mailcow-dockerized-docs/docs/u_e-postfix-extra_cf.md

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