Mail::Box::MH(3pm)

NAME
Mail::Box::MH - handle MH folders

INHERITANCE
 Mail::Box::MH
   is a Mail::Box::Dir
   is a Mail::Box
   is a Mail::Reporter

SYNOPSIS
  use Mail::Box::MH;
  my $folder = Mail::Box::MH->new(folder => $ENV{MAIL}, ...);

DESCRIPTION
This documentation describes how MH mailboxes work, and what you can do with the MH folder object "Mail::Box::MH".

Extends "DESCRIPTION" in Mail::Box::Dir.

OVERLOADED
Extends "OVERLOADED" in Mail::Box::Dir.

overload: "" stringification
    Inherited, see "OVERLOADED" in Mail::Box

overload: @{} use as ARRAY
    Inherited, see "OVERLOADED" in Mail::Box

overload: cmp string comparison
    Inherited, see "OVERLOADED" in Mail::Box

METHODS
Extends "METHODS" in Mail::Box::Dir.

  Constructors
Extends "Constructors" in Mail::Box::Dir.

$class->new(%options)
    Inherited, see "Constructors" in Mail::Box

     -Option           --Defined in     --Default
      access             Mail::Box        'r'
      body_delayed_type  Mail::Box        Mail::Message::Body::Delayed
      body_type          Mail::Box        Mail::Message::Body::Lines
      coerce_options     Mail::Box        +[]
      create             Mail::Box        false
      directory          Mail::Box::Dir   <derived from folder name>
      extract            Mail::Box        10240
      field_type         Mail::Box        undef
      fix_headers        Mail::Box        false
      folder             Mail::Box        $ENV{MAIL}
      folderdir          Mail::Box        $ENV{HOME}/.mh
      head_delayed_type  Mail::Box        Mail::Message::Head::Delayed
      head_type          Mail::Box        Mail::Message::Head::Complete
      index                               undef
      index_filename                      <foldername>/.index
      index_type                          Mail::Box::MH::Index
      keep_dups          Mail::Box        false
      keep_index                          false
      labels                              undef
      labels_filename                     <foldername>/.mh_sequence
      labels_type                         Mail::Box::MH::Labels
      lock_file          Mail::Box        <index_file>
      lock_timeout       Mail::Box        1 hour
      lock_type          Mail::Box        Mail::Box::Locker::DotLock
      lock_wait          Mail::Box        10 seconds
      locker             Mail::Box        undef
      manager            Mail::Box        undef
      message_type       Mail::Box        <folder-class>::Message
      multipart_type     Mail::Box        Mail::Message::Body::Multipart
      remove_when_empty  Mail::Box        true
      save_on_exit       Mail::Box        true
      trusted            Mail::Box        <depends on folder location>

    access => $mode
    body_delayed_type => CLASS
    body_type => CLASS|CODE
    coerce_options => ARRAY
    create => BOOLEAN
    directory => $directory
    extract => INTEGER|CODE|METHOD|'LAZY'|'ALWAYS'
    field_type => CLASS
    fix_headers => BOOLEAN
    folder => $name
    folderdir => $directory
    head_delayed_type => CLASS
    head_type => CLASS
    index => $object
        You may specify an $object of a type which extends Mail::Box::MH::Index (at least implements a get() method), as alternative for an index file reader as created by "Mail::Box::MH".

    index_filename => $file
        The $file which is used in each directory to store the headers of all mails. The filename shall not contain a directory path. (e.g. Do not use "/usr/people/jan/.index", nor "subdir/.index", but say ".index".)

    index_type => $class
    keep_dups => BOOLEAN
    keep_index => BOOLEAN
        Keep an index file of the specified mailbox, one file per directory. Using an index file will speed up things considerably, because it avoids reading all the message files the moment that you open the folder. When you open a folder, you can use the index file to retrieve information such as the subject of each message, instead of having to read possibly thousands of messages.

    labels => $object
        You may specify an $object of a type which extends Mail::Box::MH::Labels (at least implements the get() method), as alternative for labels file reader as created by "Mail::Box::MH".

