X.2.3 · Mailbox
SMTP enhanced status code 5.2.3
Message length exceeds administrative limit
X.2.3 means your message is larger than that particular mailbox allows, a limit lower than the server's general size limit. It is permanent for this message, but a smaller message will go through.
| Registry code | X.2.3 |
|---|---|
| Forms in use | 5.2.3 |
| Registry name | Message length exceeds administrative limit |
| Subject | 2: Mailbox |
| Permanent or temporary | 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 552 |
| Defined in | [RFC 3463] |
What the registry says
A per-mailbox administrative message length limit has been exceeded. This status code should be used when the per-mailbox message length limit is less than the general system limit. This code should be used as a permanent failure.
IANA SMTP Enhanced Status Codes registry, X.2.3
The basic codes it travels with
- 552 Requested mail action aborted: exceeded storage allocation
In practice
X.2.3 is a per-mailbox size cap that is lower than the server's general limit, so the same message can reach one recipient on a domain and fail for another. It is most common with large attachments sent to accounts on restricted plans. The address is fine and should stay on the list. Resend a smaller version, or share the file through a link.
What the sender should do
Send the file as a link instead of an attachment, or reduce the size, then resend. The address itself is fine.
Would verifying the list have prevented 5.2.3?
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 5.2.3 mean?
- X.2.3 means your message is larger than that particular mailbox allows, a limit lower than the server's general size limit. It is permanent for this message, but a smaller message will go through.
- What should I do about 5.2.3?
- Send the file as a link instead of an attachment, or reduce the size, then resend. The address itself is fine.
- Would an email verifier have caught 5.2.3?
- 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)
- 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