This is simple and neat.
Haven't found any use for it yet but worth to have it around :)
Run our fake SMTP server:
sonic:~# python -m smtpd -n -c DebuggingServer 192.168.11.22:25
Test our fake SMTP server:
venus:~# telnet 192.168.11.22 25 Trying 192.168.11.22... Connected to 192.168.11.22. Escape character is '^]'. 220 192.168.11.22 Python SMTP proxy version 0.2 ^] telnet> q Connection closed.