# Concepts

There are two general ways you can run the SMTP server, via the command line or programmatically.

There are several dimensions in which you can extend the basic functionality of the SMTP server. You can implement an event handler which uses well defined handler hooks that are called during the various steps in the SMTP dialog. If such a hook is implemented, it assumes responsibility for the status messages returned to the client.

You can also subclass the core `SMTP` class to implement new commands, or change the semantics of existing commands.

For example, if you wanted to print the received message on the console, you could implement a handler that hooks into the `DATA` command. The contents of the message will be available on one of the hook’s arguments, and your handler could print this content to stdout.

On the other hand, if you wanted to implement an SMTP-like server that adds a new command called `PING`, you would do this by subclassing `SMTP`, adding a method that implements whatever semantics for `PING` that you want.

## Sessions and envelopes

Two classes are used during the SMTP dialog with clients. Instances of these are passed to the handler hooks.

Note

Handler Hooks MAY add new attributes to these classes for inter-hook coordination.

### Session

The session represents the state built up during a client’s socket connection to the server. Each time a client connects to the server, a new session object is created.

class Session(loop)
  Parameters:

    loop – asyncio event loop currently running `SMTP`.

  peer

    Defaulting to None, this attribute will contain the transport’s socket’s `peername` value.

  ssl

    Defaulting to None, this attribute will contain some extra information, as a dictionary, from the `asyncio.sslproto.SSLProtocol` instance. This dictionary provides additional information about the connection. It contains implementation-specific information so its contents may change, but it should roughly correspond to the information available through `asyncio.BaseTransport.get_extra_info()`

  host_name

    Defaulting to None, this attribute will contain the host name argument as seen in the `HELO` or `EHLO` (or for LMTP, the `LHLO`) command.

  extended_smtp

    Defaulting to False, this flag will be True when the `EHLO` greeting was seen, indicating ESMTP.

  loop

    This is the asyncio event loop instance.

    Handler Hooks can utilize this if needed, for instance invoking `call_later()` to set some timers.

  login_data

    Contains the login information gathered during the `AUTH` procedure. If it contains `None`, that means authentication has not taken place or has failed.

    Warning

    This is the “legacy” login_data, populated only if `auth_callback` parameter is set.

    Deprecated since version 1.3: This attribute will be removed in version 2.0.

  auth_data

    Contains the authentication data returned by the `authenticator` callback.

  authenticated: bool | None

    A tri-state flag indicating status of authentication:

    > - `None` := Authentication has not been performed

    > - `False` := Authentication has been performed, but failed

    > - `True` := Authentication has been performed, and succeeded

### Envelope

The envelope represents state built up during the client’s SMTP dialog. Each time the protocol state is reset, a new envelope is created. E.g. when the SMTP `RSET` command is sent, the state is reset and a new envelope is created. A new envelope is also created after the `DATA` command is completed, or in certain error conditions as mandated by RFC 5321.

class Envelope
  mail_from: str

    Defaulting to None, this attribute holds the email address given in the `MAIL FROM` command.

  mail_options: List[str]

    Defaulting to None, this attribute contains a list of any ESMTP mail options provided by the client, such as those passed in by `smtplib.SMTP.sendmail()`

  content: AnyStr

    Defaulting to None, this attribute will contain the contents of the message as provided by the `DATA` command. If the `decode_data` parameter to the `SMTP` constructor was True, then this attribute will contain the UTF-8 decoded string, otherwise it will contain the raw bytes.

  original_content: bytes

    Defaulting to None, this attribute will contain the contents of the message as provided by the `DATA` command. Unlike the `content` attribute, this attribute will always contain the raw bytes.

  rcpt_tos: List[str]

    Defaulting to the empty list, this attribute will contain a list of the email addresses provided in the `RCPT TO` commands.

  rcpt_options: List[str]

    Defaulting to the empty list, this attribute will contain the list of any recipient options provided by the client, such as those passed in by `smtplib.SMTP.sendmail()`