVER_USERS can be set to a colon-separated list of users that are
    never to be used for any deliveries. This is like the never_users runtime
    option, but it cannot be overridden; the runtime option adds additional
    users to the list. The default setting is "root"; this prevents a non-root
    user who is permitted to modify the runtime file from using Exim as a way
    to get root.


56.2 Root privilege
-------------------

The Exim binary is normally setuid to root, which means that it gains root
privilege (runs as root) when it starts execution. In some special cases (for
example, when the daemon is not in use and there are no local deliveries), it
may be possible to run Exim setuid to some user other than root. This is
discussed in the next section. However, in most installations, root privilege
is required for two things:

  * To set up a socket connected to the standard SMTP port (25) when
    initialising the listening daemon. If Exim is run from inetd, this
    privileged action is not required.

  * To be able to change uid and gid in order to read users' .forward files and
    perform local deliveries as the receiving user or as specified in the
    configuration.

It is not necessary to be root to do any of the other things Exim does, such as
receiving messages and delivering them externally over SMTP, and it is
obviously more secure if Exim does not run as root except when necessary. For
this reason, a user and group for Exim to use must be defined in Local/Makefile
. These are known as "the Exim user" and "the Exim group". Their values can be
changed by the runtime configuration, though this is not recommended. Often a
user called exim is used, but some sites use mail or another user name
altogether.

Exim uses setuid() whenever it gives up root privilege. This is a permanent
abdication; the process cannot regain root afterwards. Prior to release 4.00,
seteuid() was used in some circumstances, but this is no longer the case.

After a new Exim process has interpreted its command line options, it changes
uid and gid in the following cases:

  * If the -C option is used to specify an alternate configuration file, or if
    the -D option is used to define macro values for the configuration, and the
    calling process is not running as root, the uid and gid are changed to
    those of the calling process. However, if DISABLE_D_OPTION is defined in
    Local/Makefile, the -D option may not be used at all. If WHITELIST_D_MACROS
    is defined in Local/Makefile, then some macro values can be supplied if the
    calling process is running as root, the Exim run-time user or
    CONFIGURE_OWNER, if defined.

  * If the expansion test option (-be) or one of the filter testing options (
    -bf or -bF) are used, the uid and gid are changed to those of the calling
    process.

  * If the process is not a daemon process or a queue runner process or a
    delivery process or a process for testing address routing (started with -bt
    ), the uid and gid are changed to the Exim user and group. This means that
    Exim always runs under its own uid and gid when receiving messages. This
    also applies when testing address verification (the -bv option) and testing
    incoming message policy controls (the -bh option).

  * For a daemon, queue runner, delivery, or address testing process, the uid
    remains as root at this stage, but the gid is changed to the Exim group.

The processes that initially retain root privilege behave as follows:

  * A daemon process changes the gid to the Exim group and the uid to the Exim
    user after setting up one or more listening sockets. The initgroups()
    function is called, so that if the Exim user is in any additional groups,
    they will be used during message reception.

  * A queue runner process retains root privilege throughout its execution. Its
    job is to fork a controlled sequence of delivery processes.

  * A delivery process retains root privilege throughout most of its execution,
    but any actual deliveries (that is, the transports themselves) are run in
    subprocesses which always change to a non-root uid and gid. For local
    deliveries this is typically the uid and gid of the owner of the mailbox;
    for remote deliveries, the Exim uid and gid are used. Once all the delivery
    subprocesses have been run, a delivery process changes to the Exim uid and
    gid while doing post-delivery tidying up such as updating the retry
    database and generating bounce and warning messages.

    While the recipient addresses in a message are being routed, the delivery
    process runs as root. However, if a user's filter file has to be processed,
    this is done in a subprocess that runs under the individual user's uid and
    gid. A system filter is run as root unless system_filter_user is set.

  * A process that is testing addresses (the -bt option) runs as root so that
    the routing is done in the same environment as a message delivery.


56.3 Running Exim without privilege
-----------------------------------

Some installations like to run Exim in an unprivileged state for more of its
operation, for added security. Support for this mode of operation is provided
by the global option deliver_drop_privilege. When this is set, the uid and gid
are changed to the Exim user and group at the start of a delivery process (and
also queue runner and address testing processes). This means that address
routing is no longer run as root, and the deliveries themselves cannot change
to any other uid.

Leaving the binary setuid to root, but setting deliver_drop_privilege means
that the daemon can still be started in the usual way, and it can respond
correctly to SIGHUP because the re-invocation regains root privilege.

An alternative approach is to make Exim setuid to the Exim user and also setgid
to the Exim group. If you do this, the daemon must be started from a root
process. (Calling Exim from a root process makes it behave in the way it does
when it is setuid root.) However, the daemon cannot restart itself after a
SIGHUP signal because it cannot regain privilege.

It is still useful to set deliver_drop_privilege in this case, because it stops
Exim from trying to re-invoke itself to do a delivery after a message has been
received. Such a re-invocation is a waste of resources because it has no
effect.

If restarting the daemon is not an issue (for example, if mua_wrapper is set,
or inetd is being used instead of a daemon), having the binary setuid to the
Exim user seems a clean approach, but there is one complication:

In this style of operation, Exim is running with the real uid and gid set to
those of the calling process, and the effective uid/gid set to Exim's values.
Ideally, any association with the calling process' uid/gid should be dropped,
that is, the real uid/gid should be reset to the effective values so as to
discard any privileges that the caller may have. While some operating systems
have a function that permits this action for a non-root effective uid, quite a
number of them do not. Because of this lack of standardization, Exim does not
address this problem at this time.

For this reason, the recommended approach for "mostly unprivileged" running is
to keep the Exim binary setuid to root, and to set deliver_drop_privilege. This
also has the advantage of allowing a daemon to be used in the most
straightforward way.

If you configure Exim not to run delivery processes as root, there are a number
of restrictions on what you can do:

  * You can deliver only as the Exim user/group. You should explicitly use the 
    user and group options to override routers or local transports that
    normally deliver as the recipient. This makes sure that configurations that
    work in this mode function the same way in normal mode. Any implicit or
    explicit specification of another user causes an error.

  * Use of .forward files is severely restricted, such that it is usually not
    worthwhile to include them in the configuration.

  * Users who wish to use .forward would have to make their home directory and
    the file itself accessible to the Exim user. Pipe and append-to-file
    entries, and their equivalents in Exim filters, cannot be used. While they
    could be enabled in the Exim user's name, that would be insecure and not
    very useful.

  * Unless the local user mailboxes are all owned by the Exim user (possible in
    some POP3 or IMAP-only environments):

     1. They must be owned by the Exim group and be writeable by that group.
        This implies you must set mode in the appendfile configuration, as well
        as the mode of the mailbox files themselves.

     2. You must set no_check_owner, since most or all of the files will not be
        owned by the Exim user.

     3. You must set file_must_exist, because Exim cannot set the owner
        correctly on a newly created mailbox when unprivileged. This also
        implies that new mailboxes need to be created manually.

These restrictions severely restrict what can be done in local deliveries.
However, there are no restrictions on remote deliveries. If you are running a
gateway host that does no local deliveries, setting deliver_drop_privilege
gives more security at essentially no cost.

If you are using the mua_wrapper facility (see chapter 52), 
deliver_drop_privilege is forced to be true.


56.4 Delivering to local files
------------------------------

Full details of the checks applied by appendfile before it writes to a file are
given in chapter 26.


56.5 Running local commands
---------------------------

There are a number of ways in which an administrator can configure Exim to run
commands based upon received, untrustworthy, data. Further, in some
configurations a user who can control a .forward file can also arrange to run
commands. Configuration to check includes, but is not limited to:

  * Use of use_shell in the pipe transport: various forms of shell command
    injection may be possible with this option present. It is dangerous and
    should be used only with considerable caution. Consider constraints which
    whitelist allowed characters in a variable which is to be used in a pipe
    transport that has use_shell enabled.

  * A number of options such as forbid_filter_run, forbid_filter_perl, 
    forbid_filter_dlfunc and so forth which restrict facilities available to
    .forward files in a redirect router. If Exim is running on a central mail
    hub to which ordinary users do not have shell access, but home directories
    are NFS mounted (for instance) then administrators should review the list
    of these forbid options available, and should bear in mind that the options
    that may need forbidding can change as new features are added between
    releases.

  * The ${run...} expansion item does not use a shell by default, but
    administrators can configure use of /bin/sh as part of the command. Such
    invocations should be viewed with prejudicial suspicion.

  * Administrators who use embedded Perl are advised to explore how Perl's
    taint checking might apply to their usage.

  * Use of ${expand...} is somewhat analogous to shell's eval builtin and
    administrators are well advised to view its use with suspicion, in case
    (for instance) it allows a local-part to contain embedded Exim directives.

  * Use of ${match_local_part...} and friends becomes more dangerous if Exim
    was built with EXPAND_LISTMATCH_RHS defined: the second string in each can
    reference arbitrary lists and files, rather than just being a list of
    opaque strings. The EXPAND_LISTMATCH_RHS option was added and set false by
    default because of real-world security vulnerabilities caused by its use
    with untrustworthy data injected in, for SQL injection attacks. Consider
    the use of the inlisti expansion condition instead.


56.6 Trust in configuration data
--------------------------------

If configuration data for Exim can come from untrustworthy sources, there are
some issues to be aware of:

  * Use of ${expand...} may provide a path for shell injection attacks.

