X.7.11 · Security or policy
SMTP enhanced status code 5.7.11
Encryption required for requested authentication mechanism
X.7.11 means the chosen authentication mechanism may only be used over an encrypted connection. RFC 4954 keeps it for historical purposes.
| Registry code | X.7.11 |
|---|---|
| Forms in use | 5.7.11 |
| Registry name | Encryption required for requested authentication mechanism |
| Subject | 7: Security or policy |
| Permanent or temporary | 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 524, 538 |
| Defined in | [RFC 4954] |
What the registry says
This response to the AUTH command indicates that the selected authentication mechanism may only be used when the underlying SMTP connection is encrypted. Note that this response code is documented here for historical purposes only. Modern implementations SHOULD NOT advertise mechanisms that are not permitted due to lack of encryption, unless an encryption layer of sufficient strength is currently being employed.
IANA SMTP Enhanced Status Codes registry, X.7.11
The basic codes it travels with
- 538 Encryption required for requested authentication mechanism
In practice
X.7.11 says a mechanism is allowed only over encryption. Modern servers simply do not advertise such mechanisms until TLS is active. Gmail uses 501 5.7.11 for a syntax error when parameters follow a command that takes none.
What the sender should do
Enable TLS on the connection before authenticating.
Would verifying the list have prevented 5.7.11?
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.
How providers use X.7.11
Quoted from each provider’s own error reference.
- Gmail 501 5.7.11
Syntax error (no parameters allowed). For more information, go to About SMTP error messages and review RFC 3207 specifications.
Common questions
- What does 5.7.11 mean?
- X.7.11 means the chosen authentication mechanism may only be used over an encrypted connection. RFC 4954 keeps it for historical purposes.
- What should I do about 5.7.11?
- Enable TLS on the connection before authenticating.
- Would an email verifier have caught 5.7.11?
- 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)
- Google Workspace Admin Help: Gmail SMTP errors and codes (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