Enhanced SMTP status code · RFC 3463 · IANA
5.1.10 Recipient domain has a null MX
The recipient's domain publishes a null MX (RFC 7505): it declares that it accepts no mail at all.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
- Subject: X.1 · Address
About the originator or destination address: syntax or validity. The sender can usually correct it and retry.
- Associated basic codes (IANA registry)
How 5.1.10 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Microsoft 365 / Exchange Online
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5.1.10
Recipient not found
The recipient's address does not exist or is not valid.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026