X.3.1 · Mail system
SMTP enhanced status code 4.3.1
Mail system full
X.3.1 means the receiving mail system as a whole is out of storage, not just one mailbox. It is meant as a temporary error.
| Registry code | X.3.1 |
|---|---|
| Forms in use | 4.3.1 |
| Registry name | Mail system full |
| Subject | 3: Mail system |
| Permanent or temporary | 4.x.x persistent transient failure: retrying later may succeed |
| Associated basic codes | 452 |
| Defined in | [RFC 3463] |
What the registry says
Mail system storage has been exceeded. The general semantics imply that the individual recipient may not be able to delete material to make room for additional messages. This is useful only as a persistent transient error.
IANA SMTP Enhanced Status Codes registry, X.3.1
The basic codes it travels with
- 452 Requested action not taken: insufficient system storage
In practice
X.3.1 says the whole destination system is out of storage, which is different from one user's mailbox being full. It is rare on large providers and more often seen on small self-hosted servers with a full disk. Your server should keep retrying for its normal queue lifetime. If it persists for days, the domain's administrator needs to know, and the message will eventually expire with 4.4.7.
What the sender should do
Let your server retry. Nothing about the recipient address or your sending is implied.
Would verifying the list have prevented 4.3.1?
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 4.3.1 mean?
- X.3.1 means the receiving mail system as a whole is out of storage, not just one mailbox. It is meant as a temporary error.
- What should I do about 4.3.1?
- Let your server retry. Nothing about the recipient address or your sending is implied.
- Would an email verifier have caught 4.3.1?
- 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