:mod:`smtpd` --- SMTP Server
============================

.. module:: smtpd
   :synopsis: A SMTP server implementation in Python.

.. moduleauthor:: Barry Warsaw <barry@zope.com>
.. sectionauthor:: Moshe Zadka <moshez@moshez.org>

**Source code:** :source:`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 extension.


SMTPServer Objects
------------------


.. class:: SMTPServer(localaddr, remoteaddr, data_size_limit=33554432)

   Create a new :class:`SMTPServer` object, which binds to local address
   *localaddr*.  It will treat *remoteaddr* as an upstream SMTP relayer.  It
   inherits from :class:`asyncore.dispatcher`, and so will insert itself into
   :mod:`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.

   .. method:: process_message(peer, mailfrom, rcpttos, data)

      Raise :exc:`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 :attr:`_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:`2822`
      format).

   .. attribute:: channel_class

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


DebuggingServer Objects
-----------------------


.. class:: DebuggingServer(localaddr, remoteaddr)

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


PureProxy Objects
-----------------


.. class:: PureProxy(localaddr, remoteaddr)

   Create a new pure proxy server. Arguments are as per :class:`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.


MailmanProxy Objects
--------------------


.. class:: MailmanProxy(localaddr, remoteaddr)

   Create a new pure proxy server. Arguments are as per :class:`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.

SMTPChannel Objects
-------------------

.. class:: SMTPChannel(server, conn, addr)

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

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

   The :class:`SMTPChannel` has the following instance variables:

   .. attribute:: smtp_server

      Holds the :class:`SMTPServer` that spawned this channel.

   .. attribute:: conn

      Holds the socket object connecting to the client.

   .. attribute:: addr

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

   .. attribute:: 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".

   .. attribute:: smtp_state

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

   .. attribute:: seen_greeting

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

   .. attribute:: mailfrom

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

   .. attribute:: rcpttos

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

   .. attribute:: 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".

   .. attribute:: fqdn

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

   .. attribute:: peer

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

   The :class:`SMTPChannel` operates by invoking methods named ``smtp_<command>``
   upon reception of a command line from the client. Built into the base
   :class:`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
            :attr:`seen_greeting`.  Sets server to base command mode.
   EHLO     Accepts the greeting from the client and stores it in
            :attr:`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
            :attr:`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 :attr:`rcpttos` list.
   RSET     Resets the :attr:`mailfrom`, :attr:`rcpttos`, and
            :attr:`received_data`, but not the greeting.
   DATA     Sets the internal state to :attr:`DATA` and stores remaining lines
            from the client in :attr:`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.
   ======== ===================================================================