This specific error indicates that is attempting to deliver an email through a transport method (like a filter or relay) that it cannot find in its current configuration . The "upd" suffix often appears in logs as part of a service name or status update, suggesting a problem with a specific daemon or updated setting. Common Root Causes
The "unknown mail transport" message is often a summary. Look further back in /var/log/mail.log or /var/log/maillog for the specific "mail transport unavailable" or "connect to..." errors that preceded the suspension. Postfix This specific error indicates that is attempting
postqueue -f
references a custom transport (like a content filter or a specific relay service) that isn't properly defined in , Postfix will simply report it as "unknown". Configuration Cause: full disk, wrong ownership on /var/spool/postfix
mailq | grep -B1 "temporarily suspended" Check Logs for Details: The "unknown mail transport"
To apply changes, always restart the service using systemctl restart postfix or postfix reload .