Email Knowledge Bounce codes

5.2.3 — Message too large

The enhanced status code 5.2.3 typically means the message exceeds a size limit at the destination. The limit may apply to the full message size (including attachments and encoding overhead), not just the raw file size you attached.

What 5.2.3 means

Email systems enforce size limits to protect storage and processing resources. When you attach files, the message is encoded (often base64), which increases the transmitted size. That’s why “a 20 MB file” can become significantly larger in the actual email.

Providers can reject oversized mail during SMTP delivery, or they may accept it and then later generate a bounce depending on the pipeline. In both cases, the diagnostic text usually mentions “message size”, “too large”, “exceeds maximum”, or a specific limit.

What it can look like

Action: failed
Status: 5.2.3
Diagnostic-Code: smtp; 552 5.2.3 Message size exceeds fixed maximum message size

Next steps

If you are the sender

Reduce the message size: remove attachments, compress files, or send a download link instead. If you are sending documents, consider splitting them into separate emails. If you control the sending system, check the configured maximum message size on both the submission side and the outbound delivery side.

If you administer the receiving system

Confirm your maximum message size configuration and ensure the rejection message is clear. Some systems enforce different limits at different points (front-end, content filter, mailbox store). Consistent limits reduce confusion.

Related but different: 5.2.2 is usually “mailbox full”, not “message too large”.

Related codes

5.2.2 (mailbox full), and the full list: /bouncecodes/.