Because the failure happened at the "transport" layer rather than the SMTP protocol layer, Postfix does not know exactly why it failed, so it marks the status as "unknown" and places the email in a deferred queue to try again later.
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Start here, then move down the list if the problem persists. Because the failure happened at the "transport" layer
The “unknown mail transport error” is frustrating precisely because it’s generic. But 90% of the time, it’s a network handshake issue or a broken transport map following an update (the “upd” in your log). Because the failure happened at the "transport" layer