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@dalias @lispi314
Unless the 250 reply at SMTP EoD is delayed until real final delivery, it is not possible to be certain of final deliverability. Small systems running Sendmail are able do that but I don't believe that any other widespread MTA even offers synchronous final delivery during SMTP time. It's not feasible at scale.
If a message gets queued, it can always fail at the next hop. Users get deleted between accept and deliver. Mailboxes fill. LMTP receivers fall over mid-transaction.