X.6.10 · Message content or media
SMTP enhanced status code 5.6.10
Deprecated duplicate of X.6.8
X.6.10 is a deprecated duplicate of X.6.8 in the IANA registry. Servers should not send it, and if you see it, read it as X.6.8.
| Registry code | X.6.10 |
|---|---|
| Forms in use | 5.6.10, 4.6.10 |
| Registry name | Deprecated duplicate of X.6.8 |
| Subject | 6: Message content or media |
| Permanent or temporary | 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | |
| Defined in | [RFC 6531] |
What the registry says
This is a duplicate of X.6.8 and is thus deprecated.
IANA SMTP Enhanced Status Codes registry, X.6.10
In practice
The IANA registry marks X.6.10 as a duplicate of X.6.8, kept only because it was published. A server that sends it is reporting the X.6.8 condition: a UTF-8 reply was required but the client did not allow it. Follow the X.6.8 advice.
What the sender should do
Treat it as X.6.8: enable SMTPUTF8 support in the client or use an ASCII form of the address.
Would verifying the list have prevented 5.6.10?
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 5.6.10 mean?
- X.6.10 is a deprecated duplicate of X.6.8 in the IANA registry. Servers should not send it, and if you see it, read it as X.6.8.
- What is the difference between 5.6.10 and 4.6.10?
- They share the subject and detail digits, so they report the same condition, "Deprecated duplicate of X.6.8". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.
- What should I do about 5.6.10?
- Treat it as X.6.8: enable SMTPUTF8 support in the client or use an ASCII form of the address.
- Would an email verifier have caught 5.6.10?
- 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)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05