SMTP enhanced status code 4.4.1

What the registry says

The basic codes it travels with

• 451 Requested action aborted: local error in processing

In practice

X.4.1 is reported by your own server when the remote MX does not answer, for example because port 25 is filtered, the host is down, or the MX record points at an address nothing listens on. Repeated for days, it ends in an expired message. A domain whose MX hosts never answer is effectively undeliverable, and a verifier marks such addresses as unable to verify rather than valid.

What the sender should do

Let the queue retry. If every address at one domain gets it for days, check whether the domain's MX hosts are reachable at all.

How providers use X.4.1

Quoted from each provider’s own error reference.

• Microsoft 365 5.4.1

  Relay Access Denied

Common questions

What does 4.4.1 mean?

X.4.1 means your server tried to connect but the remote system did not answer, because it was busy or unreachable. It is a temporary error.

What should I do about 4.4.1?

Let the queue retry. If every address at one domain gets it for days, check whether the domain's MX hosts are reachable at all.

Would an email verifier have caught 4.4.1?

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.

Full guides for this code

• 550 5.4.1 — recipient address rejected, access denied