sudo postconf -e "debug_peer_level = 3" sudo postconf -e "debug_peer_list = domain.com" sudo systemctl restart postfix
A common trigger is referencing a transport like filter , maildrop , or a custom relay in main.cf without a corresponding entry in master.cf . Incident Report: Email Delivery Failure A
Next time you see it, don’t panic. Check the lines above the error. Test the transport. Flush the queue. You’ll likely be back in business within ten minutes. and email flow will resume.
By addressing the underlying script failure or service availability, the "unknown mail transport error" will resolve, and email flow will resume. Incident Report: Email Delivery Failure A