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 over a dockerized mailcow, you may want to add 172.22.1.1 as relayhost and remove the Docker interface from "inet_interfaces":

postconf -e 'relayhost = 172.22.1.1'
postconf -e "mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128"
postconf -e "inet_interfaces = loopback-only"

"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.