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

Insert these lines into data/conf/postfix/main.cf. "relayhost" does already exist (empty), just change its value.

relayhost = [your-relayhost]:587
smtp_sasl_password_maps = hash:/opt/postfix/conf/smarthost_passwd
smtp_sasl_auth_enable = yes

Create the credentials file:

echo "your-relayhost username:password" > data/conf/postfix/smarthost_passwd

Run:

docker-compose exec postfix-mailcow postmap /opt/postfix/conf/smarthost_passwd
docker-compose exec postfix-mailcow chown root:postfix /opt/postfix/conf/smarthost_passwd /opt/postfix/conf/smarthost_passwd.db
docker-compose exec postfix-mailcow chmod 660 /opt/postfix/conf/smarthost_passwd /opt/postfix/conf/smarthost_passwd.db
docker-compose exec postfix-mailcow postfix reload

Helper script

There is a helper script mailcow-setup-relayhost.sh you can run to setup a relayhost.

Usage:

Setup a relayhost:
./mailcow-setup-relayhost.sh relayhost port (username) (password)
Username and password are optional parameters.

Reset to defaults:
./mailcow-setup-relayhost.sh reset