SMTP error 522
Mailbox full — a non-standard label for 552 / 5.2.2.
Updated Jul 1, 2026
SMTP 522 is not a code defined in RFC 5321. It is a label some servers and tools use for a recipient mailbox that is full or over quota. The real reply code is 552 ("exceeded storage allocation"), with enhanced status 5.2.2. The recipient must free space or raise their quota; as the sender, stop retrying and verify the address.
The recipient must free space or raise their quota; stop retrying and verify the address.
What it means
There is no 522 reply code. RFC 5321 lists the permanent codes 500-504 and 550-555 (with 521 and 556 added later, and 530 from the auth extensions), none of which mean "mailbox full". Where you see 522 it is almost always a vendor label for a full or over-quota mailbox, which the standard expresses as reply code 552 with enhanced status 5.2.2. Read the full bounce string: if it contains 552 or 5.2.2 with text like "exceeded storage allocation", "mailbox full", or "over quota", treat it as the quota problem.
Common causes
The most common cause. The receiving server refuses new mail until space is freed. Per RFC 3463, 5.2.2 means the user exceeded a per-mailbox quota or physical capacity.
A big attachment pushes an almost-full mailbox past the limit. Note a per-message size limit is a different condition (5.2.3), so check the text.
Storage exhaustion on the receiving side can surface as the same class of error.
How to fix it
- If you are the recipient, free up space
Delete large or old messages, empty Trash and Spam, and clear the Sent folder. Sent mail often counts toward the quota.
- Increase the mailbox quota
Ask your provider or admin for more storage if you legitimately need it.
- If you are the sender, do not hammer-retry
A 552/5.2.2 is final for that attempt. Immediate resends will not help and excessive retries can hurt your reputation. Let your normal retry schedule run or wait until the recipient clears space.
- Verify the recipient address is active
A chronically full mailbox can be an abandoned account. Consider suppressing it to protect your bounce rate.
- Reduce message weight or use another channel
Link to large files instead of attaching them, or reach an important recipient another way while their mailbox is full.
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 522 a real SMTP reply code?
No. RFC 5321 does not define it. It is a non-standard label for a mailbox-full bounce, which the standard expresses as 552 with enhanced status 5.2.2.
What is the difference between 522, 552, and 5.2.2?
552 is the real reply code, 5.2.2 is its enhanced status ("mailbox full"), and 522 is a non-standard label some tools use for the same condition.
Will resending the email fix a 522 error?
Not until the recipient frees space or raises their quota. It is their side to resolve. Do not retry aggressively.