20.16. 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.

20.16.1. SMTPServer Objects

class smtpd.SMTPServer(localaddr, remoteaddr)

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.

process_message(peer, mailfrom, rcpttos, data)

Raise 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 2822 format).


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

20.16.2. 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.

20.16.3. 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.

20.16.4. 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.

20.16.5. SMTPChannel Objects

class smtpd.SMTPChannel(server, conn, addr)

Create a new 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 SMTPServer.channel_class of your SMTPServer.

The SMTPChannel has the following instance variables:


Holds the SMTPServer that spawned this channel.


Holds the socket object connecting to the client.


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


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".


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


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


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


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


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".


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


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.
NOOP Takes no action.
QUIT Closes the connection cleanly.
MAIL Accepts the “MAIL FROM:” syntax and stores the supplied address as mailfrom.
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.