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.

Source: https://muffinresearch.co.uk/fake-smtp-server-with-python/