X.2.2 · Mailbox
SMTP enhanced status code 4.2.2
Mailbox full
X.2.2 means the recipient's mailbox is full. RFC 3463 says it should be used as a temporary failure, since the owner can delete mail, but many servers send it with a 552 permanent reply.
| Registry code | X.2.2 |
|---|---|
| Forms in use | 4.2.2, 5.2.2 |
| Registry name | Mailbox full |
| Subject | 2: Mailbox |
| 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 | 552 |
| Defined in | [RFC 3463] |
What the registry says
The mailbox is full because the user has exceeded a per-mailbox administrative quota or physical capacity. The general semantics implies that the recipient can delete messages to make more space available. This code should be used as a persistent transient failure.
IANA SMTP Enhanced Status Codes registry, X.2.2
The basic codes it travels with
- 552 Requested mail action aborted: exceeded storage allocation
In practice
A full mailbox is common among abandoned free-mail accounts, which fill up with years of unread mail. Gmail sends 452 4.2.2 while the account is out of storage and 552 5.2.2 when it is out of storage and inactive. Most sending services count repeated full-mailbox bounces as soft bounces and suppress the address after a set number. Microsoft uses 5.2.2 for something else, a sender over its submission quota.
What the sender should do
Retry later. If the same address is still full after several sends over a few weeks, the mailbox is probably abandoned and should be suppressed.
Would verifying the list have prevented 4.2.2?
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.
How providers use X.2.2
Quoted from each provider’s own error reference.
- Gmail 452 4.2.2
The recipient's inbox is out of storage space. Please direct the recipient to Manage files in your Google Drive storage.
- Gmail 552 5.2.2
The recipient's inbox is out of storage space and inactive. Please direct the recipient to Manage files in your Google Drive storage.
- Microsoft 365 5.2.2
Submission quota exceeded
Common questions
- What does 4.2.2 mean?
- X.2.2 means the recipient's mailbox is full. RFC 3463 says it should be used as a temporary failure, since the owner can delete mail, but many servers send it with a 552 permanent reply.
- What is the difference between 4.2.2 and 5.2.2?
- They share the subject and detail digits, so they report the same condition, "Mailbox full". 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 4.2.2?
- Retry later. If the same address is still full after several sends over a few weeks, the mailbox is probably abandoned and should be suppressed.
- Would an email verifier have caught 4.2.2?
- 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.
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)
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (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