Customize/Expand main.cf
Please create a new file data/conf/postfix/extra.cf for overrides or additional content to main.cf.
Postfix will complain about duplicate values once after starting postfix-mailcow, this is intended.
Syslog-ng was configured to hide those warnings while Postfix is running, to not spam the log files with unnecessary information every time a service is used.
Restart postfix-mailcow to apply your changes:
docker compose restart postfix-mailcow
docker-compose restart postfix-mailcow