PyMOTW: smtpd

By Doug Hellmann
October 19, 2008

smtpd – Sample SMTP Servers

Purpose:Includes classes for implementing SMTP servers.
Python Version: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.

SMTPServer

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:

peer
The client’s address, a tuple containing IP and incoming port.
mailfrom
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.
rcpttos
The list of recipients from the message envelope. Again, this does not always match the To header, especially if someone is blind carbon copied.
data
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. Let’s create a simple server to print information about the messages recieved.

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)
        return

server = CustomSMTPServer(('127.0.0.1', 1025), None)

asyncore.loop()

Since SMTPServer uses asyncore, to run the server we run the asyncore.loop() function.

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', 'recipient@example.com'))
msg['From'] = email.utils.formataddr(('Author', 'author@example.com'))
msg['Subject'] = 'Simple test message'

server = smtplib.SMTP('127.0.0.1', 1025)
server.set_debuglevel(True) # show communication with the server
try:
    server.sendmail('author@example.com', ['recipient@example.com'], msg.as_string())
finally:
    server.quit()

Now if we run smtpd_custom.py in one terminal, and smtpd_senddata.py in another, we should see:

$ python smtpd_senddata.py
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:<author@example.com>\r\n'
reply: '250 Ok\r\n'
reply: retcode (250); Msg: Ok
send: 'rcpt TO:<recipient@example.com>\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 <recipient@example.com>\r\nFrom: Author <author@example.com>\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

and

$ python smtpd_custom.py
Receiving message from: ('127.0.0.1', 58541)
Message addressed from: author@example.com
Message addressed to  : ['recipient@example.com']
Message length        : 229

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

Note

To stop the server, press Ctrl-C.

DebuggingServer

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

import smtpd
import asyncore

server = smtpd.DebuggingServer(('127.0.0.1', 1025), None)

asyncore.loop()

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

$ python smtpd_debug.py
---------- MESSAGE FOLLOWS ----------
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
To: Recipient <recipient@example.com>
From: Author <author@example.com>
Subject: Simple test message
X-Peer: 127.0.0.1

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

PureProxy

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

Warning

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(('127.0.0.1', 1025), ('mail', 25))

asyncore.loop()

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=<author@example.com>, size=248, class=0, nrcpts=1, msgid=<200810192316.m9JNGXJb006785@homer.example.com>, proto=ESMTP, daemon=MTA, relay=[192.168.1.17]

MailmanProxy

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.

References

Standard library documentation: smtpd

The smtplib module provides a client interface.

See also email and asyncore.

RFC 2822

GNU Mailman mailing list software

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