SMTP error 422
Mailbox full — a misread of enhanced status 4.2.2, not a real code.
Updated Jul 1, 2026
SMTP 422 is not a real reply code. It is a misread of the enhanced status 4.2.2 ("mailbox full"), which servers send inside a 452 reply when the recipient inbox is over quota. It is a temporary soft bounce: retry with backoff, and the message delivers once the recipient frees space.
Treat as a temporary soft bounce: retry with backoff until the recipient frees space.
What it means
RFC 5321 defines no 422 reply code. Its transient codes are 421, 450, 451, 452, and 455. What people call "422" is almost always the enhanced status 4.2.2 ("mailbox full") with the dots dropped. In a real bounce it travels inside a reply line such as "452 4.2.2 The email account that you tried to reach is over quota": the 452 is the reply code and the 4.2.2 is the detail. A few ESP dashboards also use "Error 422" as a friendly name for the same over-quota condition.
Common causes
Per RFC 3463, X.2.2 means the mailbox is full because the user exceeded a quota or physical capacity. The recipient can delete messages to make space.
Logs and tooling drop the dots, turning 4.2.2 into a fictional "422".
How to fix it
- Treat it as a soft bounce
Do not suppress or delete the address. It is valid.
- Retry with exponential backoff over hours and days
Most mail servers queue and retry transient codes automatically. Check your retry policy rather than resending in a tight loop.
- Suppress only after persistence
If the same address returns 4.2.2 for several days, or later returns a 5.x.x permanent bounce, pause sending to it.
- Reach the recipient another way if urgent
Use SMS, push, or a different inbox, and ask them to clear space.
Paired code
A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:
Common questions
Is 422 a permanent failure?
No. There is no real 422 code. The condition behind it, 4.2.2 (mailbox full), is a transient failure that should deliver on retry once the recipient frees space. The permanent counterpart is 5.2.2.
Why do I see "422" instead of "4.2.2"?
Logs and tools drop the dots from the enhanced code, leaving a number that looks like a reply code but is not.
Should I remove an address that returns a 4.2.2 mailbox-full error?
Not right away. It is a valid address that is temporarily full. Suppress it only after repeated failures over several days.