"smtpd" --- SMTP 服务器
***********************

**源代码:** Lib/smtpd.py

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

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

参见:

  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 对象
===============

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 新版功能: The *map* constructor argument.

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

   3.5 新版功能: The *decode_data* and *enable_SMTPUTF8* constructor
   parameters, and the *kwargs* parameter to "process_message()" when
   *decode_data* is "False".

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


DebuggingServer 对象
====================

class smtpd.DebuggingServer(localaddr, remoteaddr)

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


PureProxy对象
=============

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 对象
=================

class smtpd.MailmanProxy(localaddr, remoteaddr)

   Deprecated since version 3.9, will be removed in version 3.11:
   "MailmanProxy" is deprecated, it depends on a "Mailman" module
   which no longer exists and therefore is already broken.

   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 对象
================

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".

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

   在 3.6 版更改: *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):

   +----------+---------------------------------------------------------------------+
   | 命令     | 所采取的行动                                                        |
   |==========|=====================================================================|
   | HELO     | 接受来自客户端的问候语，并将其存储在 "seen_greeting" 中。将服务器设 |
   |          | 置为基本命令模式。                                                  |
   +----------+---------------------------------------------------------------------+
   | EHLO     | 接受来自客户的问候并将其存储在 "seen_greeting" 中。将服务器设置为扩 |
   |          | 展命令模式。                                                        |
   +----------+---------------------------------------------------------------------+
   | NOOP     | 不采取任何措施。                                                    |
   +----------+---------------------------------------------------------------------+
   | QUIT     | 干净地关闭连接。                                                    |
   +----------+---------------------------------------------------------------------+
   | 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     | 重置 "mailfrom", "rcpttos", 和 "received_data" ，但不重置问候语。   |
   +----------+---------------------------------------------------------------------+
   | 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     | 返回有关命令语法的最少信息                                          |
   +----------+---------------------------------------------------------------------+
   | VRFY     | 返回代码252（服务器不知道该地址是否有效）                           |
   +----------+---------------------------------------------------------------------+
   | EXPN     | 报告该命令未实现。                                                  |
   +----------+---------------------------------------------------------------------+