  * Letting untrusted data provide a regular expression is unwise.

  * Using ${match...} to apply a fixed regular expression against untrusted
    data may result in pathological behaviour within PCRE2. Be aware of what
    "backtracking" means and consider options for being more strict with a
    regular expression. Avenues to explore include limiting what can match
    (avoiding "." when "[a-z0-9]" or other character class will do), use of
    atomic grouping and possessive quantifiers or just not using regular
    expressions against untrusted data.

  * It can be important to correctly use ${quote:...}, ${quote_local_part:...}
    and ${quote_<lookup-type>:...} expansion items to ensure that data is
    correctly constructed.

  * Some lookups might return multiple results, even though normal usage is
    only expected to yield one result.


56.7 IPv4 source routing
------------------------

Many operating systems suppress IP source-routed packets in the kernel, but
some cannot be made to do this, so Exim does its own check. It logs incoming
IPv4 source-routed TCP calls, and then drops them. Things are all different in
IPv6. No special checking is currently done.


56.8 The VRFY, EXPN, and ETRN commands in SMTP
----------------------------------------------

Support for these SMTP commands is disabled by default. If required, they can
be enabled by defining suitable ACLs.


56.9 Privileged users
---------------------

Exim recognizes two sets of users with special privileges. Trusted users are
able to submit new messages to Exim locally, but supply their own sender
addresses and information about a sending host. For other users submitting
local messages, Exim sets up the sender address from the uid, and doesn't
permit a remote host to be specified.

However, an untrusted user is permitted to use the -f command line option in
the special form -f <> to indicate that a delivery failure for the message
should not cause an error report. This affects the message's envelope, but it
does not affect the Sender: header. Untrusted users may also be permitted to
use specific forms of address with the -f option by setting the 
untrusted_set_sender option.

Trusted users are used to run processes that receive mail messages from some
other mail domain and pass them on to Exim for delivery either locally, or over
the Internet. Exim trusts a caller that is running as root, as the Exim user,
as any user listed in the trusted_users configuration option, or under any
group listed in the trusted_groups option.

Admin users are permitted to do things to the messages on Exim's queue. They
can freeze or thaw messages, cause them to be returned to their senders, remove
them entirely, or modify them in various ways. In addition, admin users can run
the Exim monitor and see all the information it is capable of providing, which
includes the contents of files on the spool.

By default, the use of the -M and -q options to cause Exim to attempt delivery
of messages on its queue is restricted to admin users. This restriction can be
relaxed by setting the no_prod_requires_admin option. Similarly, the use of -bp
(and its variants) to list the contents of the queue is also restricted to
admin users. This restriction can be relaxed by setting 
no_queue_list_requires_admin.

Exim recognizes an admin user if the calling process is running as root or as
the Exim user or if any of the groups associated with the calling process is
the Exim group. It is not necessary actually to be running under the Exim
group. However, if admin users who are not root or the Exim user are to access
the contents of files on the spool via the Exim monitor (which runs
unprivileged), Exim must be built to allow group read access to its spool
files.

By default, regular users are trusted to perform basic testing and
introspection commands, as themselves. This setting can be tightened by setting
the commandline_checks_require_admin option. This affects most of the checking
options, such as -be and anything else -b*.


56.10 Spool files
-----------------

Exim's spool directory and everything it contains is owned by the Exim user and
set to the Exim group. The mode for spool files is defined in the Local/
Makefile configuration file, and defaults to 0640. This means that any user who
is a member of the Exim group can access these files.


56.11 Use of argv[0]
--------------------

Exim examines the last component of argv[0], and if it matches one of a set of
specific strings, Exim assumes certain options. For example, calling Exim with
the last component of argv[0] set to "rsmtp" is exactly equivalent to calling
it with the option -bS. There are no security implications in this.


56.12 Use of %f formatting
--------------------------

The only use made of "%f" by Exim is in formatting load average values. These
are actually stored in integer variables as 1000 times the load average.
Consequently, their range is limited and so therefore is the length of the
converted output.


56.13 Embedded Exim path
------------------------

Exim uses its own path name, which is embedded in the code, only when it needs
to re-exec in order to regain root privilege. Therefore, it is not root when it
does so. If some bug allowed the path to get overwritten, it would lead to an
arbitrary program's being run as exim, not as root.


56.14 Dynamic module directory
------------------------------

Any dynamically loadable modules must be installed into the directory defined
in "LOOKUP_MODULE_DIR" in Local/Makefile for Exim to permit loading it.


56.15 Use of sprintf()
----------------------

A large number of occurrences of "sprintf" in the code are actually calls to 
string_sprintf(), a function that returns the result in malloc'd store. The
intermediate formatting is done into a large fixed buffer by a function that
runs through the format string itself, and checks the length of each conversion
before performing it, thus preventing buffer overruns.

The remaining uses of sprintf() happen in controlled circumstances where the
output buffer is known to be sufficiently long to contain the converted string.


56.16 Use of debug_printf() and log_write()
-------------------------------------------

Arbitrary strings are passed to both these functions, but they do their
formatting by calling the function string_vformat(), which runs through the
format string itself, and checks the length of each conversion.


56.17 Use of strcat() and strcpy()
----------------------------------

These are used only in cases where the output buffer is known to be large
enough to hold the result.



===============================================================================
57. FORMAT OF SPOOL FILES

A message on Exim's queue consists of two files, whose names are the message id
followed by -D and -H, respectively. The data portion of the message is kept in
the -D file on its own. The message's envelope, status, and headers are all
kept in the -H file, whose format is described in this chapter. Each of these
two files contains the final component of its own name as its first line. This
is insurance against disk crashes where the directory is lost but the files
themselves are recoverable.

The file formats may be changed, or new formats added, at any release. Spool
files are not intended as an interface to other programs and should not be used
as such.

Some people are tempted into editing -D files in order to modify messages. You
need to be extremely careful if you do this; it is not recommended and you are
on your own if you do it. Here are some of the pitfalls:

  * You must ensure that Exim does not try to deliver the message while you are
    fiddling with it. The safest way is to take out a write lock on the -D
    file, which is what Exim itself does, using fcntl(). If you update the file
    in place, the lock will be retained. If you write a new file and rename it,
    the lock will be lost at the instant of rename.

  * If you change the number of lines in the file, the value of $body_linecount
    , which is stored in the -H file, will be incorrect and can cause
    incomplete transmission of messages or undeliverable messages.

  * If the message is in MIME format, you must take care not to break it.

  * If the message is cryptographically signed, any change will invalidate the
    signature.

All in all, modifying -D files is fraught with danger.

Files whose names end with -J may also be seen in the input directory (or its
subdirectories when split_spool_directory is set). These are journal files,
used to record addresses to which the message has been delivered during the
course of a delivery attempt. If there are still undelivered recipients at the
end, the -H file is updated, and the -J file is deleted. If, however, there is
some kind of crash (for example, a power outage) before this happens, the -J
file remains in existence. When Exim next processes the message, it notices the
-J file and uses it to update the -H file before starting the next delivery
attempt.

Files whose names end with -K or .eml may also be seen in the spool. These are
temporaries used for DKIM or malware processing, when that is used. They should
be tidied up by normal operations; any old ones are probably relics of crashes
and can be removed.


57.1 Format of the -H file
--------------------------

The second line of the -H file contains the login name for the uid of the
process that called Exim to read the message, followed by the numerical uid and
gid. For a locally generated message, this is normally the user who sent the
message. For a message received over TCP/IP via the daemon, it is normally the
Exim user.

The third line of the file contains the address of the message's sender as
transmitted in the envelope, contained in angle brackets. The sender address is
empty for bounce messages. For incoming SMTP mail, the sender address is given
in the MAIL command. For locally generated mail, the sender address is created
by Exim from the login name of the current user and the configured 
qualify_domain. However, this can be overridden by the -f option or a leading
"From " line if the caller is trusted, or if the supplied address is "<>" or an
address that matches untrusted_set_senders.

The fourth line contains two numbers. The first is the time that the message
was received, in the conventional Unix form - the number of seconds since the
start of the epoch. The second number is a count of the number of messages
warning of delayed delivery that have been sent to the sender.

There follow a number of lines starting with a hyphen. These contain variables,
can appear in any order, and are omitted when not relevant.

If there is a second hyphen after the first, the corresponding data is tainted.
If there is a value in parentheses, the data is quoted for a lookup.

The following word specifies a variable, and the remainder of the item depends
on the variable.

-acl <number> <length>

    This item is obsolete, and is not generated from Exim release 4.61 onwards;
    -aclc and -aclm are used instead. However, -acl is still recognized, to
    provide backward compatibility. In the old format, a line of this form is
    present for every ACL variable that is not empty. The number identifies the
    variable; the acl_cx variables are numbered 0-9 and the acl_mx variables
    are numbered 10-19. The length is the length of the data string for the
    variable. The string itself starts at the beginning of the next line, and
    is followed by a newline character. It may contain internal newlines.

-aclc <rest-of-name> <length>

    A line of this form is present for every ACL connection variable that is
    defined. Note that there is a space between -aclc and the rest of the name.
    The length is the length of the data string for the variable. The string
    itself starts at the beginning of the next line, and is followed by a
    newline character. It may contain internal newlines.

-aclm <rest-of-name> <length>

    A line of this form is present for every ACL message variable that is
    defined. Note that there is a space between -aclm and the rest of the name.
    The length is the length of the data string for the variable. The string
    itself starts at the beginning of the next line, and is followed by a
    newline character. It may contain internal newlines.

