X.6.1 · Message content or media
SMTP enhanced status code 5.6.1
Media not supported
X.6.1 means the message's media type is not supported by the delivery protocol or the next system in the path.
| Registry code | X.6.1 |
|---|---|
| Forms in use | 5.6.1, 4.6.1 |
| Registry name | Media not supported |
| 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 | Not given |
| Defined in | [RFC 3463] |
What the registry says
The media of the message is not supported by either the delivery protocol or the next system in the forwarding path. This is useful only as a permanent error.
IANA SMTP Enhanced Status Codes registry, X.6.1
In practice
X.6.1 appears when the next hop cannot carry the message's media type, for example on gateways to systems that only accept plain text. It is rare on the public internet. Attaching content in a common format, or linking to it, avoids the problem.
What the sender should do
Send the content in a more common format, or as a link.
Would verifying the list have prevented 5.6.1?
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 5.6.1 mean?
- X.6.1 means the message's media type is not supported by the delivery protocol or the next system in the path.
- What is the difference between 5.6.1 and 4.6.1?
- They share the subject and detail digits, so they report the same condition, "Media not supported". 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 5.6.1?
- Send the content in a more common format, or as a link.
- Would an email verifier have caught 5.6.1?
- 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