    labels_filename => $file
        In MH-folders, messages can be labeled, for instance based on the sender or whether it is read or not. This status is kept in a $file which is usually called ".mh_sequences", but that name can be overruled with this flag.

    labels_type => $class
    lock_file => $file
    lock_timeout => $seconds
    lock_type => CLASS|STRING|ARRAY
    lock_wait => $seconds
    locker => OBJECT
    manager => $manager
    message_type => CLASS
    multipart_type => CLASS
    remove_when_empty => BOOLEAN
    save_on_exit => BOOLEAN
    trusted => BOOLEAN

  Attributes
Extends "Attributes" in Mail::Box::Dir.

$obj->fixHeaders()
    Inherited, see "Attributes" in Mail::Box

$obj->folderdir( [$directory] )
    Inherited, see "Attributes" in Mail::Box

$obj->isTrusted()
    Inherited, see "Attributes" in Mail::Box

$obj->name()
    Inherited, see "Attributes" in Mail::Box

$obj->removeEmpty()
    Inherited, see "Attributes" in Mail::Box

$obj->type()
    Inherited, see "Attributes" in Mail::Box

  The folder
Extends "The folder" in Mail::Box::Dir.

$obj->addMessage($message, %options)
    Inherited, see "The folder" in Mail::Box

$obj->addMessages(@messages)
    Inherited, see "The folder" in Mail::Box

$class->appendMessages(%options)
    Append a message to a folder which is not open. Improves base, see "The folder" in Mail::Box

     -Option  --Defined in     --Default
      folder    Mail::Box        <required>
      message   Mail::Box        undef
      messages  Mail::Box        undef
      share     Mail::Box        false

    folder => $name
    message => $message
    messages => $message|\@messages
    share => BOOLEAN
$obj->close(%options)
    Inherited, see "The folder" in Mail::Box

$obj->copyTo($folder, %options)
    Inherited, see "The folder" in Mail::Box

$obj->delete(%options)
    Inherited, see "The folder" in Mail::Box

$obj->directory()
    Inherited, see "The folder" in Mail::Box::Dir

$obj->organization()
    Inherited, see "The folder" in Mail::Box

$obj->size()
    Inherited, see "The folder" in Mail::Box

$obj->update(%options)
    Inherited, see "The folder" in Mail::Box

$obj->url()
    Inherited, see "The folder" in Mail::Box

  Folder flags
Extends "Folder flags" in Mail::Box::Dir.

$obj->access()
    Inherited, see "Folder flags" in Mail::Box

$obj->isModified()
    Inherited, see "Folder flags" in Mail::Box

$obj->modified( [BOOLEAN] )
    Inherited, see "Folder flags" in Mail::Box

$obj->writable()
    Inherited, see "Folder flags" in Mail::Box

  The messages
Extends "The messages" in Mail::Box::Dir.

$obj->current( [$number|$message|$message_id] )
    Inherited, see "The messages" in Mail::Box

$obj->find($message_id)
    Inherited, see "The messages" in Mail::Box

$obj->findFirstLabeled( $label, [BOOLEAN, [\@msgs]] )
    Inherited, see "The messages" in Mail::Box

$obj->message( $index, [$message] )
    Inherited, see "The messages" in Mail::Box

$obj->messageId( $message_id, [$message] )
    Inherited, see "The messages" in Mail::Box

$obj->messageIds()
    Inherited, see "The messages" in Mail::Box

$obj->messages( <'ALL'|$range|'ACTIVE'|'DELETED'|$label| !$label|$filter> )
    Inherited, see "The messages" in Mail::Box

$obj->nrMessages(%options)
    Inherited, see "The messages" in Mail::Box

$obj->scanForMessages($message, $message_ids, $timespan, $window)
    Inherited, see "The messages" in Mail::Box

  Sub-folders
Extends "Sub-folders" in Mail::Box::Dir.

