The Postfix error "delivery temporarily suspended: unknown mail transport error" is

: If you recently added a spam filter or antivirus (like Amavis or ClamAV), ensure their service lines are present and correctly typed. HowtoForge 3. Inspect the Logs for the Root Cause

Or a recursive lookup (domain points to a transport that points back to the same domain).

"delivery temporarily suspended: unknown mail transport error" in Postfix generally means the system is trying to use a transport service

Verify the path ( argv ). Does the file exist?