Enhanced SMTP status code · RFC 3463 · IANA
5.7.9 Authentication mechanism too weak
The chosen authentication mechanism is weaker than the server's policy allows; the client must choose another (RFC 4954).
- 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.
- Associated basic codes (IANA registry)
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026