This option is not best-practice and should only be implemented when there is no other option available to achieve whatever you are trying to do. Simply create a file `data/conf/postfix/check_sasl_access` and enter the following content. This user must exist in your installation and needs to authenticate before sending mail. ``` user-to-allow-everything@example.com OK ``` Open `data/conf/postfix/main.cf` and find `smtpd_sender_restrictions`. Prepend `check_sasl_access hash:/opt/postfix/conf/check_sasl_access` like this: ``` smtpd_sender_restrictions = check_sasl_access hash:/opt/postfix/conf/check_sasl_access reject_authenticated_sender_login_mismatch [...] ``` Run postmap on check_sasl_access: ``` docker-compose exec postfix-mailcow postmap /opt/postfix/conf/check_sasl_access ``` Restart the Postfix container.