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# 21.18. `smtpd` — SMTP Server

Source code: Lib/smtpd.py

This module offers several classes to implement SMTP (email) servers.

Several server implementations are present; one is a generic do-nothing implementation, which can be overridden, while the other two offer specific mail-sending strategies.

Additionally the SMTPChannel may be extended to implement very specific interaction behaviour with SMTP clients.

The code supports RFC 5321, plus the RFC 1870 SIZE and RFC 6531 SMTPUTF8 extensions.

## 21.18.1. SMTPServer Objects

class `smtpd.``SMTPServer`(localaddr, remoteaddr, data_size_limit=33554432, map=None, enable_SMTPUTF8=False, decode_data=True)

  Create a new `SMTPServer` object, which binds to local address localaddr. It will treat remoteaddr as an upstream SMTP relayer. It inherits from `asyncore.dispatcher`, and so will insert itself into `asyncore`‘s event loop on instantiation.

  data_size_limit specifies the maximum number of bytes that will be accepted in a `DATA` command. A value of `None` or `0` means no limit.

  map is the socket map to use for connections (an initially empty dictionary is a suitable value). If not specified the `asyncore` global socket map is used.

  enable_SMTPUTF8 determins whether the `SMTPUTF8` extension (as defined in RFC 6531) should be enabled. The default is `False`. If set to `True`, decode_data must be `False` (otherwise an error is raised). When `True`, `SMTPUTF8` is accepted as a parameter to the `MAIL` command and when present is passed to `process_message()` in the `kwargs['mail_options']` list.

  decode_data specifies whether the data portion of the SMTP transaction should be decoded using UTF-8. The default is `True` for backward compatibility reasons, but will change to `False` in Python 3.6; specify the keyword value explicitly to avoid the `DeprecationWarning`. When decode_data is set to `False` the server advertises the `8BITMIME` extension (RFC 6152), accepts the `BODY=8BITMIME` parameter to the `MAIL` command, and when present passes it to `process_message()` in the `kwargs['mail_options']` list.

  `process_message`(peer, mailfrom, rcpttos, data, **kwargs)

    Raise a `NotImplementedError` exception. Override this in subclasses to do something useful with this message. Whatever was passed in the constructor as remoteaddr will be available as the `_remoteaddr` attribute. peer is the remote host’s address, mailfrom is the envelope originator, rcpttos are the envelope recipients and data is a string containing the contents of the e-mail (which should be in RFC 5321 format).

    If the decode_data constructor keyword is set to `True`, the data argument will be a unicode string. If it is set to `False`, it will be a bytes object.

    kwargs is a dictionary containing additional information. It is empty unless at least one of `decode_data=False` or `enable_SMTPUTF8=True` was given as an init parameter, in which case it contains the following keys:

    > mail_options:
    >   a list of all received parameters to the `MAIL` command (the elements are uppercase strings; example: `['BODY=8BITMIME', 'SMTPUTF8']`).
    > rcpt_options:
    >   same as mail_options but for the `RCPT` command. Currently no `RCPT TO` options are supported, so for now this will always be an empty list.

    Implementations of `process_message` should use the `**kwargs` signature to accept arbitrary keyword arguments, since future feature enhancements may add keys to the kwargs dictionary.

    Return `None` to request a normal `250 Ok` response; otherwise return the desired response string in RFC 5321 format.

  `channel_class`

    Override this in subclasses to use a custom `SMTPChannel` for managing SMTP clients.

  New in version 3.4: The map constructor argument.

  Changed in version 3.5: localaddr and remoteaddr may now contain IPv6 addresses.

  New in version 3.5: the decode_data and enable_SMTPUTF8 constructor arguments, and the kwargs argument to `process_message()` when one or more of these is specified.

## 21.18.2. DebuggingServer Objects

class `smtpd.``DebuggingServer`(localaddr, remoteaddr)

  Create a new debugging server. Arguments are as per `SMTPServer`. Messages will be discarded, and printed on stdout.

## 21.18.3. PureProxy Objects

class `smtpd.``PureProxy`(localaddr, remoteaddr)

  Create a new pure proxy server. Arguments are as per `SMTPServer`. Everything will be relayed to remoteaddr. Note that running this has a good chance to make you into an open relay, so please be careful.

