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=False)

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. 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. The default is False. When decode_data is not set to True 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 parameter 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.

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 parameters, and the kwargs parameter to process_message() when one or more of these is specified.

Changed in version 3.6: decode_data is now False by default.

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=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 determins 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.

Changed in version 3.5: The decode_data and enable_SMTPUTF8 parameters were added.

Changed in version 3.6: 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.