X.6.5 · Message content or media
SMTP enhanced status code 5.6.5
Conversion Failed
X.6.5 means a required content conversion was attempted and failed. It can be temporary or permanent.
| Registry code | X.6.5 |
|---|---|
| Forms in use | 5.6.5, 4.6.5 |
| Registry name | Conversion Failed |
| 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
A conversion was required but was unsuccessful. This may be useful as a permanent or persistent temporary notification.
IANA SMTP Enhanced Status Codes registry, X.6.5
In practice
X.6.5 means a conversion was attempted and failed. It is usually seen on gateways and security appliances that rewrite messages. Simplifying the message, for example fewer nested parts and standard encodings, usually helps.
What the sender should do
Simplify the message encoding and resend.
Would verifying the list have prevented 5.6.5?
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 5.6.5 mean?
- X.6.5 means a required content conversion was attempted and failed. It can be temporary or permanent.
- What is the difference between 5.6.5 and 4.6.5?
- They share the subject and detail digits, so they report the same condition, "Conversion Failed". 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 5.6.5?
- Simplify the message encoding and resend.
- Would an email verifier have caught 5.6.5?
- 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