552 5.3.4: message too big for system
Your message is larger than the receiving system's per-message limit. Attachments are the usual cause, and resending the same message will fail again.
552 5.3.4 Your message exceeded Google's message size limits.
Start of Gmail's text, as listed in Google's SMTP errors reference.
What 552 5.3.4 means
RFC 3463 defines X.3.4 as "Message too big for system": the message is larger than the per-message size limit, which may be physical or administrative. It is useful only as a permanent error.
Gmail uses 552 5.3.4 for several limits: overall message size, the number of attachments, total header size, and the size of a single header such as Subject. The text says which limit you hit.
Size limits apply to the encoded message, not the files on your disk. Email usually carries attachments in Base64 (RFC 2045), which turns every 3 bytes into 4 characters, so a message grows by about a third on the way. Each server along the path can also have its own limit, including the one that sends on your behalf, so the smallest limit wins. A server can also advertise its limit up front with the SIZE extension; the bounce may come from your own server if it saw that limit.
Likely causes
- Attachments push the encoded message over the receiver's limit.
- Too many attachments for the receiver.
- Very large headers, often from automated systems or long recipient lists.
How to fix it
If you sent the message
- Share large files through a link to cloud storage instead of attaching them.
- Compress or split attachments into several messages.
- For automated mail, check what is inflating the headers.
If you're the recipient or their admin
- If partners regularly need to send large files, agree on a file-sharing service or raise your own limit where your provider allows.
Frequently asked questions
My attachment is under the limit. Why did it bounce?
Encoding makes it larger: Base64 adds about a third. Other parts of the message and every server on the path count too, and the smallest limit along the way applies.
How do I send a file that is too large for email?
Upload it to a file-sharing or cloud storage service and email a link. That also avoids filling the recipient's mailbox.
Sources
- RFC 3463: Enhanced Mail System Status Codes
- Gmail SMTP errors and codes (Google Workspace Admin Help)
- RFC 2045: MIME part one, section 6.8 (Base64 encoding)
Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.
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