-active_hostname <hostname>

    This is present if, when the message was received over SMTP, the value of
    $smtp_active_hostname was different to the value of $primary_hostname.

-allow_unqualified_recipient

    This is present if unqualified recipient addresses are permitted in header
    lines (to stop such addresses from being qualified if rewriting occurs at
    transport time). Local messages that were input using -bnq and remote
    messages from hosts that match recipient_unqualified_hosts set this flag.

-allow_unqualified_sender

    This is present if unqualified sender addresses are permitted in header
    lines (to stop such addresses from being qualified if rewriting occurs at
    transport time). Local messages that were input using -bnq and remote
    messages from hosts that match sender_unqualified_hosts set this flag.

-auth_id <text>

    The id information for a message received on an authenticated SMTP
    connection - the value of the $authenticated_id variable.

-auth_sender <address>

    The address of an authenticated sender - the value of the
    $authenticated_sender variable.

-body_linecount <number>

    This records the number of lines in the body of the message, and is present
    unless -spool_file_wireformat is.

-body_zerocount <number>

    This records the number of binary zero bytes in the body of the message,
    and is present if the number is greater than zero.

-deliver_firsttime

    This is written when a new message is first added to the spool. When the
    spool file is updated after a deferral, it is omitted.

-frozen <time>

    The message is frozen, and the freezing happened at <time>.

-helo_name <text>

    This records the host name as specified by a remote host in a HELO or EHLO
    command.

-host_address <address>.<port>

    This records the IP address of the host from which the message was received
    and the remote port number that was used. It is omitted for locally
    generated messages.

-host_auth <text>

    If the message was received on an authenticated SMTP connection, this
    records the name of the authenticator - the value of the
    $sender_host_authenticated variable.

-host_lookup_failed

    This is present if an attempt to look up the sending host's name from its
    IP address failed. It corresponds to the $host_lookup_failed variable.

-host_name <text>

    This records the name of the remote host from which the message was
    received, if the host name was looked up from the IP address when the
    message was being received. It is not present if no reverse lookup was
    done.

-ident <text>

    For locally submitted messages, this records the login of the originating
    user, unless it was a trusted user and the -oMt option was used to specify
    an ident value. For messages received over TCP/IP, this records the ident
    string supplied by the remote host, if any.

-interface_address <address>.<port>

    This records the IP address of the local interface and the port number
    through which a message was received from a remote host. It is omitted for
    locally generated messages.

-local

    The message is from a local sender.

-localerror

    The message is a locally-generated bounce message.

-local_scan <string>

    This records the data string that was returned by the local_scan() function
    when the message was received - the value of the $local_scan_data variable.
    It is omitted if no data was returned.

-manual_thaw

    The message was frozen but has been thawed manually, that is, by an
    explicit Exim command rather than via the auto-thaw process.

-N

    A testing delivery process was started using the -N option to suppress any
    actual deliveries, but delivery was deferred. At any further delivery
    attempts, -N is assumed.

-received_protocol

    This records the value of the $received_protocol variable, which contains
    the name of the protocol by which the message was received.

-sender_set_untrusted

    The envelope sender of this message was set by an untrusted local caller
    (used to ensure that the caller is displayed in queue listings).

-spam_score_int <number>

    If a message was scanned by SpamAssassin, this is present. It records the
    value of $spam_score_int.

-spool_file_wireformat

    The -D file for this message is in wire-format (for ESMTP CHUNKING) rather
    than Unix-format. The line-ending is CRLF rather than newline. There is
    still, however, no leading-dot-stuffing.

-tls_certificate_verified

    A TLS certificate was received from the client that sent this message, and
    the certificate was verified by the server.

-tls_cipher <cipher name>

    When the message was received over an encrypted connection, this records
    the name of the cipher suite that was used.

-tls_peerdn <peer DN>

    When the message was received over an encrypted connection, and a
    certificate was received from the client, this records the Distinguished
    Name from that certificate.

Following the options there is a list of those addresses to which the message
is not to be delivered. This set of addresses is initialized from the command
line when the -t option is used and extract_addresses_remove_arguments is set;
otherwise it starts out empty. Whenever a successful delivery is made, the
address is added to this set. The addresses are kept internally as a balanced
binary tree, and it is a representation of that tree which is written to the
spool file. If an address is expanded via an alias or forward file, the
original address is added to the tree when deliveries to all its child
addresses are complete.

If the tree is empty, there is a single line in the spool file containing just
the text "XX". Otherwise, each line consists of two letters, which are either Y
or N, followed by an address. The address is the value for the node of the
tree, and the letters indicate whether the node has a left branch and/or a
right branch attached to it, respectively. If branches exist, they immediately
follow. Here is an example of a three-node tree:

YY darcy@austen.fict.example
NN alice@wonderland.fict.example
NN editor@thesaurus.ref.example

After the non-recipients tree, there is a list of the message's recipients.
This is a simple list, preceded by a count. It includes all the original
recipients of the message, including those to whom the message has already been
delivered. In the simplest case, the list contains one address per line. For
example:

4
editor@thesaurus.ref.example
darcy@austen.fict.example
rdo@foundation
alice@wonderland.fict.example

However, when a child address has been added to the top-level addresses as a
result of the use of the one_time option on a redirect router, each line is of
the following form:

<top-level address> <errors_to address> <length>,<parent number>#<flag bits>

The 01 flag bit indicates the presence of the three other fields that follow
the top-level address. Other bits may be used in future to support additional
fields. The <parent number> is the offset in the recipients list of the
original parent of the "one time" address. The first two fields are the
envelope sender that is associated with this address and its length. If the
length is zero, there is no special envelope sender (there are then two space
characters in the line). A non-empty field can arise from a redirect router
that has an errors_to setting.

A blank line separates the envelope and status information from the headers
which follow. A header may occupy several lines of the file, and to save effort
when reading it in, each header is preceded by a number and an identifying
character. The number is the number of characters in the header, including any
embedded newlines and the terminating newline. The character is one of the
following:

<blank> header in which Exim has no special interest
"B"     Bcc: header
"C"     Cc: header
"F"     From: header
"I"     Message-id: header
"P"     Received: header - P for "postmark"
"R"     Reply-To: header
"S"     Sender: header
"T"     To: header
"*"     replaced or deleted header

Deleted or replaced (rewritten) headers remain in the spool file for debugging
purposes. They are not transmitted when the message is delivered. Here is a
typical set of headers:

111P Received: by hobbit.fict.example with local (Exim 4.00)
id 14y9EI-00026G-00; Fri, 11 May 2001 10:28:59 +0100
049  Message-Id: <E14y9EI-00026G-00@hobbit.fict.example>
038* X-rewrote-sender: bb@hobbit.fict.example
042* From: Bilbo Baggins <bb@hobbit.fict.example>
049F From: Bilbo Baggins <B.Baggins@hobbit.fict.example>
099* To: alice@wonderland.fict.example, rdo@foundation,
darcy@austen.fict.example, editor@thesaurus.ref.example
104T To: alice@wonderland.fict.example, rdo@foundation.example,
darcy@austen.fict.example, editor@thesaurus.ref.example
038  Date: Fri, 11 May 2001 10:28:59 +0100

The asterisked headers indicate that the envelope sender, From: header, and To:
header have been rewritten, the last one because routing expanded the
unqualified domain foundation.


57.2 Format of the -D file
--------------------------

The data file is traditionally in Unix-standard format: lines are ended with an
ASCII newline character. However, when the spool_wireformat main option is used
some -D files can have an alternate format. This is flagged by a 
-spool_file_wireformat line in the corresponding -H file. The -D file lines
(not including the first name-component line) are suitable for direct copying
to the wire when transmitting using the ESMTP CHUNKING option, meaning lower
processing overhead. Lines are terminated with an ASCII CRLF pair. There is no
dot-stuffing (and no dot-termination).



===============================================================================
58. DKIM, SPF, SRS AND DMARC


58.1 DKIM (DomainKeys Identified Mail)
--------------------------------------

DKIM is a mechanism by which messages sent by some entity can be provably
linked to a domain which that entity controls. It permits reputation to be
tracked on a per-domain basis, rather than merely upon source IP address. DKIM
is documented in RFC 6376.

As DKIM relies on the message being unchanged in transit, messages handled by a
mailing-list (which traditionally adds to the message) will not match any
original DKIM signature.

DKIM support is compiled into Exim by default if TLS support is present. It can
be disabled by setting DISABLE_DKIM=yes in Local/Makefile.

Exim's DKIM implementation allows for

 1. Signing outgoing messages: This function is implemented in the SMTP
    transport. It can co-exist with all other Exim features (including
    transport filters) except cutthrough delivery.

 2. Verifying signatures in incoming messages: This is implemented by an
    additional ACL (acl_smtp_dkim), which can be called several times per
    message, with different signature contexts.

In typical Exim style, the verification implementation does not include any
default "policy". Instead it enables you to build your own policy using Exim's
standard controls.

Please note that verification of DKIM signatures in incoming mail is turned on
by default for logging (in the <= line) purposes.

