"smtpd" --- SMTP Server
***********************

**Source code:** Lib/smtpd.py

======================================================================

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

也參考: The aiosmtpd package is a recommended replacement for this
  module. It is based on "asyncio" and provides a more straightforward
  API. "smtpd" should be considered deprecated.

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.


SMTPServer Objects
==================

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

   Create a new "SMTPServer" object, which binds to local address
   *localaddr*.  It will treat *remoteaddr* as an upstream SMTP
   relayer.  Both *localaddr* and *remoteaddr* should be a (host,
   port) tuple.  The object 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* determines whether the "SMTPUTF8" extension (as
   defined in **RFC 6531**) should be enabled.  The default is
   "False". 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* and
   *enable_SMTPUTF8* cannot be set to "True" at the same time.

   *decode_data* specifies whether the data portion of the SMTP
   transaction should be decoded using UTF-8.  When *decode_data* is
   "False" (the default), 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.
   *decode_data* and *enable_SMTPUTF8* cannot be set to "True" at the
   same time.

   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 if "decode_data=True" was given as an init argument,
      otherwise 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.

   3.4 版新加入: The *map* constructor argument.

   3.5 版更變: *localaddr* and *remoteaddr* may now contain IPv6
   addresses.

   3.5 版新加入: The *decode_data* and *enable_SMTPUTF8* constructor
   parameters, and the *kwargs* parameter to "process_message()" when
   *decode_data* is "False".

   3.6 版更變: *decode_data* is now "False" by default.


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.


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.


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.


SMTPChannel Objects
===================

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

   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* determines whether the "SMTPUTF8" extension (as
   defined in **RFC 6531**) should be enabled.  The default is
   "False". *decode_data* and *enable_SMTPUTF8* cannot be 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 "False".
   *decode_data* and *enable_SMTPUTF8* cannot be set to "True" at the
   same time.

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

   3.5 版更變: The *decode_data* and *enable_SMTPUTF8* parameters were
   added.

   3.6 版更變: *decode_data* is now "False" by default.

   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.                        |
   +----------+---------------------------------------------------------------------+
