Enhanced SMTP status code · RFC 3463 · IANA
5.7.506 Access Denied, Bad HELO
The server introduces itself in HELO as the server it connects to, instead of by its own name.
- 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 5.7.506 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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5.7.506
Access Denied, Bad HELO
The server introduces itself in HELO as the server it connects to, instead of by its own name. Check with Mail delivery test.
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