## 21.18.4. MailmanProxy Objects

class `smtpd.``MailmanProxy`(localaddr, remoteaddr)

  Create a new pure proxy server. Arguments are as per `SMTPServer`. Everything will be relayed to remoteaddr, unless local mailman configurations knows about an address, in which case it will be handled via mailman. Note that running this has a good chance to make you into an open relay, so please be careful.

## 21.18.5. SMTPChannel Objects

class `smtpd.``SMTPChannel`(server, conn, addr, data_size_limit=33554432, map=None, enable_SMTPUTF8=False, decode_data=True)

  Create a new `SMTPChannel` object which manages the communication between the server and a single SMTP client.

  conn and addr are as per the instance variables described below.

  data_size_limit specifies the maximum number of bytes that will be accepted in a `DATA` command. A value of `None` or `0` means no limit.

  enable_SMTPUTF8 determins whether the `SMTPUTF8` extension (as defined in RFC 6531) should be enabled. The default is `False`. A `ValueError` is raised if both enable_SMTPUTF8 and decode_data are set to `True` at the same time.

  A dictionary can be specified in map to avoid using a global socket map.

  decode_data specifies whether the data portion of the SMTP transaction should be decoded using UTF-8. The default is `True` for backward compatibility reasons, but will change to `False` in Python 3.6. Specify the keyword value explicitly to avoid the `DeprecationWarning`.

  To use a custom SMTPChannel implementation you need to override the `SMTPServer.channel_class` of your `SMTPServer`.

  Changed in version 3.5: the decode_data and enable_SMTPUTF8 arguments were added.

  The `SMTPChannel` has the following instance variables:

  `smtp_server`

    Holds the `SMTPServer` that spawned this channel.

  `conn`

    Holds the socket object connecting to the client.

  `addr`

    Holds the address of the client, the second value returned by `socket.accept`

  `received_lines`

    Holds a list of the line strings (decoded using UTF-8) received from the client. The lines have their `"\r\n"` line ending translated to `"\n"`.

  `smtp_state`

    Holds the current state of the channel. This will be either `COMMAND` initially and then `DATA` after the client sends a “DATA” line.

  `seen_greeting`

    Holds a string containing the greeting sent by the client in its “HELO”.

  `mailfrom`

    Holds a string containing the address identified in the “MAIL FROM:” line from the client.

  `rcpttos`

    Holds a list of strings containing the addresses identified in the “RCPT TO:” lines from the client.

  `received_data`

    Holds a string containing all of the data sent by the client during the DATA state, up to but not including the terminating `"\r\n.\r\n"`.

  `fqdn`

    Holds the fully-qualified domain name of the server as returned by `socket.getfqdn()`.

  `peer`

    Holds the name of the client peer as returned by `conn.getpeername()` where `conn` is `conn`.

  The `SMTPChannel` operates by invoking methods named `smtp_<command>` upon reception of a command line from the client. Built into the base `SMTPChannel` class are methods for handling the following commands (and responding to them appropriately):

  Command | Action taken
  --- | ---
  HELO | Accepts the greeting from the client and stores it in `seen_greeting`. Sets server to base command mode.
  EHLO | Accepts the greeting from the client and stores it in `seen_greeting`. Sets server to extended command mode.
  NOOP | Takes no action.
  QUIT | Closes the connection cleanly.
  MAIL | Accepts the “MAIL FROM:” syntax and stores the supplied address as `mailfrom`. In extended command mode, accepts the RFC 1870 SIZE attribute and responds appropriately based on the value of data_size_limit.
  RCPT | Accepts the “RCPT TO:” syntax and stores the supplied addresses in the `rcpttos` list.
  RSET | Resets the `mailfrom`, `rcpttos`, and `received_data`, but not the greeting.
  DATA | Sets the internal state to `DATA` and stores remaining lines from the client in `received_data` until the terminator `"\r\n.\r\n"` is received.
  HELP | Returns minimal information on command syntax
  VRFY | Returns code 252 (the server doesn’t know if the address is valid)
  EXPN | Reports that the command is not implemented.

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