SMTP enhanced status code 5.1.10

What the registry says

The basic codes it travels with

• 556 Domain does not accept mail (the relay can tell the destination never accepts mail, for example from a null MX)

In practice

A null MX is a single MX record with priority 0 and a target of ".", published by domains that never receive mail, often parked or web-only domains. RFC 7505 says senders should not try to deliver to them, and some sending servers fail these addresses before any connection is made, with 556 5.1.10. Microsoft uses 550 5.1.10 differently, for a recipient not found in Exchange Online, so read the text after the code.

What the sender should do

Remove every address on that domain. A verifier that reads MX records sees the null MX and marks the addresses invalid before sending.

How providers use X.1.10

Quoted from each provider’s own error reference.

• Microsoft 365 5.1.10

  Recipient not found

Common questions

What does 5.1.10 mean?

X.1.10 means the recipient domain publishes a null MX, a single MX record of "." defined in RFC 7505, which declares that the domain accepts no mail at all. No address on it can ever receive a message.

What should I do about 5.1.10?

Remove every address on that domain. A verifier that reads MX records sees the null MX and marks the addresses invalid before sending.

Would an email verifier have caught 5.1.10?

Yes, verification catches this. A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.

Full guides for this code

• 550 5.1.10 — recipient not found (Microsoft 365)