X.5.6 · Mail delivery protocol
SMTP enhanced status code 5.5.6
Authentication Exchange line is too long
X.5.6 means an AUTH response from the client was longer than the server's buffer for that authentication mechanism, as defined in RFC 4954.
| Registry code | X.5.6 |
|---|---|
| Forms in use | 5.5.6 |
| Registry name | Authentication Exchange line is too long |
| Subject | 5: Mail delivery protocol |
| Permanent or temporary | 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 500 |
| Defined in | [RFC 4954] |
What the registry says
This enhanced status code SHOULD be returned when the server fails the AUTH command due to the client sending a [BASE64] response which is longer than the maximum buffer size available for the currently selected SASL mechanism. This is useful for both permanent and persistent transient errors.
IANA SMTP Enhanced Status Codes registry, X.5.6
The basic codes it travels with
- 500 Syntax error, command unrecognized
In practice
X.5.6 can occur with OAuth tokens or certificates passed in AUTH that exceed the server's line limit for that mechanism. It is rare and specific to authentication. Updating the client library, or using a mechanism that sends credentials in shorter pieces, avoids it. If you see it from a provider that supports OAuth, check that the client sends the XOAUTH2 token in the form the provider documents.
What the sender should do
Use a different authentication mechanism or a shorter token, or update the client library.
Would verifying the list have prevented 5.5.6?
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 5.5.6 mean?
- X.5.6 means an AUTH response from the client was longer than the server's buffer for that authentication mechanism, as defined in RFC 4954.
- What should I do about 5.5.6?
- Use a different authentication mechanism or a shorter token, or update the client library.
- Would an email verifier have caught 5.5.6?
- 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 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply 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