5.6.8: UTF-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.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.6.8
- 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 the server says
5.6.8 UTF-8 string reply is required, but not permitted by the SMTP client
What it means
This code comes from RFC 6531 and concerns the VRFY and EXPN commands, which ask a server to confirm or expand an address. If the answer contains a non-ASCII mailbox name and the client did not ask for UTF-8 replies (the SMTPUTF8 parameter on the command), the server cannot give it and uses X.6.8 instead. The registry associates it with reply codes 252, 550 and 553. It has nothing to do with normal message delivery, and many public servers switch VRFY and EXPN off, so the code is rarely seen. X.6.10 is a deprecated duplicate of this code.
Common causes
- A VRFY or EXPN query without the SMTPUTF8 parameter about a mailbox whose name has non-ASCII characters.
- An address-checking tool that uses VRFY and does not support SMTPUTF8.
How to fix it
- 01Repeat the query with the SMTPUTF8 parameter if your client supports it.
- 02Use a client or library with SMTPUTF8 support.
- 03Do not rely on VRFY to check addresses; many servers switch it off or answer it vaguely.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.6.7Non-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.
- 5.6.10Deprecated 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.
- 5.6.9Internationalised message cannot be delivered.The message has UTF-8 (internationalised) headers and could not be passed on to one or more recipients whose path cannot handle them, so it was rejected.
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