Additional log detail can be enabled using the dkim_verbose log_selector. When
set, for each signature in incoming email, exim will log a line displaying the
most important signature details, and the signature status. Here is an example
(with line-breaks added for clarity):

2009-09-09 10:22:28 1MlIRf-0003LU-U3 DKIM:
    d=facebookmail.com s=q1-2009b
    c=relaxed/relaxed a=rsa-sha1
    i=@facebookmail.com t=1252484542 [verification succeeded]

You might want to turn off DKIM verification processing entirely for internal
or relay mail sources. To do that, set the dkim_disable_verify ACL control
modifier. This should typically be done in the RCPT ACL, at points where you
accept mail from relay sources (internal hosts or authenticated senders).

58.1.1 Signing outgoing messages

For signing to be usable you must have published a DKIM record in DNS. Note
that RFC 8301 (which does not cover EC keys) says:

rsa-sha1 MUST NOT be used for signing or verifying.

Signers MUST use RSA keys of at least 1024 bits for all keys.
Signers SHOULD use RSA keys of at least 2048 bits.

Note also that the key content (the 'p=' field) in the DNS record is different
between RSA and EC keys; for the former it is the base64 of the ASN.1 for the
RSA public key (equivalent to the private-key .pem with the header/trailer
stripped) but for EC keys it is the base64 of the pure key; no ASN.1 wrapping.

Signing is enabled by setting private options on the SMTP transport. These
options take (expandable) strings as arguments.

+-------------------------------------------------------+
|dkim_domain|Use: smtp|Type: string list*|Default: unset|
+-------------------------------------------------------+

The domain(s) you want to sign with. After expansion, this can be a list. Each
element in turn, lowercased, is put into the $dkim_domain expansion variable
while expanding the remaining signing options. If it is empty after expansion,
DKIM signing is not done, and no error will result even if dkim_strict is set.

+---------------------------------------------------------+
|dkim_selector|Use: smtp|Type: string list*|Default: unset|
+---------------------------------------------------------+

This sets the key selector string. After expansion, which can use $dkim_domain,
this can be a list. Each element in turn is put in the expansion variable 
$dkim_selector which may be used in the dkim_private_key option along with 
$dkim_domain. If the option is empty after expansion, DKIM signing is not done
for this domain, and no error will result even if dkim_strict is set.

To do, for example, dual-signing with RSA and EC keys this could be be used:

dkim_selector = ec_sel : rsa_sel
dkim_private_key = KEYS_DIR/$dkim_selector

+-------------------------------------------------------+
|dkim_private_key|Use: smtp|Type: string*|Default: unset|
+-------------------------------------------------------+

This sets the private key to use. You can use the $dkim_domain and 
$dkim_selector expansion variables to determine the private key to use. The
result can either

  * be a valid RSA private key in ASCII armor (.pem file), including line
    breaks

  * with GnuTLS 3.6.0 or OpenSSL 1.1.1 or later, be a valid Ed25519 private key
    (same format as above)

  * start with a slash, in which case it is treated as a file that contains the
    private key

  * be "0", "false" or the empty string, in which case the message will not be
    signed. This case will not result in an error, even if dkim_strict is set.

To generate keys under OpenSSL:

openssl genrsa -out dkim_rsa.private 2048
openssl rsa -in dkim_rsa.private -out /dev/stdout -pubout -outform PEM

The result file from the first command should be retained, and this option set
to use it. Take the base-64 lines from the output of the second command,
concatenated, for the DNS TXT record. See section 3.6 of RFC6376 for the record
specification.

Under GnuTLS:

certtool --generate-privkey --rsa --bits=2048 --password='' -8 --outfile=dkim_rsa.private
certtool --load-privkey=dkim_rsa.private --pubkey-info

Note that RFC 8301 says:

Signers MUST use RSA keys of at least 1024 bits for all keys.
Signers SHOULD use RSA keys of at least 2048 bits.

EC keys for DKIM are defined by RFC 8463. They are considerably smaller than
RSA keys for equivalent protection. As they are a recent development, users
should consider dual-signing (by setting a list of selectors, and an expansion
for this option) for some transition period. The "_CRYPTO_SIGN_ED25519" macro
will be defined if support is present for EC keys.

OpenSSL 1.1.1 and GnuTLS 3.6.0 can create Ed25519 private keys:

openssl genpkey -algorithm ed25519 -out dkim_ed25519.private
certtool --generate-privkey --key-type=ed25519 --outfile=dkim_ed25519.private

To produce the required public key value for a DNS record:

openssl pkey -outform DER -pubout -in dkim_ed25519.private | tail -c +13 | base64
certtool --load_privkey=dkim_ed25519.private --pubkey_info --outder | tail -c +13 | base64

Exim also supports an alternate format of Ed25519 keys in DNS which was a
candidate during development of the standard, but not adopted. A future release
will probably drop that support.

+-------------------------------------------------+
|dkim_hash|Use: smtp|Type: string*|Default: sha256|
+-------------------------------------------------+

Can be set to any one of the supported hash methods, which are:

  * "sha1" - should not be used, is old and insecure

  * "sha256" - the default

  * "sha512" - possibly more secure but less well supported

Note that RFC 8301 says:

rsa-sha1 MUST NOT be used for signing or verifying.

+----------------------------------------------------+
|dkim_identity|Use: smtp|Type: string*|Default: unset|
+----------------------------------------------------+

If set after expansion, the value is used to set an "i=" tag in the signing
header. The DKIM standards restrict the permissible syntax of this optional tag
to a mail address, with possibly-empty local part, an @, and a domain identical
to or subdomain of the "d=" tag value. Note that Exim does not check the value.

+-------------------------------------------------+
|dkim_canon|Use: smtp|Type: string*|Default: unset|
+-------------------------------------------------+

This option sets the canonicalization method used when signing a message. The
DKIM RFC currently supports two methods: "simple" and "relaxed". The option
defaults to "relaxed" when unset. Note: the current implementation only
supports signing with the same canonicalization method for both headers and
body.

+--------------------------------------------------+
|dkim_strict|Use: smtp|Type: string*|Default: unset|
+--------------------------------------------------+

This option defines how Exim behaves when signing a message that should be
signed fails for some reason. When the expansion evaluates to either "1" or
"true", Exim will defer. Otherwise Exim will send the message unsigned. You can
use the $dkim_domain and $dkim_selector expansion variables here.

+------------------------------------------------------------+
|dkim_sign_headers|Use: smtp|Type: string*|Default: see below|
+------------------------------------------------------------+

If set, this option must expand to a colon-separated list of header names.
Headers with these names, or the absence or such a header, will be included in
the message signature. When unspecified, the header names listed in RFC4871
will be used, whether or not each header is present in the message. The default
list is available for the expansion in the macro "_DKIM_SIGN_HEADERS" and an
oversigning variant is in "_DKIM_OVERSIGN_HEADERS".

If a name is repeated, multiple headers by that name (or the absence thereof)
will be signed. The textually later headers in the headers part of the message
are signed first, if there are multiples.

A name can be prefixed with either an "=" or a "+" character. If an "=" prefix
is used, all headers that are present with this name will be signed. If a "+"
prefix if used, all headers that are present with this name will be signed, and
one signature added for a missing header with the name will be appended.

+-------------------------------------------------------+
|dkim_timestamps|Use: smtp|Type: integer*|Default: unset|
+-------------------------------------------------------+

This option controls the inclusion of timestamp information in the signature.
If not set, no such information will be included. Otherwise, must be an
unsigned number giving an offset in seconds from the current time for the
expiry tag (eg. 1209600 for two weeks); both creation (t=) and expiry (x=) tags
will be included.

RFC 6376 lists these tags as RECOMMENDED.

58.1.2 Verifying DKIM signatures in incoming mail

Verification of DKIM signatures in SMTP incoming email is done for all messages
for which an ACL control dkim_disable_verify has not been set. Individual
classes of signature algorithm can be ignored by changing the main options 
dkim_verify_hashes or dkim_verify_keytypes. The dkim_verify_minimal option can
be set to cease verification processing for a message once the first passing
signature is found.

Performing verification sets up information used by the authresults expansion
item.

For most purposes the default option settings suffice and the remainder of this
section can be ignored.

The results of verification are made available to the acl_smtp_dkim ACL, which
can examine and modify them. A missing ACL definition defaults to accept. By
default, the ACL is called once for each syntactically(!) correct signature in
the incoming message. If any ACL call does not accept, the message is not
accepted. If a cutthrough delivery was in progress for the message, that is
summarily dropped (having wasted the transmission effort).

To evaluate the verification result in the ACL a large number of expansion
variables containing the signature status and its details are set up during the
runtime of the ACL.

Calling the ACL only for existing signatures is not sufficient to build more
advanced policies. For that reason, the main option dkim_verify_signers, and an
expansion variable $dkim_signers exist.

The main option dkim_verify_signers can be set to a colon-separated list of
DKIM domains or identities for which the ACL acl_smtp_dkim is called. It is
expanded when the message has been received. At this point, the expansion
variable $dkim_signers already contains a colon-separated list of signer
domains and identities for the message. When dkim_verify_signers is not
specified in the main configuration, it defaults as:

dkim_verify_signers = $dkim_signers

This leads to the default behaviour of calling acl_smtp_dkim for each DKIM
signature in the message. Current DKIM verifiers may want to explicitly call
the ACL for known domains or identities. This would be achieved as follows:

dkim_verify_signers = paypal.com:ebay.com:$dkim_signers

This would result in acl_smtp_dkim always being called for "paypal.com" and
"ebay.com", plus all domains and identities that have signatures in the
message. You can also be more creative in constructing your policy. For
example:

dkim_verify_signers = $sender_address_domain:$dkim_signers

If a domain or identity is listed several times in the (expanded) value of 
dkim_verify_signers, the ACL is only called once for that domain or identity.

