What it looks like in your logs
552 5.3.4 Message size exceeds fixed maximum message size (message is 38 MB, limit is 25 MB)What does 552 mean?
RFC 5321 defines 552 as a requested mail action aborted because of exceeded storage allocation. In practice it shows up in two forms. With the enhanced code 5.3.4, the message itself is bigger than the receiving server accepts. With 5.2.2, the recipient mailbox is over its quota and has no room for anything new. Your server treats the reply as final for this attempt, so nothing is retried automatically.
Size limits apply to the encoded message, not the files on your disk. Attachments are Base64 encoded in transit, which adds roughly a third to their size, so a 20 MB attachment can arrive as a 27 MB message. Gmail and Microsoft 365 accept around 25 MB for external mail by default, while many corporate servers set 10 MB or less.
Gmail returns 552 5.2.2 when a recipient is over quota and 552 5.3.4 for oversized messages. Microsoft 365 uses 552 5.3.4 for size limits and sometimes 552 with 5.2.2 for full mailboxes. Your server may also advertise its own limit in the SIZE extension of the EHLO response.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 552 | ✓ this code |
Common causes
- Attachments exceed the receiving server's maximum message size.
- Base64 encoding pushed a borderline attachment over the limit.
- Several attachments together, or large embedded images, exceed the limit.
- The recipient organization sets a conservative size cap.
- The recipient mailbox is full and returned 552 5.2.2.
How to fix 552
- Check the enhanced code: 5.3.4 is a message size problem, 5.2.2 is a full mailbox.
- Replace large attachments with links to a file sharing service.
- Compress images and keep marketing emails well under 1 MB, including embedded images.
- For transactional mail, check the SIZE value in the receiver's EHLO response and cap attachments below it.
- For full mailboxes, retry on a later send and suppress after repeated failures; see 5.2.2.
552 questions
What does SMTP error 552 mean?
It means the receiving server aborted delivery because of storage. Either your message is larger than the server permits, or the recipient mailbox has no space left. The enhanced code tells you which: 5.3.4 for message size and 5.2.2 for a full mailbox.
Why does my email bounce when the attachment is under 25 MB?
Because the limit applies to the encoded message. Base64 encoding adds about 33% to attachment size, and headers and body add more. A 20 MB file can easily become a 27 MB message. Many business servers also use lower limits than Gmail or Outlook, sometimes 10 MB.
Can a 552 error mean the recipient mailbox is full?
Yes. When 552 is paired with 5.2.2, the recipient is over quota. That condition may clear if they delete mail, so retry on a later send. If the same address keeps failing for days, suppress it. Our 5.2.2 page covers the thresholds.