Generic SMTP message-size error
SMTP 5.3.4: message too large
5.3.4 means the message exceeds a size limit on the receiving system or delivery path. Reduce the final encoded message or share large files by an approved link.
A server can return:
552 5.3.4 Message size exceeds fixed maximum message size
The 5.x.x result is permanent for this message. Retrying unchanged content will not help.
Why the limit is exceeded
- Attachments are too large after MIME/base64 encoding.
- The sender limit is higher than a receiving or gateway limit.
- Embedded images, long threads, or headers add unexpected size.
- A transport rule imposes a lower organization limit.
How to resolve it
- Read the bounce for the reported size and limit.
- Measure the final MIME message, not only source files.
- Remove or reduce attachments and quoted history.
- Use an organization-approved file-sharing link where appropriate.
- Generate and send a new, smaller message.
Frequently asked questions
What does SMTP 5.3.4 mean?
The message is too large for the receiving system or a server on the route.
Why is the email larger than its attachments?
MIME and base64 encoding add overhead, while headers and body content also count toward transmitted size.
Will compressing the attachment always help?
Only if the final encoded message falls below every limit and the attachment type remains permitted.
Related email diagnostics
Primary sources: IANA SMTP Enhanced Status Codes registry and RFC 3463.