X.3.3 · Mail system
SMTP enhanced status code 5.3.3
System not capable of selected features
X.3.3 means the message asked for a feature the destination system does not support, often in gateways between different mail systems.
| Registry code | X.3.3 |
|---|---|
| Forms in use | 5.3.3, 4.3.3 |
| Registry name | System not capable of selected features |
| Subject | 3: Mail system |
| 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
Selected features specified for the message are not supported by the destination system. This can occur in gateways when features from one domain cannot be mapped onto the supported feature in another.
IANA SMTP Enhanced Status Codes registry, X.3.3
In practice
X.3.3 appears when a message asks for an extension or feature, such as delivery status notifications, 8-bit MIME or a priority, that a gateway further along cannot honour. It is mostly seen with older gateways and specialised systems. Sending the message again without the feature usually works. It has nothing to do with the recipient address. If you control the sending software, look for options such as DSN requests, delivery-by deadlines or priority headers, and turn them off for that destination. Most ordinary mail does not need them.
What the sender should do
Send the message without the extension or feature that was requested, or ask the recipient's administrator which features their gateway supports.
Would verifying the list have prevented 5.3.3?
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 5.3.3 mean?
- X.3.3 means the message asked for a feature the destination system does not support, often in gateways between different mail systems.
- What is the difference between 5.3.3 and 4.3.3?
- They share the subject and detail digits, so they report the same condition, "System not capable of selected features". 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 5.3.3?
- Send the message without the extension or feature that was requested, or ask the recipient's administrator which features their gateway supports.
- Would an email verifier have caught 5.3.3?
- 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)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05