550 5.1.10 — recipient not found

This one is worth getting right, because the registry and the practice disagree. In the IANA registry the enhanced code X.1.10 is "Recipient address has null MX" — it belongs to RFC 7505, which defines the null MX record a domain publishes to say it accepts no mail at all.

In practice, the overwhelming majority of 550 5.1.10 replies people actually see come from Microsoft 365, where it means the recipient was not found by an address lookup. Both are permanent and both mean "this will never be delivered", so the handling is the same. But if you are writing a bounce processor, the code alone does not tell you which of the two you got — the reply text does.

The three-digit reply code comes from RFC 5321: a 2yz is a success, a 4yz is a transient failure the sender should retry, and a 5yz is a permanent failure the sender should not repeat in the same form. The dotted code beside it comes from RFC 3463 and has three parts — class.subject.detail — where the class repeats 2, 4 or 5, the subject says which part of the system had the problem, and the detail says what it was. The full list is an IANA registry established by RFC 5248, which is the authoritative source and the one nothing else ranking for these queries cites.

Operationally, not at all: both are permanent, both mean the recipient does not exist there, both should be suppressed. Microsoft’s own documentation covers the whole 5.1.1 to 5.1.20 range in one article for exactly that reason.

Where Microsoft’s guidance is specific is on backscatter: it notes that a 5.1.10 bounce can arrive because a spammer forged your address as the sender of a message to a non-existent recipient. If you are receiving these for messages you did not send, the bounce is not about your list at all and can be ignored.