My file is 20 MB and the limit is 25 MB. Why did it fail?

Base64 encoding grows the attachment by about a third, so a 20 MB file becomes roughly 27 MB on the wire.

How do I tell 5.2.3 from 5.3.4?

5.2.3 points at a mailbox or policy limit set by an admin. 5.3.4 points at the server’s overall maximum message size.

5.2.3 bounce: message exceeds administrative limit

5.2.3Message length exceeds administrative limitPermanent: retrying will not help

Typical wording

552 5.2.3 Message length exceeds administrative limit

What it means

X.2.3 is "Message length exceeds administrative limit" in RFC 3463. An admin set a cap on message size for this mailbox, group, or connector, and your message is over it. Because the subject is 2 (mailbox), the limit belongs to a specific recipient or policy rather than to the whole server.

Microsoft Exchange and Exchange Online often use 5.2.3 for per-recipient or per-sender size limits, and some gateways use it for size policies on distribution lists. That is the practical difference from 5.3.4, which says the message is too big for the mail system as a whole. Remember that size is measured after encoding: base64 makes attachments roughly 33 percent larger than the file on disk.

Because the message will not shrink by itself, the code is normally permanent. A 4.2.3 is uncommon and only makes sense if the limit is about to change.

Likely causes

An attachment that is under the limit on disk but over it once base64 encoded.
A recipient mailbox or group with a lower size limit than the organisation default.
A send connector or transport rule on your own side that caps outgoing message size.
Several attachments added to a reply chain that already carried earlier files.

How to fix it

Check the size of the message as sent, including all attachments, and allow about a third more for encoding.
Share large files through a link to cloud storage instead of attaching them.
If you must attach, compress files or split them across several messages, each well under the limit.
If the limit is in your own organisation, ask your admin to review the size limits on your mailbox, send connector, and transport rules.
If you are the recipient’s admin, decide whether the mailbox or group limit is still appropriate and raise it if needed.

Checks that apply

Your domain or sending IP (fills in the checks below)

SMTP Diagnostics Shows the SIZE value the recipient server advertises in its EHLO reply, the upper bound the whole system accepts.
Email Header Analyzer Reveals which hop in the path rejected the message, so you know whether the limit is yours or theirs.

FAQ

My file is 20 MB and the limit is 25 MB. Why did it fail?: Base64 encoding grows the attachment by about a third, so a 20 MB file becomes roughly 27 MB on the wire.
How do I tell 5.2.3 from 5.3.4?: 5.2.3 points at a mailbox or policy limit set by an admin. 5.3.4 points at the server’s overall maximum message size.

All 191 bounce codes