Enhanced SMTP status code · RFC 3463 · IANA
5.7.7 Message integrity failure
The message could not be validated because it was corrupted or altered on the way.
- 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 can be temporary or permanent.
Also seen as:
4.7.7 - Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026