Enhanced SMTP status code · RFC 3463 · IANA
5.4.8 MX hosts of <domain> failed MTA-STS validation
The recipient's MX host is not one allowed by its domain's MTA-STS policy; the policy must include the MX host.
- 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).
Also seen as:
4.4.8 - 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.
How 5.4.8 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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5.4.8
MX hosts of <domain> failed MTA-STS validation
The recipient's MX host is not one allowed by its domain's MTA-STS policy; the policy must include the MX host. Check with MTA-STS generator.
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4.4.8
MX hosts of <domain> failed MTA-STS validation
The recipient's MX host is not one allowed by its domain's MTA-STS policy; the policy must include the MX host. Check with MTA-STS generator.
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