Your first and most important action is to look beyond the initial error and find the preceding log messages. Postfix almost always logs a specific reason for the failure just before the generic "unknown mail transport error".
Inspect Postfix logs (usually located in /var/log/postfix/ ) for more detailed error messages. Files like mail.log , postfix.log , or syslog might contain clues. Your first and most important action is to
postsuper -r ALL deferred
Set up log parsers like Logwatch or integration daemons to scan your mail logs for the word suspended or deferred , alerting your team before users notice delivery delays. Your first and most important action is to