Permanent (5xx)
552 5.3.4
Message too large
The message (including attachments) exceeds the receiving server's maximum accepted size.
552 5.3.4 Message size exceeds fixed maximum message size
What it means in practice
The message is bigger than the receiving server accepts. Limits vary by provider and by the recipient organisation's own settings, and a message's size on the wire is roughly a third larger than its attachments, because MIME base64 encoding adds overhead.
Transactional email rarely needs attachments at all. A signed link to the invoice or file keeps the message small, works on every provider, and lets you revoke access later. If you must attach, keep the whole message to a few megabytes.
Common causes
- Large attachments.
- Inline images or embedded content pushing total message size over the limit.
What to do
- →Keep transactional email attachments small; link to hosted files instead of attaching large ones directly.