$any->listSubFolders(%options)
    Inherited, see "Sub-folders" in Mail::Box

$any->nameOfSubFolder( $subname, [$parentname] )
    Inherited, see "Sub-folders" in Mail::Box

$obj->openRelatedFolder(%options)
    Inherited, see "Sub-folders" in Mail::Box

$obj->openSubFolder()
    Inherited, see "Sub-folders" in Mail::Box

$any->topFolderWithMessages()
    Inherited, see "Sub-folders" in Mail::Box

  Internals
Extends "Internals" in Mail::Box::Dir.

$obj->coerce($message, %options)
    Inherited, see "Internals" in Mail::Box

$any->create($foldername, %options)
    Inherited, see "Internals" in Mail::Box

     -Option   --Defined in--Default
      folderdir  Mail::Box   undef

    folderdir => $directory
$obj->determineBodyType($message, $head)
    Inherited, see "Internals" in Mail::Box

$obj->folderToDirectory($foldername, $folderdir)
    Inherited, see "Internals" in Mail::Box::Dir

$class->foundIn( [$foldername], %options )
    Inherited, see "Internals" in Mail::Box

$obj->highestMessageNumber()
    Returns the highest number which is used in the folder to store a file. This method may be called when the folder is read (then this number can be derived without file-system access), but also when the folder is not read (yet).

$obj->index()
    Create a index reader/writer object.

$obj->labels()
    Create a label reader/writer object.

$obj->lineSeparator( [<STRING|'CR'|'LF'|'CRLF'>] )
    Inherited, see "Internals" in Mail::Box

$obj->locker()
    Inherited, see "Internals" in Mail::Box

$obj->messageInFile($filename)
    Inherited, see "Internals" in Mail::Box::Dir

$obj->read(%options)
    Inherited, see "Internals" in Mail::Box

$obj->readMessageFilenames($directory)
    Inherited, see "Internals" in Mail::Box::Dir

$obj->readMessages(%options)
    Inherited, see "Internals" in Mail::Box

$obj->storeMessage($message)
    Inherited, see "Internals" in Mail::Box

$obj->toBeThreaded($messages)
    Inherited, see "Internals" in Mail::Box

$obj->toBeUnthreaded($messages)
    Inherited, see "Internals" in Mail::Box

$obj->updateMessages(%options)
    Inherited, see "Internals" in Mail::Box

$obj->write(%options)
    Inherited, see "Internals" in Mail::Box

$obj->writeMessages(%options)
    Inherited, see "Internals" in Mail::Box

     -Option  --Defined in     --Default
      messages  Mail::Box        <required>
      renumber                   true

    messages => ARRAY
    renumber => BOOLEAN
        Permit renumbering of message. By default this is "true", but for some unknown reason, you may be thinking that messages should not be renumbered.

  Other methods
Extends "Other methods" in Mail::Box::Dir.

$any->timespan2seconds($time)
    Inherited, see "Other methods" in Mail::Box

  Error handling
Extends "Error handling" in Mail::Box::Dir.

$obj->AUTOLOAD()
    Inherited, see "Error handling" in Mail::Reporter

$obj->notImplemented()
    Inherited, see "Error handling" in Mail::Reporter

  Cleanup
Extends "Cleanup" in Mail::Box::Dir.

$obj->DESTROY()
    Inherited, see "Cleanup" in Mail::Box

DETAILS
Extends "DETAILS" in Mail::Box::Dir.

  Different kinds of folders
Extends "Different kinds of folders" in Mail::Box::Dir.

  Available folder types
Extends "Available folder types" in Mail::Box::Dir.

  Folder class implementation
Extends "Folder class implementation" in Mail::Box::Dir.

  How MH folders work
MH-type folders use a directory to store the messages of one folder. Each message is stored in a separate file. This seems useful, because changes in a folder change only a few of these small files, in contrast with file-based folders where changes in a folder cause rewrites of huge folder files.

However, MH-based folders perform very bad if you need header information of all messages. For instance, if you want to have full knowledge about all message-threads (see Mail::Box::Thread::Manager) in the folder, it requires to read all header lines in all message files. And usually, reading your messages in threads is desired.

So, each message is written in a separate file. The filenames are numbers, which count from 1. Next to these message files, a directory may contain a file named ".mh_sequences", storing labels which relate to the messages. Furthermore, a folder-directory may contain sub-directories, which are seen as sub-folders.

