Disable Sender Addresses Verification

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.