Do not blindly restart services. Follow this forensic approach.
Before fixing the problem, you must understand what Postfix is trying to say. Do not blindly restart services
: Run df -h to ensure you haven't run out of space on the /var or root partition. : Run df -h to ensure you haven't
The "unknown mail transport error" is a summary. The actual error usually appears 1–3 lines above it in the logs. Do not blindly restart services
"delivery temporarily suspended: unknown mail transport error" Accompanied by: "status=deferred" Often paired with: "postfix/upd" process mentions
: In master.cf , the 5th column indicates whether a service should run in a "chroot" jail. If this is enabled ( y ) but the necessary environment (like /etc/services or library files) isn't correctly mirrored inside the chroot directory, services like the smtp client may fail to resolve protocols or hosts.
