X.6.6 · Message content or media
SMTP enhanced status code 5.6.6
Message content not available
X.6.6 means the message content could not be fetched from a remote system, as used with the BURL extension of RFC 4468.
| Registry code | X.6.6 |
|---|---|
| Forms in use | 5.6.6 |
| Registry name | Message content not available |
| Subject | 6: Message content or media |
| Permanent or temporary | 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 554 |
| Defined in | [RFC 4468] |
What the registry says
The message content could not be fetched from a remote system. This may be useful as a permanent or persistent temporary notification.
IANA SMTP Enhanced Status Codes registry, X.6.6
The basic codes it travels with
- 554 Transaction failed (Or, in the case of a connection-opening response, "No SMTP service here")
In practice
X.6.6 comes from the BURL extension, where a client asks a submission server to fetch content from an IMAP server instead of uploading it. It is very rare in ordinary sending. If you see it, the referenced content was not available to the submission server. Ordinary SMTP clients do not use BURL, so this code points to a specialised mail client or a mobile mail system. It does not indicate any problem with the recipient.
What the sender should do
Check that the content your client referenced is still available to the submission server, or send the content directly.
Would verifying the list have prevented 5.6.6?
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 5.6.6 mean?
- X.6.6 means the message content could not be fetched from a remote system, as used with the BURL extension of RFC 4468.
- What should I do about 5.6.6?
- Check that the content your client referenced is still available to the submission server, or send the content directly.
- Would an email verifier have caught 5.6.6?
- 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