Actually I would appreciate advice on how to do this on an internal environment.
google's your friend on this one
https://www.google.com/search?q=postfix+docker+testing
first link,
https://fabianlee.org/2019/10/23/docker-running-a-postfix-container-for-testing-mail-during-development/
looks like a reasonable place to start
start simple, demonstrate basic setup works inbound & outbound, then worry
about adding proxies, etc, and finally opening yourself up to the 'net -- and the
'net to you.
