Enhanced SMTP status code · RFC 3463 · IANA
5.7.512 Access denied, message must be RFC 5322 section 3.6.2 compliant
The message has no valid From address (RFC 5322 §3.6.2).
- 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).
- 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 5.7.512 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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5.7.512
Access denied, message must be RFC 5322 section 3.6.2 compliant
The message has no valid From address (RFC 5322 §3.6.2). Check with Header analyzer.
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