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Using Python smtplib and smptd

A Basic Guide
28 January 2017


This is a basic guide for an area where I wasn’t able to find any particularly great tutorials, perhaps because this stuff is obvious to more experienced developers. Well, it wasn’t to me.

OK, to begin: If you want to send an email using Python, you need two things:

1) An SMTP client

2) An SMTP server

In most cases, you will connect to a remote SMTP server, such as gmail’s, or perhaps your company’s SMTP server. However, it is also possible to create your own. In this post I will explain both cases.

Creating the SMTP Client with smtplib

For basic use of smtplib, you need to create an SMTP instance, which initializes the connection to an SMTP server, then you can use the sendmail() and quit() methods.


import smtplib

client = smtplib.SMTP('localhost')

fromaddr = ''
toaddrs = ''
msg = 'Hello'

client.sendmail(fromaddr, toaddrs, msg)

If you run this code locally and do not have an SMTP server running, you will get a connection failed error. On Linux, the easiest thing to do is to install something like Postfix which creates an SMTP server for your. Once you setup and start that server, the above code will work. Alternatively, you could connect to an external service (such as gmail), and in that case your code will change to look like this:

def send_email(user, pwd, recipient, subject, body):
    import smtplib

    gmail_user = user
    gmail_pwd = pwd
    FROM = user
    TO = recipient if type(recipient) is list else [recipient]
    SUBJECT = subject
    TEXT = body

    # Prepare actual message
    message = """From: %s\nTo: %s\nSubject: %s\n\n%s
    """ % (FROM, ", ".join(TO), SUBJECT, TEXT)
        server = smtplib.SMTP("", 587)
        server.login(gmail_user, gmail_pwd)
        server.sendmail(FROM, TO, message)
        print 'successfully sent the mail'
        print "failed to send mail"

Note that when connecting to external services, we need to employ more authentication methods such as ehlo() and login()

Setting up your own SMTP Server with smtpd

If you want to do everything yourself, you can setup your own SMTP server using smtpd

import smtpd
import asyncore

# expects a pair of tuples, passing in None for the second
server = smtpd.SMTPServer(('localhost', 1025), None)


The smtpd.SMTP class inherits from asyncore dispatcher meaning that the server is created when we call asyncore.loop()

A gotcha I found was that the asycore.loop() method enters a polling loop that then blocks any subsequent code. The solution to this is to either place your SMTP server code in a separate file and run it via a separate terminal, or to run it in its own thread.

Bringing it Together

Here’s a complete working example, combining both libraries

import threading
import smtpd
import asyncore
import smtplib

server = smtpd.SMTPServer(('localhost', 1025), None)

loop_thread = threading.Thread(target=asyncore.loop, name="Asyncore Loop")
# If you want to make the thread a daemon
# loop_thread.daemon = True

# port should match your SMTP server
client = smtplib.SMTP('localhost', port=1025)

fromaddr = ''
toaddrs = ''
msg = 'Hello'

server.sendmail(fromaddr, toaddrs, msg)

Note that if you try and run this code using your gmail/outlook etc. email addresses, you will get permission denied errors because the login process is required.