SMTP 552 5.2.2: mailbox full (recipient over quota)
A 552 5.2.2 reply means the recipient mailbox is over its storage quota and cannot accept more mail. RFC 3463 defines X.2.2 as "Mailbox full": the account has hit a per-mailbox quota or the disk behind it is out of space. Sent as a 5.x code, the receiver is treating this as a hard failure rather than asking you to retry.
The enhanced status code 5.2.2 is defined by RFC 3463 as "Mailbox full": the recipient has exceeded a per-mailbox administrative quota or the physical storage is full. The address is valid, so this is not a user-unknown case.
Why it happens
The recipient never clears the mailbox and has hit the account quota.
A shared or role mailbox filled up and nobody is pruning it.
The receiving server enforces a hard quota and returns 5.2.2 instead of deferring.
What you can do
There is little the sending side can fix, since the space problem belongs to the recipient. Reach the person through another channel so they can free space, then resend. Many servers return the transient form 4.2.2 instead, which tells you to retry later; a 5.2.2 is the receiver deciding to fail immediately.
Common questions
Does 552 5.2.2 mean the address is wrong?→
No. The mailbox exists but is over quota. A wrong address returns 5.1.1 instead.
Can I fix 552 5.2.2 from my side?→
Not directly. The recipient has to free space. Tell them through another channel, then resend.
What is the difference between 5.2.2 and 4.2.2?→
Both mean mailbox full. 5.2.2 is permanent and fails now; 4.2.2 is transient and your server retries later.
Will my server retry a 552 5.2.2?→
No. The leading 5 marks it permanent, so the message bounces instead of being queued for retry.