: A transport name specified in main.cf (e.g., transport_maps , relayhost , or virtual_transport ) is missing its definition in master.cf .
Check logs (primary debugging step)
environment (common on Debian/Ubuntu), it might not be able to find system files like /etc/services or resolver libraries. Server Fault : A transport name specified in main
The Postfix error "delivery temporarily suspended: unknown mail transport error" : A transport name specified in main
"delivery temporarily suspended: unknown mail transport error" : A transport name specified in main
A malformed entry in your transport file can cause this. For example: