552 5.3.4 Message size exceeds fixed maximum message size
Sample bounce line
552 5.3.4 Message size exceeds fixed maximum message size
What it means
The message is larger than the receiving system's per-message size limit. RFC 1870 (the SIZE extension) specifies the reply 'message size exceeds fixed maximum message size' with code 552, and RFC 3463 defines X.3.4 as Message too big for system, useful only as a permanent error.
Why it happens
- Attachments push the message past the server's maximum size; in RFC 1870 the server announces its limit in the EHLO SIZE keyword and compares it with the SIZE declared on MAIL FROM
- Postfix: message_size_limit (default 10240000 bytes) is the maximal size of a message including envelope information
- Dovecot submission: submission_max_mail_size (default 40M) is announced as the SMTP SIZE capability
How to fix it
- Sender: shrink or compress the attachments, or share a download link instead
- Admin: raise message_size_limit (Postfix) or submission_max_mail_size (Dovecot submission) consistently across every hop
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