A simple SMTP server (in Python)

Question

Could you please suggest a simple SMTP server with the very basic APIs (by very basic I mean, to read, write, delete email), that could be run on a linux box? I just need to

Answer 1

I also wanted to start a smtp server in Python and send emails with Python. I wanted to run all this in a Flask web application in a single process, which means the smtp server must be non-blocking. Here's the solution I eventually came to [gist]:

app.py

from flask import Flask, render_template
from smtp_client import send_email
from smtp_server import SMTPServer

app = Flask(__name__)

@app.route('/send_email')
def email():
  server = SMTPServer()
  server.start()
  try:
    send_email()
  finally:
    server.stop()
  return 'OK'

@app.route('/')
def index():
  return 'Woohoo'

if __name__ == '__main__':
  app.run(debug=True, host='0.0.0.0')

smtp_server.py

# smtp_server.py
import smtpd
import asyncore
import threading

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

class SMTPServer():
  def __init__(self):
    self.port = 1025

  def start(self):
    '''Start listening on self.port'''
    # create an instance of the SMTP server, derived from  asyncore.dispatcher
    self.smtp = CustomSMTPServer(('0.0.0.0', self.port), None)
    # start the asyncore loop, listening for SMTP connection, within a thread
    # timeout parameter is important, otherwise code will block 30 seconds
    # after the smtp channel has been closed
    kwargs = {'timeout':1, 'use_poll': True}
    self.thread = threading.Thread(target=asyncore.loop, kwargs=kwargs)
    self.thread.start()

  def stop(self):
    '''Stop listening to self.port'''
    # close the SMTPserver to ensure no channels connect to asyncore
    self.smtp.close()
    # now it is safe to wait for asyncore.loop() to exit
    self.thread.join()

  # check for emails in a non-blocking way
  def get(self):
    '''Return all emails received so far'''
    return self.smtp.emails

if __name__ == '__main__':
  server = CustomSMTPServer(('0.0.0.0', 1025), None)
  asyncore.loop()

smtp_client.py

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

def send_email():
  sender='author@example.com'
  recipient='6142546977@tmomail.net'

  msg = MIMEText('This is the body of the message.')
  msg['To'] = email.utils.formataddr(('Recipient', recipient))
  msg['From'] = email.utils.formataddr(('Author', 'author@example.com'))
  msg['Subject'] = 'Simple test message'

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

Then start the server with python app.py and in another request simulate a request to /send_email with curl localhost:5000/send_email. Note that to actually send the email (or sms) you'll need to jump through other hoops detailed here: https://blog.codinghorror.com/so-youd-like-to-send-some-email-through-code/.

Answer 2

There are really 2 things required to send an email:

• An SMTP Server - This can either be the Python SMTP Server or you can use GMail or your ISP's server. Chances are you don't need to run your own.
• An SMTP Library - Something that will send an email request to the SMTP server. Python ships with a library called smtplib that can do that for you. There is tons of information on how to use it here: http://docs.python.org/library/smtplib.html

For reading, there are two options depending on what server you are reading the email from.

• For a POP Email Server - You can use the poplib python library: http://docs.python.org/library/poplib.html
• For an IMAP Email Server - You can use the imaplib python library: http://docs.python.org/library/imaplib.html

Answer 3

To get Hasen's script working in Python 3 I had to tweak it slightly:

from datetime import datetime
import asyncore
from smtpd import SMTPServer

class EmlServer(SMTPServer):
    no = 0
    def process_message(self, peer, mailfrom, rcpttos, data, **kwargs):
        filename = '%s-%d.eml' % (datetime.now().strftime('%Y%m%d%H%M%S'),
            self.no)
        print(filename)
        f = open(filename, 'wb')
        f.write(data)
        f.close
        print('%s saved.' % filename)
        self.no += 1

def run():
    EmlServer(('localhost', 25), None)
    try:
        asyncore.loop()
    except KeyboardInterrupt:
        pass

if __name__ == '__main__':
    run()