X.7.9 · Security or policy
SMTP enhanced status code 5.7.9
Authentication mechanism is too weak
X.7.9 means the authentication mechanism your client chose is weaker than the server allows for that user.
| Registry code | X.7.9 |
|---|---|
| Forms in use | 5.7.9 |
| Registry name | Authentication mechanism is too weak |
| Subject | 7: Security or policy |
| Permanent or temporary | 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 534 |
| Defined in | [RFC 4954] |
What the registry says
This response to the AUTH command indicates that the selected authentication mechanism is weaker than server policy permits for that user. The client SHOULD retry with a new authentication mechanism.
IANA SMTP Enhanced Status Codes registry, X.7.9
The basic codes it travels with
- 534 Authentication mechanism is too weak
In practice
X.7.9 is returned when the chosen AUTH mechanism is weaker than the server allows for that user, for example plain-text login on an account that requires OAuth. The client should try another mechanism. Updating the app or switching to OAuth usually fixes it.
What the sender should do
Switch to a stronger mechanism, usually OAuth or an app password over TLS, as the provider documents.
Would verifying the list have prevented 5.7.9?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does 5.7.9 mean?
- X.7.9 means the authentication mechanism your client chose is weaker than the server allows for that user.
- What should I do about 5.7.9?
- Switch to a stronger mechanism, usually OAuth or an app password over TLS, as the provider documents.
- Would an email verifier have caught 5.7.9?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
- RFC 4954: SMTP Service Extension for Authentication (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05