In other words, the error is not in Postfix. The error is that the other server failed at conversation. And Postfix, like a baffled diplomat, can only report: "They said something , but it was not language."
This status is a "soft failure" or deferral, meaning Postfix will keep the mail in the queue and try again later, but it signal that a underlying configuration or service issue is blocking the path. Primary Causes of the "Unknown Mail Transport Error" In other words, the error is not in Postfix
In short: Postfix tried to look up how to deliver an email (via upd ), got garbage or nothing back, and panicked into a "temporarily suspended" state. In other words