smtpd – Sample SMTP Servers

Purpose:Includes classes for implementing SMTP servers.
Available In:2.1 and later

The smtpd module includes classes for building simple mail transport protocol servers. It is the server-side of the protocol used by smtplib.


The base class for all of the provided example servers is SMTPServer. It handles communicating with the client, receiving the data, and provides a convenient hook to override to handle the message once it is fully available.

The constructor arguments are the local address to listen for connections and the remote address for proxying. The method process_message() is provided as a hook to be overridden by your derived class. It is called when the message is completely received, and given these arguments:


The client’s address, a tuple containing IP and incoming port.


The “from” information out of the message envelope, given to the server by the client when the message is delivered. This does not necessarily match the From header in all cases.


The list of recipients from the message envelope. Again, this does not always match the To header, especially if someone is blind carbon copied.


The full RFC 2822 message body.

Since the default implementation of process_message() raises NotImplementedError, to demonstrate using SMTPServer we need to create a subclass and provide a useful implementation. This first example defines a server that prints information about the messages it receives.

import smtpd
import asyncore

class CustomSMTPServer(smtpd.SMTPServer):
    def process_message(self, peer, mailfrom, rcpttos, data):
        print 'Receiving message from:', peer
        print 'Message addressed from:', mailfrom
        print 'Message addressed to  :', rcpttos
        print 'Message length        :', len(data)

server = CustomSMTPServer(('', 1025), None)


SMTPServer uses asyncore, so to run the server we call asyncore.loop().

Now, we need a client to send data. By adapting one of the examples from the smtplib page, we can set up a client to send data to our test server running locally on port 1025.

import smtplib
import email.utils
from email.mime.text import MIMEText

# Create the message
msg = MIMEText('This is the body of the message.')
msg['To'] = email.utils.formataddr(('Recipient', ''))
msg['From'] = email.utils.formataddr(('Author', ''))
msg['Subject'] = 'Simple test message'

server = smtplib.SMTP('', 1025)
server.set_debuglevel(True) # show communication with the server
    server.sendmail('', [''], msg.as_string())

Now if we run in one terminal, and in another, we should see:

$ python
send: 'ehlo farnsworth.local\r\n'
reply: '502 Error: command "EHLO" not implemented\r\n'
reply: retcode (502); Msg: Error: command "EHLO" not implemented
send: 'helo farnsworth.local\r\n'
reply: '250 farnsworth.local\r\n'
reply: retcode (250); Msg: farnsworth.local
send: 'mail FROM:<>\r\n'
reply: '250 Ok\r\n'
reply: retcode (250); Msg: Ok
send: 'rcpt TO:<>\r\n'
reply: '250 Ok\r\n'
reply: retcode (250); Msg: Ok
send: 'data\r\n'
reply: '354 End data with <CR><LF>.<CR><LF>\r\n'
reply: retcode (354); Msg: End data with <CR><LF>.<CR><LF>
data: (354, 'End data with <CR><LF>.<CR><LF>')
send: 'Content-Type: text/plain; charset="us-ascii"\r\nMIME-Version: 1.0\r\nContent-Transfer-Encoding: 7bit\r\nTo: Recipient <>\r\nFrom: Author <>\r\nSubject: Simple test message\r\n\r\nThis is the body of the message.\r\n.\r\n'
reply: '250 Ok\r\n'
reply: retcode (250); Msg: Ok
data: (250, 'Ok')
send: 'quit\r\n'
reply: '221 Bye\r\n'
reply: retcode (221); Msg: Bye


$ python
Receiving message from: ('', 58541)
Message addressed from:
Message addressed to  : ['']
Message length        : 229

The port number for the incoming message will vary each time. Notice that the rcpttos argument is a list of values and mailfrom is a single string.


To stop the server, press Ctrl-C.


The example above shows the arguments to process_message(), but smtpd also includes a server specifically designed for more complete debugging, called DebuggingServer. It prints the entire incoming message to stdout and then stops processing (it does not proxy the message to a real mail server).

import smtpd
import asyncore

server = smtpd.DebuggingServer(('', 1025), None)


Using the client program from above, the output of the DebuggingServer is:

$ python
---------- MESSAGE FOLLOWS ----------
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
To: Recipient <>
From: Author <>
Subject: Simple test message

This is the body of the message.
------------ END MESSAGE ------------


The PureProxy class implements a straightforward proxy server. Incoming messages are forwarded upstream to the server given as argument to the constructor.


The stdlib docs say, “running this has a good chance to make you into an open relay, so please be careful.”

Setting up the proxy server is just as easy as the debug server:

import smtpd
import asyncore

server = smtpd.PureProxy(('', 1025), ('mail', 25))


It prints no output, though, so to verify that it is working we need to look at the mail server logs.

Oct 19 19:16:34 homer sendmail[6785]: m9JNGXJb006785: from=<>, size=248, class=0, nrcpts=1, msgid=<>, proto=ESMTP, daemon=MTA, relay=[]


smtpd also includes a special proxy that acts as a front-end for Mailman. If the local Mailman configuration recognizes the address, it is handled directly. Otherwise the message is delivered to the proxy.

See also

Standard library documentation for this module.
Provides a client interface.
Parses email messages.
Base module for writing asynchronous servers.
RFC 2822
Defines the email message format.
GNU Mailman mailing list software
An excellent example of Python software that works with email messages.