Note that if the option is set using untrustworthy data (such as the From:
header) care should be taken to force lowercase for domains and for the domain
part if identities. The default setting can be regarded as trustworthy in this
respect.

If multiple signatures match a domain (or identity), the ACL is called once for
each matching signature.

Inside the DKIM ACL, the following expansion variables are available (from most
to least important):

$dkim_cur_signer

    The signer that is being evaluated in this ACL run. This can be a domain or
    an identity. This is one of the list items from the expanded main option 
    dkim_verify_signers (see above).

$dkim_verify_status

    Within the DKIM ACL, a string describing the general status of the
    signature. One of

      o none: There is no signature in the message for the current domain or
        identity (as reflected by $dkim_cur_signer).

      o invalid: The signature could not be verified due to a processing error.
        More detail is available in $dkim_verify_reason.

      o fail: Verification of the signature failed. More detail is available in
        $dkim_verify_reason.

      o pass: The signature passed verification. It is valid.

    This variable can be overwritten using an ACL 'set' modifier. This might,
    for instance, be done to enforce a policy restriction on hash-method or
    key-size:

      warn condition       = ${if eq {$dkim_verify_status}{pass}}
           condition       = ${if eq {${length_3:$dkim_algo}}{rsa}}
           condition       = ${if or {{eq {$dkim_algo}{rsa-sha1}} \
                                      {< {$dkim_key_length}{1024}}}}
           logwrite        = NOTE: forcing DKIM verify fail (was pass)
           set dkim_verify_status = fail
           set dkim_verify_reason = hash too weak or key too short

    So long as a DKIM ACL is defined (it need do no more than accept), after
    all the DKIM ACL runs have completed, the value becomes a colon-separated
    list of the values after each run. This is maintained for the mime, prdr
    and data ACLs.

$dkim_verify_reason

    A string giving a little bit more detail when $dkim_verify_status is either
    "fail" or "invalid". One of

      o pubkey_unavailable (when $dkim_verify_status="invalid"): The public key
        for the domain could not be retrieved. This may be a temporary problem.

      o pubkey_syntax (when $dkim_verify_status="invalid"): The public key
        record for the domain is syntactically invalid.

      o bodyhash_mismatch (when $dkim_verify_status="fail"): The calculated
        body hash does not match the one specified in the signature header.
        This means that the message body was modified in transit.

      o signature_incorrect (when $dkim_verify_status="fail"): The signature
        could not be verified. This may mean that headers were modified,
        re-written or otherwise changed in a way which is incompatible with
        DKIM verification. It may of course also mean that the signature is
        forged.

    This variable can be overwritten, with any value, using an ACL 'set'
    modifier.

$dkim_domain

    The signing domain. IMPORTANT: This variable is only populated if there is
    an actual signature in the message for the current domain or identity (as
    reflected by $dkim_cur_signer).

$dkim_identity

    The signing identity, if present. IMPORTANT: This variable is only
    populated if there is an actual signature in the message for the current
    domain or identity (as reflected by $dkim_cur_signer).

$dkim_selector

    The key record selector string.

$dkim_algo

    The algorithm used. One of 'rsa-sha1' or 'rsa-sha256'. If running under
    GnuTLS 3.6.0 or OpenSSL 1.1.1 or later, may also be 'ed25519-sha256'. The
    "_CRYPTO_SIGN_ED25519" macro will be defined if support is present for EC
    keys.

    Note that RFC 8301 says:

    rsa-sha1 MUST NOT be used for signing or verifying.

    DKIM signatures identified as having been signed with historic
    algorithms (currently, rsa-sha1) have permanently failed evaluation

    To enforce this you must either have a DKIM ACL which checks this variable
    and overwrites the $dkim_verify_status variable as discussed above, or have
    set the main option dkim_verify_hashes to exclude processing of such
    signatures.

$dkim_canon_body

    The body canonicalization method. One of 'relaxed' or 'simple'.

$dkim_canon_headers

    The header canonicalization method. One of 'relaxed' or 'simple'.

$dkim_copiedheaders

    A transcript of headers and their values which are included in the
    signature (copied from the 'z=' tag of the signature). Note that RFC6376
    requires that verification fail if the From: header is not included in the
    signature. Exim does not enforce this; sites wishing strict enforcement
    should code the check explicitly.

$dkim_bodylength

    The number of signed body bytes. If zero ("0"), the body is unsigned. If no
    limit was set by the signer, "9999999999999" is returned. This makes sure
    that this variable always expands to an integer value. Note: The presence
    of the signature tag specifying a signing body length is one possible route
    to spoofing of valid DKIM signatures. A paranoid implementation might wish
    to regard signature where this variable shows less than the "no limit"
    return as being invalid.

$dkim_created

    UNIX timestamp reflecting the date and time when the signature was created.
    When this was not specified by the signer, "0" is returned.

$dkim_expires

    UNIX timestamp reflecting the date and time when the signer wants the
    signature to be treated as "expired". When this was not specified by the
    signer, "9999999999999" is returned. This makes it possible to do useful
    integer size comparisons against this value. Note that Exim does not check
    this value.

$dkim_headernames

    A colon-separated list of names of headers included in the signature.

$dkim_key_testing

    "1" if the key record has the "testing" flag set, "0" if not.

$dkim_key_nosubdomains

    "1" if the key record forbids subdomaining, "0" otherwise.

$dkim_key_srvtype

    Service type (tag s=) from the key record. Defaults to "*" if not specified
    in the key record.

$dkim_key_granularity

    Key granularity (tag g=) from the key record. Defaults to "*" if not
    specified in the key record.

$dkim_key_notes

    Notes from the key record (tag n=).

$dkim_key_length

    Number of bits in the key. Valid only once the key is loaded, which is at
    the time the header signature is verified, which is after the body hash is.

    Note that RFC 8301 says:

    Verifiers MUST NOT consider signatures using RSA keys of
    less than 1024 bits as valid signatures.

    This is enforced by the default setting for the dkim_verify_min_keysizes
    option.

In addition, two ACL conditions are provided, usable only in a DKIM ACL:

dkim_signers

    ACL condition that checks a colon-separated list of domains or identities
    for a match against the domain or identity that the ACL is currently
    verifying (reflected by $dkim_cur_signer). This is typically used to
    restrict an ACL verb to a group of domains or identities. For example:

    # Warn when Mail purportedly from GMail has no gmail signature
    warn sender_domains = gmail.com
         dkim_signers = gmail.com
         dkim_status = none
         log_message = GMail sender without gmail.com DKIM signature

    Note that the above does not check for a total lack of DKIM signing; for
    that check for empty $h_DKIM-Signature: in the data ACL.

dkim_status

    ACL condition that checks a colon-separated list of possible DKIM
    verification results against the actual result of verification. This is
    typically used to restrict an ACL verb to a list of verification outcomes,
    for example:

    deny sender_domains = paypal.com:paypal.de
         dkim_signers = paypal.com:paypal.de
         dkim_status = none:invalid:fail
         message = Mail from Paypal with invalid/missing signature

    The possible status keywords are: 'none','invalid','fail' and 'pass'.
    Please see the documentation of the $dkim_verify_status expansion variable
    above for more information of what they mean.


58.2 SPF (Sender Policy Framework)
----------------------------------

SPF is a mechanism whereby a domain may assert which IP addresses may transmit
messages with its domain in the envelope from, documented by RFC 7208. For more
information on SPF see http://www.open-spf.org, a static copy of the http://
openspf.org.

Messages sent by a system not authorised will fail checking of such assertions.
This includes retransmissions done by traditional forwarders.

SPF verification support is built into Exim if SUPPORT_SPF=yes is set in Local/
Makefile. The support uses the libspf2 library https://www.libspf2.org/. There
is no Exim involvement in the transmission of messages; publishing certain DNS
records is all that is required.

For verification, an ACL condition and an expansion lookup are provided. 
Performing verification sets up information used by the authresults expansion
item.

The ACL condition "spf" can be used at or after the MAIL ACL. It takes as an
argument a list of strings giving the outcome of the SPF check, and will
succeed for any matching outcome. Valid strings are:

pass

    The SPF check passed, the sending host is positively verified by SPF.

fail

    The SPF check failed, the sending host is NOT allowed to send mail for the
    domain in the envelope-from address.

softfail

    The SPF check failed, but the queried domain can't absolutely confirm that
    this is a forgery.

none

    The queried domain does not publish SPF records.

neutral

    The SPF check returned a "neutral" state. This means the queried domain has
    published a SPF record, but wants to allow outside servers to send mail
    under its domain as well. This should be treated like "none".

permerror

    This indicates a syntax error in the SPF record of the queried domain. You
    may deny messages when this occurs.

temperror

    This indicates a temporary error during all processing, including Exim's
    SPF processing. You may defer messages when this occurs.

invalid

    There was an error during processing of the SPF lookup

You can prefix each string with an exclamation mark to invert its meaning, for
example "!fail" will match all results but "fail". The string list is evaluated
left-to-right, in a short-circuit fashion.

Example:

deny spf = fail
     message = $sender_host_address is not allowed to send mail from \
               ${if def:sender_address_domain \
                    {$sender_address_domain}{$sender_helo_name}}.  \
               Please see http://www.open-spf.org/Why;\
               identity=${if def:sender_address_domain \
                             {$sender_address}{$sender_helo_name}};\
               ip=$sender_host_address

Note: The above mentioned URL may not be as helpful as expected. You are
encouraged to replace the link with a link to a site with more explanations.

