X.0.0 · Other or undefined
SMTP enhanced status code 5.0.0
Other undefined Status
X.0.0 is the catch-all enhanced status code: the server is only telling you the class of the failure, 4 for temporary or 5 for permanent, and nothing about the cause. The reason, if any, is in the free text after the code.
| Registry code | X.0.0 |
|---|---|
| Forms in use | 5.0.0, 4.0.0 |
| Registry name | Other undefined Status |
| Subject | 0: Other or undefined |
| 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 | Any |
| Defined in | [RFC 3463] |
What the registry says
Other undefined status is the only undefined error code. It should be used for all errors for which only the class of the error is known.
IANA SMTP Enhanced Status Codes registry, X.0.0
In practice
In logs, 5.0.0 often appears when the receiving software has no better mapping for its internal error, so the same code can mean a rejected recipient on one server and a content block on another. Because the code carries no diagnosis, two bounces with 5.0.0 are not necessarily the same problem. Group them by the text that follows, and by the receiving domain, before deciding whether the address or your sending is at fault.
What the sender should do
Read the text that follows the code, since that is the only diagnosis you get. Treat 5.0.0 as a hard failure for that send, and if it repeats for one address, suppress the address until you know why.
Would verifying the list have prevented 5.0.0?
Partly
The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
Common questions
- What does 5.0.0 mean?
- X.0.0 is the catch-all enhanced status code: the server is only telling you the class of the failure, 4 for temporary or 5 for permanent, and nothing about the cause. The reason, if any, is in the free text after the code.
- What is the difference between 5.0.0 and 4.0.0?
- They share the subject and detail digits, so they report the same condition, "Other undefined Status". 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 5.0.0?
- Read the text that follows the code, since that is the only diagnosis you get. Treat 5.0.0 as a hard failure for that send, and if it repeats for one address, suppress the address until you know why.
- Would an email verifier have caught 5.0.0?
- Partly. The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
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