SMTP 552 5.3.4: message too big for system (over size limit)
A 552 5.3.4 reply means your message is larger than the size limit the receiving system will accept. RFC 3463 defines X.3.4 as "Message too big for system": the total message exceeds a per-message limit set for physical or administrative reasons. The usual culprit is an attachment, made larger still by MIME encoding.
The enhanced status code 5.3.4 is defined by RFC 3463 as "Message too big for system": the message is larger than the per-message size limit on the receiving side, whether that limit comes from storage or from policy.
Why the size is bigger than you think
Attachments are encoded as base64 for transport, which inflates them by roughly a third. A 20 MB file leaves your outbox closer to 27 MB on the wire, so a message that looks under the limit locally can still trip the far-side cap.
The limit belongs to the receiver
Each server sets its own maximum, and many advertise it in the SMTP SIZE extension during the connection. Your own outbound limit does not matter here; the message crossed the limit on the far side.
Common questions
Whose size limit does 552 5.3.4 refer to?→
The receiving side. Each server sets its own per-message cap, and many advertise it through the SMTP SIZE extension.
Why does my message exceed the limit when the file looked smaller?→
Attachments are base64 encoded for transport, which adds about a third. A 20 MB file becomes roughly 27 MB on the wire.
Is 552 5.3.4 permanent?→
Yes. The leading 5 marks it permanent, so the same oversized message keeps failing until you shrink it.
How do I send a large file after a 5.3.4?→
Upload it to file storage and send a link, or compress it below the limit on the receiving side before resending.