X.3.0 · Mail system
SMTP enhanced status code 5.3.0
Other or undefined mail system status
X.3.0 says the destination system normally accepts mail but something about the system itself caused the failure. As 451 4.3.0 it is a common generic temporary error; as 5.3.0 it is a permanent system refusal.
| Registry code | X.3.0 |
|---|---|
| Forms in use | 5.3.0, 2.3.0, 4.3.0 |
| Registry name | Other or undefined mail system status |
| Subject | 3: Mail system |
| Permanent or temporary | 2.x.x success; 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 221, 250, 421, 451, 550, 554 |
| Defined in | [RFC 3463] |
What the registry says
The destination system exists and normally accepts mail, but something about the system has caused the generation of this DSN.
IANA SMTP Enhanced Status Codes registry, X.3.0
The basic codes it travels with
- 221 <domain> Service closing transmission channel
- 250 Requested mail action okay, completed
- 421 <domain> Service not available, closing transmission channel
- 451 Requested action aborted: local error in processing
- 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
- 554 Transaction failed (Or, in the case of a connection-opening response, "No SMTP service here")
In practice
X.3.0 is one of the most common generic codes. Gmail sends 451 4.3.0 when its servers temporarily reject a message, and 421 4.3.0 for a temporary system problem. Many MTAs use 5.3.0 for an unexpected failure in local delivery. A 4.3.0 that clears on retry needs no action. A 4.3.0 that repeats for hours for one domain is worth looking at, since it can hide a policy or configuration problem.
What the sender should do
Let a 4.3.0 retry. For 5.3.0, read the text after the code; it usually names the real reason.
Would verifying the list have prevented 5.3.0?
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.
How providers use X.3.0
Quoted from each provider’s own error reference.
- Gmail 451 4.3.0
Email server has temporarily rejected this message. Go to About SMTP error messages for more information.
- Gmail 421 4.3.0
Temporary System Problem. Try again later. For more information, go to About SMTP error messages.
Common questions
- What does 5.3.0 mean?
- X.3.0 says the destination system normally accepts mail but something about the system itself caused the failure. As 451 4.3.0 it is a common generic temporary error; as 5.3.0 it is a permanent system refusal.
- What is the difference between 5.3.0 and 2.3.0 and 4.3.0?
- They share the subject and detail digits, so they report the same condition, "Other or undefined mail system status". The first digit is the class: 2.x.x success; 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 5.3.0?
- Let a 4.3.0 retry. For 5.3.0, read the text after the code; it usually names the real reason.
- Would an email verifier have caught 5.3.0?
- 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.
Full guides for this code
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)
- Google Workspace Admin Help: Gmail SMTP errors and 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