X.6.3 · Message content or media
SMTP enhanced status code 5.6.3
Conversion required but not supported
X.6.3 means the content had to be converted to be forwarded but no host in the path could do it, typically when 8-bit content meets a 7-bit-only hop.
| Registry code | X.6.3 |
|---|---|
| Forms in use | 5.6.3 |
| Registry name | Conversion required but not supported |
| 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 3463] |
What the registry says
The message content must be converted in order to be forwarded but such conversion is not possible or is not practical by a host in the forwarding path. This condition may result when an ESMTP gateway supports 8bit transport but is not able to downgrade the message to 7 bit as required for the next hop.
IANA SMTP Enhanced Status Codes registry, X.6.3
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.6.3 typically involves 8-bit content meeting a server that only accepts 7-bit. Modern mail libraries encode bodies as quoted-printable or base64 by default, so the error usually points to a raw script or an old relay. Encoding the content correctly fixes it.
What the sender should do
Encode the message as 7-bit MIME, using quoted-printable or base64, before sending.
Would verifying the list have prevented 5.6.3?
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 5.6.3 mean?
- X.6.3 means the content had to be converted to be forwarded but no host in the path could do it, typically when 8-bit content meets a 7-bit-only hop.
- What should I do about 5.6.3?
- Encode the message as 7-bit MIME, using quoted-printable or base64, before sending.
- Would an email verifier have caught 5.6.3?
- 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