Delivery Temporarily Suspended Unknown Mail Transport Error Postfix Upd -

Find the specific email causing the error:

So the "unknown transport error" often means the remote server broke the rules. It might have: Find the specific email causing the error: So

Look for lines mentioning warning , error , or fatal . Find the specific email causing the error: So

If you recently ran apt upgrade postfix or yum update postfix , old process binaries may linger. You might have a new master.cf but an old upd daemon still running in memory. This mismatch triggers "unknown transport error" because the IPC protocol changed between versions. Find the specific email causing the error: So