X.6.8 · Message content or media
SMTP enhanced status code 5.6.8
UTF-8 string reply is required, but not permitted by the SMTP client
X.6.8 means a UTF-8 reply was needed to show the mailbox name, but the SMTP client does not allow that form of response.
| Registry code | X.6.8 |
|---|---|
| Forms in use | 5.6.8, 2.6.8 |
| Registry name | UTF-8 string reply is required, but not permitted by the SMTP client |
| Subject | 6: Message content or media |
| Permanent or temporary | 2.x.x success; 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 252, 553, 550 |
| Defined in | [RFC 6531] |
What the registry says
This indicates that a reply containing a UTF-8 string is required to show the mailbox name, but that form of response is not permitted by the SMTP client.
IANA SMTP Enhanced Status Codes registry, X.6.8
The basic codes it travels with
- 252 Cannot VRFY user, but will accept message and attempt delivery
- 553 Requested action not taken: mailbox name not allowed (e.g., mailbox syntax incorrect)
- 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
In practice
X.6.8 occurs when a server needs to show a UTF-8 mailbox name in a reply, for example to a VRFY command, but the client did not announce SMTPUTF8 support. It is rare outside internationalised mail. Enabling SMTPUTF8 in the client, or using an ASCII alias, avoids it.
What the sender should do
Enable SMTPUTF8 support in the sending client, or use an ASCII form of the address.
Would verifying the list have prevented 5.6.8?
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 5.6.8 mean?
- X.6.8 means a UTF-8 reply was needed to show the mailbox name, but the SMTP client does not allow that form of response.
- What is the difference between 5.6.8 and 2.6.8?
- They share the subject and detail digits, so they report the same condition, "UTF-8 string reply is required, but not permitted by the SMTP client". The first digit is the class: 2.x.x success; 5.x.x permanent failure: resending unchanged will not succeed.
- What should I do about 5.6.8?
- Enable SMTPUTF8 support in the sending client, or use an ASCII form of the address.
- Would an email verifier have caught 5.6.8?
- 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