...will you please implement proper queueing and retransmission mechanisms that do not claim that a mail is undeliverable on a Transient Negative Completion (RFC 2821, section 4.2.1), as this response does not mean that the mail has been
rejected, but only that it cannot be delivered at this time. It is used by the Greylisting service of my mail server and significantly keeps down the amount of spam coming from spam bot networks.
And the only senders who cannot cope with a 4XX status messages are travel booking systems. You could get the impression that the authors of these things might be under one blanket with the spam bot authors... On the other hand, I probably shouldn't assume malice when it can be explained with utter incompetence.