SMTP 452 4.2.2: recipient over quota (temporary mailbox full)
A 452 4.2.2 reply means the recipient mailbox is over its quota right now, so the message is deferred and retried rather than bounced. RFC 3463 defines X.2.2 as "Mailbox full": a per-mailbox quota or the storage behind it is exhausted. The 4.x form is the receiver way of saying try again once space frees up.
The enhanced status code 4.2.2 is defined by RFC 3463 as "Mailbox full": the recipient has hit a per-mailbox quota or the physical storage is out of space. With the transient 452 basic code, the receiver is deferring rather than failing, in the hope the recipient clears room.
What is happening
The recipient mailbox is at its quota and has not been pruned.
A shared or archive mailbox filled up and is not being cleared.
Storage on the receiving side is temporarily full.
What to do
Your server keeps the message queued and retries on its own. If it matters, reach the recipient through another channel so they can delete mail and make space. Should the mailbox stay full past the retry window, the message escalates to the permanent 552 5.2.2.
Common questions
Is 452 4.2.2 permanent?→
No. The 4 marks it transient. The message stays queued and retries while the receiver waits for space to free up.
What is the difference from 552 5.2.2?→
Both mean mailbox full. 452 4.2.2 is temporary and retried; 552 5.2.2 is permanent and bounces now.
Can I clear 452 4.2.2 from my side?→
No. The recipient has to free space. You can let them know through another channel, then let retries resume.
What happens if the mailbox stays full?→
Once the retry window expires, the receiver returns the permanent 552 5.2.2 and the message bounces.