: Postfix might be trying to use a transport service (e.g., private/amavis or private/filter ) that is referenced in main.cf but missing from master.cf .
Edit /etc/postfix/master.cf and modify the transport line from:
Restart Postfix: sudo systemctl restart postfix
Once you find the Queue ID (e.g., 4SgLz90z ), search for everything related to it: grep "4SgLz90z" /var/log/mail.log Use code with caution. : Postfix might be trying to use a transport service (e
You can monitor the real-time delivery attempts to ensure everything is working correctly by tailing your log file: tail -f /var/log/mail.log | grep postfix Use code with caution. Summary Checklist
If it works in permissive mode, restore or fix the context:
Check logs (primary debugging step)
Scenario C: Exhausted System Resources (Disk Space or Inodes)
If logs remain unclear, increase Postfix verbosity temporarily
setenforce 0 (temporarily). If the error stops, SELinux is blocking the transport. Summary Checklist If it works in permissive mode,
If DNS fails, check /etc/resolv.conf and ensure your firewall is not blocking port 25. If you are running Postfix in a chroot, remember that it will use /var/spool/postfix/etc/resolv.conf instead.
The most frequent culprit is a typo or a missing service definition in your Postfix configuration files ( /etc/postfix/main.cf or /etc/postfix/master.cf ). If Postfix tries to route mail through a transport service that isn't defined or has incorrect arguments, it throws this error.