5.6.10: deprecated duplicate of 5.6.8
A registered code that duplicates X.6.8 (UTF-8 reply required but not permitted) and is deprecated, so servers should send X.6.8 instead.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.6.10
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
What it means
X.6.10 appears in the registry with a single note: it duplicates X.6.8 and is deprecated. RFC 6531, which registered the internationalisation codes, lists it the same way. If a server still sends 5.6.10, read it as 5.6.8: the server needed to answer a VRFY or EXPN query with a UTF-8 mailbox name, and the client had not asked for UTF-8 replies. The registry gives it no sample text and no associated reply code. Software that still generates it should be updated to send X.6.8.
Common causes
- Mail server software that still emits the deprecated code for the X.6.8 situation.
- A VRFY or EXPN query without the SMTPUTF8 parameter about a mailbox with a non-ASCII name.
How to fix it
- 01Follow the steps for 5.6.8.
- 02Repeat the query with the SMTPUTF8 parameter if your client supports it.
- 03Server admins: update the software so it reports X.6.8.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.6.8UTF-8 reply needed but not allowed.To answer a VRFY or EXPN query the server would have to show a mailbox name in UTF-8, but the client did not say it can handle UTF-8 replies.
- 5.6.7Non-ASCII address not allowed.An address in the envelope contains non-ASCII characters (an internationalised address), and the receiving server will not accept that for this sender or recipient.
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