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Local MTA on Docker host

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

Furthermore, to relay your local mail over the dockerized mailcow, you may want to add 172.22.1.1 as relayhost:

postconf -e 'relayhost = 172.22.1.1'

"172.22.1.1" is the mailcow created network gateway in Docker. Relaying over this interface is necessary (instead of - for example - relaying directly over ${MAILCOW_HOSTNAME}) to relay over a known internal network.

Restart Postfix after applying your changes.