When the spf condition has run, it sets up several expansion variables:

$spf_header_comment

    This contains a human-readable string describing the outcome of the SPF
    check. You can add it to a custom header or use it for logging purposes.

$spf_received

    This contains a complete Received-SPF: header (name and content) that can
    be added to the message. Please note that according to the SPF draft, this
    header must be added at the top of the header list, i.e. with

    add_header = :at_start:$spf_received

    See section 44.15 for further details.

    Note: in case of "Best-guess" (see below), the convention is to put this
    string in a header called X-SPF-Guess: instead.

$spf_result

    This contains the outcome of the SPF check in string form, currently one of
    pass, fail, softfail, none, neutral, permerror, temperror, or "(invalid)".

$spf_result_guessed

    This boolean is true only if a best-guess operation was used and required
    in order to obtain a result.

$spf_smtp_comment

    This contains a string that can be used in a SMTP response to the calling
    party. Useful for "fail". The string is generated by the SPF library from
    the template configured in the main config option spf_smtp_comment_template
    .

In addition to SPF, you can also perform checks for so-called "Best-guess".
Strictly speaking, "Best-guess" is not standard SPF, but it is supported by the
same framework that enables SPF capability. Refer to http://www.open-spf.org/
FAQ/Best_guess_record for a description of what it means.

To access this feature, simply use the spf_guess condition in place of the spf
one. For example:

deny spf_guess = fail
     message = $sender_host_address doesn't look trustworthy to me

In case you decide to reject messages based on this check, you should note that
although it uses the same framework, "Best-guess" is not SPF, and therefore you
should not mention SPF at all in your reject message.

When the spf_guess condition has run, it sets up the same expansion variables
as when spf condition is run, described above.

Additionally, since Best-guess is not standardized, you may redefine what
"Best-guess" means to you by redefining the main configuration spf_guess
option. For example, the following:

spf_guess = v=spf1 a/16 mx/16 ptr ?all

would relax host matching rules to a broader network range.

A lookup expansion is also available. It takes an email address as the key and
an IP address (v4 or v6) as the database:

  ${lookup {username@domain} spf {ip.ip.ip.ip}}

The lookup will return the same result strings as can appear in $spf_result
(pass,fail,softfail,neutral,none,err_perm,err_temp).

58.2.1 SRS (Sender Rewriting Scheme)

SRS can be used to modify sender addresses when forwarding so that SPF
verification does not object to them. It operates by encoding the original
envelope sender in a new sender local part and using a domain run by the
forwarding site as the new domain for the sender. Any DSN message should be
returned to this new sender at the forwarding site, which can extract the
original sender from the coded local part and forward the DSN to the
originator.

This is a way of avoiding the breakage that SPF does to forwarding. The
constructed local-part will be longer than the original, leading to possible
problems with very long addresses. The changing of the sender address also
hinders the tracing of mail problems.

Exim can be built to include native SRS support. To do this SUPPORT_SRS=yes
must be defined in Local/Makefile. If this has been done, the macros _HAVE_SRS
and _HAVE_NATIVE_SRS will be defined. The support is limited to SRS0-encoding;
SRS1 is not supported.

To encode an address use this expansion item:

${srs_encode {<secret>}{<return path>}{<original domain>}}

    The first argument should be a secret known and used by all systems
    handling the recipient domain for the original message. There is no need to
    periodically change this key; a timestamp is also encoded. The second
    argument should be given as the envelope sender address before this
    encoding operation. If this value is empty the the expansion result will be
    empty. The third argument should be the recipient domain of the message
    when it arrived at this system. All arguments are expanded before use.

    The result of the expansion is the replacement envelope-from (return path)
    to be used.

To decode an address use this expansion condition:

inbound_srs {<local part>}{<secret>}

    The first argument should be the recipient local part as it was received.
    The second argument is the site secret. Both arguments are expanded before
    use.

    If the messages is not for an SRS-encoded recipient the condition will
    return false. If it is, the condition will return true and the variable
    $srs_recipient will be set to the decoded (original) value.

    If the second argument is empty then the condition returns true if the
    first argument is in valid SRS formet, else false. The variable
    $srs_recipient is not set for this case.

Example usage:

  #macro
  SRS_SECRET = <pick something unique for your site for this. Use on all MXs.>

  #routers

  outbound:
    driver =    dnslookup
    # if outbound, and forwarding has been done, use an alternate transport
    domains =   ! +my_domains
    transport = ${if eq {$local_part@$domain} \
                        {$original_local_part@$original_domain} \
                     {remote_smtp} {remote_forwarded_smtp}}

  inbound_srs:
    driver =    redirect
    senders =   :
    domains =   +my_domains
    # detect inbound bounces which are SRS'd, and decode them
    condition = ${if inbound_srs {$local_part} {SRS_SECRET}}
    data =      $srs_recipient

  inbound_srs_failure:
    driver =    redirect
    senders =   :
    domains =   +my_domains
    # detect inbound bounces which look SRS'd but are invalid
    condition = ${if inbound_srs {$local_part} {}}
    allow_fail
    data =      :fail: Invalid SRS recipient address

  #... further routers here

  # transport; should look like the non-forward outbound
  # one, plus the max_rcpt and return_path options
  remote_forwarded_smtp:
    driver =              smtp
    # single-recipient so that $original_domain is valid
    max_rcpt =            1
    # modify the envelope from, for mails that we forward
    return_path =         ${srs_encode {SRS_SECRET} {$return_path} {$original_domain}}


58.3 DMARC
----------

DMARC combines feedback from SPF, DKIM, and header From: in order to attempt to
provide better indicators of the authenticity of an email. This document does
not explain the fundamentals; you should read and understand how it works by
visiting the website at http://www.dmarc.org/.

If Exim is built with DMARC support, the libopendmarc library is used.

For building Exim yourself, obtain the library from http://sourceforge.net/
projects/opendmarc/ to obtain a copy, or find it in your favorite package
repository. You will need to attend to the local/Makefile feature SUPPORT_DMARC
and the associated LDFLAGS addition. This description assumes that headers will
be in /usr/local/include, and that the libraries are in /usr/local/lib.

58.3.1 Configuration

There are three main-configuration options:

The dmarc_tld_file option defines the location of a text file of valid top
level domains the opendmarc library uses during domain parsing. Maintained by
Mozilla, the most current version can be downloaded from a link at https://
publicsuffix.org/list/public_suffix_list.dat. See also the util/
renew-opendmarc-tlds.sh script. The default for the option is unset. If not
set, DMARC processing is disabled.

The dmarc_history_file option, if set defines the location of a file to log
results of dmarc verification on inbound emails. The contents are importable by
the opendmarc tools which will manage the data, send out DMARC reports, and
expire the data. Make sure the directory of this file is writable by the user
exim runs as. The default is unset.

The dmarc_forensic_sender option defines an alternate email address to use when
sending a forensic report detailing alignment failures if a sender domain's
dmarc record specifies it and you have configured Exim to send them. If set,
this is expanded and used for the From: header line; the address is extracted
from it and used for the envelope from. If not set (the default), the From:
header is expanded from the dsn_from option, and <> is used for the envelope
from.

58.3.2 Controls

By default, the DMARC processing will run for any remote, non-authenticated
user. It makes sense to only verify DMARC status of messages coming from
remote, untrusted sources. You can use standard conditions such as hosts,
senders, etc, to decide that DMARC verification should *not* be performed for
them and disable DMARC with an ACL control modifier:

  control = dmarc_disable_verify

A DMARC record can also specify a "forensic address", which gives exim an email
address to submit reports about failed alignment. Exim does not do this by
default because in certain conditions it results in unintended information
leakage (what lists a user might be subscribed to, etc). You must configure
exim to submit forensic reports to the owner of the domain. If the DMARC record
contains a forensic address and you specify the control statement below, then
exim will send these forensic emails. It is also advised that you configure a 
dmarc_forensic_sender because the default sender address construction might be
inadequate.

  control = dmarc_enable_forensic

(AGAIN: You can choose not to send these forensic reports by simply not putting
the dmarc_enable_forensic control line at any point in your exim config. If you
don't tell exim to send them, it will not send them.)

There are no options to either control. Both must appear before the DATA acl.

58.3.3 ACL

DMARC checks can be run on incoming SMTP messages by using the "dmarc_status"
ACL condition in the DATA ACL. You are required to call the "spf" condition
first in the ACLs, then the "dmarc_status" condition. Putting this condition in
the ACLs is required in order for a DMARC check to actually occur. All of the
variables are set up before the DATA ACL, but there is no actual DMARC check
that occurs until a "dmarc_status" condition is encountered in the ACLs.

The "dmarc_status" condition takes a list of strings on its right-hand side.
These strings describe recommended action based on the DMARC check. To
understand what the policy recommendations mean, refer to the DMARC website
above. Valid strings are:

    accept     The DMARC check passed and the library recommends accepting the
               email
    reject     The DMARC check failed and the library recommends rejecting the
               email
    quarantine The DMARC check failed and the library recommends keeping it for
               further inspection
    none       The DMARC check passed and the library recommends no specific
               action, neutral
    norecord   No policy section in the DMARC record for this RFC5322.From
               field
    nofrom     Unable to determine the domain of the sender
    temperror  Library error or dns error
    off        The DMARC check was disabled for this email

You can prefix each string with an exclamation mark to invert its meaning, for
example "!accept" will match all results but "accept". The string list is
evaluated left-to-right in a short-circuit fashion. When a string matches the
outcome of the DMARC check, the condition succeeds. If none of the listed
strings matches the outcome of the DMARC check, the condition fails.

