X.6.4 · Message content or media
SMTP enhanced status code 2.6.4
Conversion with loss performed
X.6.4 is a warning: the message was delivered, but a conversion lost some data on the way. It becomes a failure only if the sender asked that lossy conversion be refused.
| Registry code | X.6.4 |
|---|---|
| Forms in use | 2.6.4 |
| Registry name | Conversion with loss performed |
| Subject | 6: Message content or media |
| Permanent or temporary | 2.x.x success |
| Associated basic codes | 250 |
| Defined in | [RFC 3463] |
What the registry says
This is a warning sent to the sender when message delivery was successfully but when the delivery required a conversion in which some data was lost. This may also be a permanent error if the sender has indicated that conversion with loss is prohibited for the message.
IANA SMTP Enhanced Status Codes registry, X.6.4
The basic codes it travels with
- 250 Requested mail action okay, completed
In practice
X.6.4 is a warning, not a bounce. You might see it in a delivery report as 2.6.4 when a gateway had to convert the message and lost formatting or attachments. If the content matters, check with the recipient and avoid formats that need conversion. Lossy conversion is most often seen with older gateways that strip HTML or attachments for plain-text systems.
What the sender should do
Check with the recipient that the content arrived intact, and avoid formats that need conversion.
Would verifying the list have prevented 2.6.4?
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 2.6.4 mean?
- X.6.4 is a warning: the message was delivered, but a conversion lost some data on the way. It becomes a failure only if the sender asked that lossy conversion be refused.
- What should I do about 2.6.4?
- Check with the recipient that the content arrived intact, and avoid formats that need conversion.
- Would an email verifier have caught 2.6.4?
- 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