SMTP enhanced status code 5.3.2

What the registry says

The basic codes it travels with

• 521 Server does not accept mail (the host never accepts mail)

In practice

X.3.2 covers hosts that are not taking mail, for maintenance, overload or shutdown. RFC 7504 adds the permanent case: 521 is for hosts that never accept mail, and 556 for a relay that can tell a destination never accepts mail. A temporary 4.3.2 should clear on retry. A permanent 5.3.2 for every address on a domain is a strong sign the domain has no working mail service, and its addresses should be removed.

What the sender should do

Retry a 4.3.2 later. If a 5.3.2 comes back for every address at a domain, that domain does not accept mail and its addresses should be removed.

How providers use X.3.2

Quoted from each provider’s own error reference.

• Microsoft 365 432 4.3.2

  STOREDRV.Deliver; recipient thread limit exceeded

Common questions

What does 5.3.2 mean?

X.3.2 means the receiving host is not accepting messages, for reasons like shutdown, heavy load or maintenance. As 521 or 556 under RFC 7504 it can be permanent, meaning the host never accepts mail.

What is the difference between 5.3.2 and 4.3.2?

They share the subject and detail digits, so they report the same condition, "System not accepting network messages". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.

What should I do about 5.3.2?

Retry a 4.3.2 later. If a 5.3.2 comes back for every address at a domain, that domain does not accept mail and its addresses should be removed.

Would an email verifier have caught 5.3.2?

Partly. The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.