Of course, you can also use any other lookup method that Exim supports,
including LDAP, Postgres, MySQL, etc, as long as the result is a list of
colon-separated strings.

Performing the check sets up information used by the authresults expansion
item.

Several expansion variables are set before the DATA ACL is processed, and you
can use them in this ACL. The following expansion variables are available:

$dmarc_status

    A one word status indicating what the DMARC library thinks of the email. It
    is a combination of the results of DMARC record lookup and the SPF/DKIM/
    DMARC processing results (if a DMARC record was found). The actual policy
    declared in the DMARC record is in a separate expansion variable.

$dmarc_status_text

    Slightly longer, human readable status.

$dmarc_used_domain

    The domain which DMARC used to look up the DMARC policy record.

$dmarc_domain_policy

    The policy declared in the DMARC record. Valid values are "none", "reject"
    and "quarantine". It is blank when there is any error, including no DMARC
    record.

58.3.4 Logging

By default, Exim's DMARC configuration is intended to be non-intrusive and
conservative. To facilitate this, Exim will not create any type of logging
files without explicit configuration by you, the admin. Nor will Exim send out
any emails/reports about DMARC issues without explicit configuration by you,
the admin (other than typical bounce messages that may come about due to ACL
processing or failure delivery issues).

In order to log statistics suitable to be imported by the opendmarc tools, you
need to:

  * Configure the global option dmarc_history_file

  * Configure cron jobs to call the appropriate opendmarc history import
    scripts and truncating the dmarc_history_file

In order to send forensic reports, you need to:

  * Configure the global option dmarc_forensic_sender

  * Configure, somewhere before the DATA ACL, the control option to enable
    sending DMARC forensic reports

58.3.5 Example

Example usage:

(RCPT ACL)
  warn    domains        = +local_domains
          hosts          = +local_hosts
          control        = dmarc_disable_verify

  warn    !domains       = +screwed_up_dmarc_records
          control        = dmarc_enable_forensic

  warn    condition      = (lookup if destined to mailing list)
          set acl_m_mailing_list = 1

(DATA ACL)
  warn    dmarc_status   = accept : none : off
          !authenticated = *
          log_message    = DMARC DEBUG: $dmarc_status $dmarc_used_domain

  warn    dmarc_status   = !accept
          !authenticated = *
          log_message    = DMARC DEBUG: '$dmarc_status' for $dmarc_used_domain

  warn    dmarc_status   = quarantine
          !authenticated = *
          set $acl_m_quarantine = 1
          # Do something in a transport with this flag variable

  deny    condition      = ${if eq{$dmarc_domain_policy}{reject}}
          condition      = ${if eq{$acl_m_mailing_list}{1}}
          message        = Messages from $dmarc_used_domain break mailing lists

  deny    dmarc_status   = reject
          !authenticated = *
          message        = Message from $dmarc_used_domain failed sender's DMARC policy, REJECT

  warn    add_header     = :at_start:${authresults {$primary_hostname}}



===============================================================================
59. PROXIES

A proxy is an intermediate system through which communication is passed.
Proxies may provide a security, availability or load-distribution function.


59.1 Inbound proxies
--------------------

Exim has support for receiving inbound SMTP connections via a proxy that uses
"Proxy Protocol" to speak to it. To include this support, include
"SUPPORT_PROXY=yes" in Local/Makefile.

It was built on the HAProxy specification, found at https://www.haproxy.org/
download/1.8/doc/proxy-protocol.txt.

The purpose of this facility is so that an application load balancer, such as
HAProxy, can sit in front of several Exim servers to distribute load. Exim uses
the local protocol communication with the proxy to obtain the remote SMTP
system IP address and port information. There is no logging if a host passes or
fails Proxy Protocol negotiation, but it can easily be determined and recorded
in an ACL (example is below).

Use of a proxy is enabled by setting the hosts_proxy main configuration option
to a hostlist; connections from these hosts will use Proxy Protocol. Exim
supports both version 1 and version 2 of the Proxy Protocol and automatically
determines which version is in use.

The Proxy Protocol header is the first data received on a TCP connection and is
inserted before any TLS-on-connect handshake from the client; Exim negotiates
TLS between Exim-as-server and the remote client, not between Exim and the
proxy server. The Proxy Protocol header must be received within 
proxy_protocol_timeout, which defaults to 3s.

The following expansion variables are usable ("internal" and "external" here
refer to the interfaces of the proxy):

    $proxy_external_address IP of host being proxied or IP of remote interface
                            of proxy
    $proxy_external_port    Port of host being proxied or Port on remote
                            interface of proxy
    $proxy_local_address    IP of proxy server inbound or IP of local interface
                            of proxy
    $proxy_local_port       Port of proxy server inbound or Port on local
                            interface of proxy
    $proxy_session          boolean: SMTP connection via proxy

If $proxy_session is set but $proxy_external_address is empty there was a
protocol error. The variables $sender_host_address and $sender_host_port will
have values for the actual client system, not the proxy.

Since the real connections are all coming from the proxy, and the per host
connection tracking is done before Proxy Protocol is evaluated, 
smtp_accept_max_per_host must be set high enough to handle all of the parallel
volume you expect per inbound proxy. With the option set so high, you lose the
ability to protect your server from many connections from one IP. In order to
prevent your server from overload, you need to add a per connection ratelimit
to your connect ACL. A possible solution is:

  # Set max number of connections per host
  LIMIT   = 5
  # Or do some kind of IP lookup in a flat file or database
  # LIMIT = ${lookup{$sender_host_address}iplsearch{/etc/exim/proxy_limits}}

  defer   ratelimit      = LIMIT / 5s / per_conn / strict
          message        = Too many connections from this IP right now


59.2 Outbound proxies
---------------------

Exim has support for sending outbound SMTP via a proxy using a protocol called
SOCKS5 (defined by RFC1928). The support can be optionally included by defining
SUPPORT_SOCKS=yes in Local/Makefile.

Use of a proxy is enabled by setting the socks_proxy option on an smtp
transport. The option value is expanded and should then be a list
(colon-separated by default) of proxy specifiers. Each proxy specifier is a
list (space-separated by default) where the initial element is an IP address
and any subsequent elements are options.

Options are a string <name>=<value>. The list of options is in the following
table:

    auth   authentication method
    name   authentication username
    pass   authentication password
    port   tcp port
    tmo    connection timeout
    pri    priority
    weight selection bias

More details on each of these options follows:

  * auth: Either "none" (default) or "name". Using "name" selects username/
    password authentication per RFC 1929 for access to the proxy. Default is
    "none".

  * name: sets the username for the "name" authentication method. Default is
    empty.

  * pass: sets the password for the "name" authentication method. Default is
    empty.

  * port: the TCP port number to use for the connection to the proxy. Default
    is 1080.

  * tmo: sets a connection timeout in seconds for this proxy. Default is 5.

  * pri: specifies a priority for the proxy within the list, higher values
    being tried first. The default priority is 1.

  * weight: specifies a selection bias. Within a priority set servers are
    queried in a random fashion, weighted by this value. The default value for
    selection bias is 1.

Proxies from the list are tried according to their priority and weight settings
until one responds. The timeout for the overall connection applies to the set
of proxied attempts.


59.3 Logging
------------

To log the (local) IP of a proxy in the incoming or delivery logline, add
"+proxy" to the log_selector option. This will add a component tagged with "PRX
=" to the line.



===============================================================================
60. INTERNATIONALISATION

Exim has support for Internationalised mail names. To include this it must be
built with SUPPORT_I18N and the libidn library. Standards supported are RFCs
2060, 5890, 6530 and 6533.

If Exim is built with SUPPORT_I18N_2008 (in addition to SUPPORT_I18N, not
instead of it) then IDNA2008 is supported; this adds an extra library
requirement, upon libidn2.


60.1 MTA operations
-------------------

The main configuration option smtputf8_advertise_hosts specifies a host list.
If this matches the sending host and accept_8bitmime is true (the default) then
the ESMTP option SMTPUTF8 will be advertised.

If the sender specifies the SMTPUTF8 option on a MAIL command international
handling for the message is enabled and the expansion variable
$message_smtputf8 will have value TRUE.

The option allow_utf8_domains is set to true for this message. All DNS lookups
are converted to a-label form whatever the setting of allow_utf8_domains when
Exim is built with SUPPORT_I18N.

Both localparts and domain are maintained as the original UTF-8 form
internally; any comparison or regular-expression use will require appropriate
care. Filenames created, eg. by the appendfile transport, will have UTF-8
names.

HELO names sent by the smtp transport will have any UTF-8 components expanded
to a-label form, and any certificate name checks will be done using the a-label
form of the name.

Log lines and Received-by: header lines will acquire a "utf8" prefix on the
protocol element, eg. utf8esmtp.

The following expansion operators can be used:

${utf8_domain_to_alabel:str}
${utf8_domain_from_alabel:str}
${utf8_localpart_to_alabel:str}
${utf8_localpart_from_alabel:str}

The RCPT ACL may use the following modifier:

control = utf8_downconvert
control = utf8_downconvert/<value>

This sets a flag requiring that envelope addresses are converted to a-label
form before smtp delivery. This is usually for use in a Message Submission
Agent context, but could be used for any message.