  This implementation
This implementation supports the ".mh-sequences" file and sub-folders. Next to this, considerable effort it made to avoid reading each message-file. This should boost performance of the MailBox distribution over other Perl-modules which are able to read folders.

Folder types which store their messages each in one file, together in one directory, are bad for performance. Consider that you want to know the subjects of all messages, while browser through a folder with your mail-reading client. This would cause all message-files to be read.

Mail::Box::MH has two ways to try improve performance. You can use an index-file, and use on delay-loading. The combination performs even better. Both are explained in the next sections.

  An index-file
If you specify new(keep_index), then all header-lines of all messages from the folder which have been read once, will also be written into one dedicated index-file (one file per folder). The default filename is ".index"

However, index-files are not supported by any other reader which supports MH (as far as I know). If you read the folders with such I client, it will not cause unrecoverable conflicts with this index-file, but at most be bad for performance.

If you do not (want to) use an index-file, then delay-loading may save your day.

DIAGNOSTICS

Error: Folder $name is opened read-only
    You can not write to this folder unless you have opened the folder to write or append with new(access), or the "force" option is set "true". Cast by write()

Error: cannot append message without lock on $folder.
    It is impossible to append one or more messages to the folder which is not opened, because locking it fails. The folder may be in use by an other application, or you may need to specify some lock related options (see new()). Cast by appendMessages()

Fault: cannot create MH folder $name: $!
    For some reason, it is impossible to create the folder. Check the permissions and the $name of the folder. Does the path to the directory to be created exist? Cast by create()

Fault: cannot create directory $dir for subfolder $name: $!
    Cast by openSubFolder()

Error: cannot write folder $name without lock.
    It is impossible to get a lock on the folder, which means that the changes can not be made. You may need to tune the lock related options which are available at folder creation. Cast by writeMessages()

Warning: changes not written to read-only folder $name.
    You have opened the folder read-only --which is the default set by new(access)--, made modifications, and now want to close it. Set close(force) if you want to overrule the access mode, or close the folder with close(write) set to "NEVER". Cast by close()

Error: class $package does not implement method $method.
    Fatal error: the specific $package (or one of its superclasses) does not implement this method where it should. This message means that some other related classes do implement this method however the class at hand does not. Probably you should investigate this and probably inform the author of the package. Cast by notImplemented()

Error: copying failed for one message.
    For some reason, for instance disk full, removed by external process, or read-protection, it is impossible to copy one of the messages. Copying will proceed for the other messages. Cast by copyTo()

Error: destination folder $name is not writable.
    The folder where the messages are copied to is not opened with write access (see new(access)). This has no relation with write permission to the folder which is controlled by your operating system. Cast by copyTo()

Warning: different messages with id $msgid.
    The message id is discovered more than once within the same folder, but the content of the message seems to be different. This should not be possible: each message must be unique. Cast by messageId()

Warning: folder $name not deleted: not writable.
    The folder must be opened with write access via new(access), otherwise removing it will be refused. So, you may have write-access according to the operating system, but that will not automatically mean that this "delete" method permits you to. The reverse remark is valid as well. Cast by delete()

Error: invalid timespan '$span'.
    The string does not follow the strict rules of the time span syntax which is permitted as parameter. Cast by timespan2seconds()

Warning: message-id '$msgid' does not contain a domain.
    According to the RFCs, message-ids need to contain a unique random part, then an "@", and then a domain name. This is made to avoid the creation of two messages with the same id. The warning emerges when the "@" is missing from the string. Cast by messageId()

Error: unknown value to folder->close(write => $when).
    Cast by close()

Error: you cannot add a message which is already part of a folder to a new one.  Please use moveTo or copyTo.
    Cast by addMessage()

SEE ALSO
This module is part of Mail-Box version 4.01, built on December 13, 2025. Website: http://perl.overmeer.net/CPAN/

LICENSE
For contributors see file ChangeLog.

This software is copyright (c) 2001-2025 by Mark Overmeer.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.