Enhanced SMTP status code · RFC 3463 · IANA
4.3.1 Mail system full
The server's storage is exhausted; the recipient cannot make room themselves.
- Class: Temporary failure
Class 4: a persistent transient failure. The message was fine, but a passing condition delayed or stopped delivery; a later attempt may succeed (RFC 3463 §3.1). The sending server retries on its own for a while. The registry says this code is only useful as a temporary failure.
- Subject: X.3 · Mail system
Something about the destination server; under its administrator's control.
- Associated basic codes (IANA registry)
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026