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SMTP codes

SMTP 5.1.10 Recipient not found (null MX)

Microsoft 365: the recipient does not exist, often after a mailbox was deleted or a domain moved.

5.1.10
Class
5xx · Permanent failure
Retried?
No, the message bounces
Defined in
RFC 7372 / provider

What to do

How it appears in a bounce

550 5.1.10 Recipient not found (null MX)

Paste the full bounce into the decoder — it finds every code and recipient and says what to check.

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