SMTP error 552 means the destination has refused the message because it exceeded a storage or size limit. Two distinct cases hide behind it: the recipient’s mailbox is full, or the message itself is too big for what the receiving server accepts. The fix is different in each case.
The two real situations
| Variant | Cause | Action |
|---|---|---|
| 5.2.2 mailbox full | Recipient is out of storage | Wait, retry once or twice. If still failing, treat as soft bounce and remove eventually. |
| 5.3.4 / message too big | Total message (HTML + images + attachments) exceeds the limit | Reduce total weight: compress images, host attachments externally. |
| Quota exceeded on your side | Your sending plan reached its monthly limit | Check your Mailpro plan and credits. |
Keep messages light by design
A well-designed campaign rarely exceeds 100 KB. Compress every image, avoid embedding large attachments, and use a minimalist design. Heavy emails not only trigger 552 — they also slow rendering and hurt engagement.
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Beyond the 552 error
Automatic bounce handling, no mailbox surprises
Mailpro auto-detects bounces like 552, suppresses bad addresses from future sends and keeps your list clean — so you don't waste sends on full mailboxes or harm your sender reputation.