mailcow-dockerized-docs/docs/firststeps-local_mta.md

The easiest option would be to disable the listener on port 25/tcp.

Postfix users disable the listener by commenting the following line (starting with smtp or 25) in /etc/postfix/master.cf:

#smtp      inet  n       -       -       -       -       smtpd

Restart Postfix after applying your changes.