Enhanced SMTP status code · RFC 3463 · IANA
5.7.5 Cryptographic failure
A system meant to validate or decrypt the message could not, because a key was missing or invalid.
- 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:
4.7.5 - Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
How 5.7.5 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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5.7.5
Remote certificate failed MTA-STS validation. Reason: <validityStatus>
The recipient server's certificate fails MTA-STS validation: it must chain to a trusted authority and contain the host name in the policy. Check with SMTP server check.
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4.7.5
Remote certificate failed MTA-STS validation. Reason: <validityStatus>
The recipient server's certificate fails MTA-STS validation: it must chain to a trusted authority and contain the host name in the policy. Check with SMTP server check.
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