Enhanced SMTP status code · RFC 3463 · IANA
5.4.1 No answer from host
The connection to the next server got no answer: it was busy or could not take the call.
- 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). The registry says this code is only useful as a temporary failure.
Also seen as:
4.4.1 - Subject: X.4 · Network and routing
Network and routing status (DNS included); under the control of the destination's or an intermediate server's administrator.
- Associated basic codes (IANA registry)
How 5.4.1 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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5.4.1
Relay Access Denied
The server that answered does not accept mail for the recipient's domain; usually a server or DNS (MX) misconfiguration. Check with Email DNS check.
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5.4.1
Recipient address rejected: Access denied
The recipient's address does not exist or is not valid.
What you can check here
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026