If a value is appended it may be:

     "1" mandatory downconversion
     "0" no downconversion
    "-1" if SMTPUTF8 not supported by destination host

If no value is given, 1 is used.

If mua_wrapper is set, the utf8_downconvert control is initially set to -1.

The smtp transport has an option utf8_downconvert. If set it must expand to one
of the three values described above, or an empty string. If non-empty it
overrides value previously set (due to mua_wrapper or by an ACL control).

There is no explicit support for VRFY and EXPN. Configurations supporting these
should inspect $smtp_command_argument for an SMTPUTF8 argument.

There is no support for LMTP on Unix sockets. Using the "lmtp" protocol option
on an smtp transport, for LMTP over TCP, should work as expected.

There is no support for DSN unitext handling, and no provision for converting
logging from or to UTF-8.


60.2 MDA operations
-------------------

To aid in constructing names suitable for IMAP folders the following expansion
operator can be used:

${imapfolder {<string>} {<sep>} {<specials>}}

The string is converted from the charset specified by the "headers charset"
command (in a filter file) or headers_charset main configuration option
(otherwise), to the modified UTF-7 encoding specified by RFC 2060, with the
following exception: All occurrences of <sep> (which has to be a single
character) are replaced with periods ("."), and all periods and slashes that
are not <sep> and are not in the <specials> string are BASE64 encoded.

The third argument can be omitted, defaulting to an empty string. The second
argument can be omitted, defaulting to "/".

This is the encoding used by Courier for Maildir names on disk, and followed by
many other IMAP servers.

Examples:

${imapfolder {Foo/Bar}}        yields Foo.Bar
${imapfolder {Foo/Bar}{.}{/}}  yields Foo&AC8-Bar
${imapfolder {R?ksm?rg?s}}     yields R&AOQ-ksm&APY-rg&AOU-s

Note that the source charset setting is vital, and also that characters must be
representable in UTF-16.



===============================================================================
61. EVENTS

The events mechanism in Exim can be used to intercept processing at a number of
points. It was originally invented to give a way to do customised logging
actions (for example, to a database) but can also be used to modify some
processing actions.

Most installations will never need to use Events. The support can be left out
of a build by defining DISABLE_EVENT=yes in Local/Makefile.

There are two major classes of events: main and transport. The main
configuration option event_action controls reception events; a transport option
event_action controls delivery events.

Both options are a string which is expanded when the event fires. An example
might look like:

event_action = ${if eq {msg:delivery}{$event_name} \
{${lookup pgsql {SELECT * FROM record_Delivery( \
    '${quote_pgsql:$sender_address_domain}',\
    '${quote_pgsql:${lc:$sender_address_local_part}}', \
    '${quote_pgsql:$domain}', \
    '${quote_pgsql:${lc:$local_part}}', \
    '${quote_pgsql:$host_address}', \
    '${quote_pgsql:${lc:$host}}', \
    '${quote_pgsql:$message_exim_id}')}} \
} {}}

Events have names which correspond to the point in process at which they fire.
The name is placed in the variable $event_name and the event action expansion
must check this, as it will be called for every possible event type.

The current list of events is:

+---------------------------------------------------------------------------+
|auth:fail          |after |  both   |per driver per authentication attempt |
|-------------------+------+---------+--------------------------------------|
|dane:fail          |after |transport|per connection                        |
|-------------------+------+---------+--------------------------------------|
|msg:complete       |after |  main   |per message                           |
|-------------------+------+---------+--------------------------------------|
|msg:defer          |after |transport|per message per delivery try          |
|-------------------+------+---------+--------------------------------------|
|msg:delivery       |after |transport|per recipient                         |
|-------------------+------+---------+--------------------------------------|
|msg:rcpt:host:defer|after |transport|per recipient per host                |
|-------------------+------+---------+--------------------------------------|
|msg:rcpt:defer     |after |transport|per recipient                         |
|-------------------+------+---------+--------------------------------------|
|msg:host:defer     |after |transport|per host per delivery try; host errors|
|-------------------+------+---------+--------------------------------------|
|msg:fail:delivery  |after |transport|per recipient                         |
|-------------------+------+---------+--------------------------------------|
|msg:fail:internal  |after |  main   |per recipient                         |
|-------------------+------+---------+--------------------------------------|
|tcp:connect        |before|transport|per connection                        |
|-------------------+------+---------+--------------------------------------|
|tcp:close          |after |transport|per connection                        |
|-------------------+------+---------+--------------------------------------|
|tls:cert           |before|  both   |per certificate in verification chain |
|-------------------+------+---------+--------------------------------------|
|tls:fail:connect   |after |  main   |per connection                        |
|-------------------+------+---------+--------------------------------------|
|smtp:connect       |after |transport|per connection                        |
|-------------------+------+---------+--------------------------------------|
|smtp:ehlo          |after |transport|per connection                        |
+---------------------------------------------------------------------------+

New event types may be added in future.

The event name is a colon-separated list, defining the type of event in a tree
of possibilities. It may be used as a list or just matched on as a whole. There
will be no spaces in the name.

The second column in the table above describes whether the event fires before
or after the action is associates with. Those which fire before can be used to
affect that action (more on this below).

The third column in the table above says what section of the configuration
should define the event action.

An additional variable, $event_data, is filled with information varying with
the event type:

+---------------------------------------------+
|auth:fail          |smtp response            |
|-------------------+-------------------------|
|dane:fail          |failure reason           |
|-------------------+-------------------------|
|msg:defer          |error string             |
|-------------------+-------------------------|
|msg:delivery       |smtp confirmation message|
|-------------------+-------------------------|
|msg:fail:internal  |failure reason           |
|-------------------+-------------------------|
|msg:fail:delivery  |smtp error message       |
|-------------------+-------------------------|
|msg:host:defer     |error string             |
|-------------------+-------------------------|
|msg:rcpt:host:defer|error string             |
|-------------------+-------------------------|
|msg:rcpt:defer     |error string             |
|-------------------+-------------------------|
|tls:cert           |verification chain depth |
|-------------------+-------------------------|
|tls:fail:connect   |error string             |
|-------------------+-------------------------|
|smtp:connect       |smtp banner              |
|-------------------+-------------------------|
|smtp:ehlo          |smtp ehlo response       |
+---------------------------------------------+

The :defer events populate one extra variable: $event_defer_errno.

For complex operations an ACL expansion can be used in event_action, however
due to the multiple contexts that Exim operates in during the course of its
processing:

  * variables set in transport events will not be visible outside that
    transport call

  * acl_m variables in a server context are lost on a new connection, and after
    smtp helo/ehlo/mail/starttls/rset commands

Using an ACL expansion with the logwrite modifier can be a useful way of
writing to the main log.

The expansion of the event_action option should normally return an empty
string. Should it return anything else the following will be forced:

+-------------------------------------+
|auth:fail   |log information to write|
|------------+------------------------|
|tcp:connect |do not connect          |
|------------+------------------------|
|tls:cert    |refuse verification     |
|------------+------------------------|
|smtp:connect|close connection        |
+-------------------------------------+

All other message types ignore the result string, and no other use is made of
it.

For a tcp:connect event, if the connection is being made to a proxy then the
$host_address and $host_port variables will be that of the proxy and not the
target system.

For tls:cert events, if GnuTLS is in use this will trigger only per chain
element received on the connection. For OpenSSL it will trigger for every chain
element including those loaded locally.



===============================================================================
62. ADDING NEW DRIVERS OR LOOKUP TYPES

The following actions have to be taken in order to add a new router, transport,
authenticator, or lookup type to Exim:

 1. Choose a name for the driver or lookup type that does not conflict with any
    existing name; I will use "newdriver" in what follows.

 2. Add to src/EDITME the line:

    <type>_NEWDRIVER=yes

    where <type> is ROUTER, TRANSPORT, AUTH, or LOOKUP. If the code is not to
    be included in the binary by default, comment this line out. You should
    also add any relevant comments about the driver or lookup type.

 3. Add to src/config.h.defaults the line:

    #define <type>_NEWDRIVER

 4. Edit src/drtables.c, adding conditional code to pull in the private header
    and create a table entry as is done for all the other drivers and lookup
    types.

 5. Edit scripts/lookups-Makefile if this is a new lookup; there is a for-loop
    near the bottom, ranging the "name_mod" variable over a list of all
    lookups. Add your "NEWDRIVER" to that list. As long as the dynamic module
    would be named newdriver.so, you can use the simple form that most lookups
    have.

 6. Edit Makefile in the appropriate sub-directory (src/routers, src/transports
    , src/auths, or src/lookups); add a line for the new driver or lookup type
    and add it to the definition of OBJ.

 7. Edit OS/Makefile-Base adding a .o file for the predefined-macros, to the
    definition of OBJ_MACRO. Add a set of line to do the compile also.

 8. Create newdriver.h and newdriver.c in the appropriate sub-directory of src.

 9. Edit scripts/MakeLinks and add commands to link the .h and .c files as for
    other drivers and lookups.

Then all you need to do is write the code! A good way to start is to make a
proforma by copying an existing module of the same type, globally changing all
occurrences of the name, and cutting out most of the code. Note that any
options you create must be listed in alphabetical order, because the tables are
searched using a binary chop procedure.

There is a README file in each of the sub-directories of src describing the
interface that is expected.

