5.2.3: message too large for this mailbox

What it means

RFC 3463 separates two size limits. X.3.4 is the limit for the whole system; X.2.3 is a per-mailbox limit that an administrator set lower than that, for example on a shared or restricted mailbox. The RFC says it should be a permanent failure, so 5.2.3 is the form you see and the message is returned, usually with reply code 552. Sending the same message again fails the same way. The size that counts is the encoded message, and base64 encoding makes attachments roughly a third larger, so a file that looks under the limit on disk can be over it in transit.

Common causes

• A large attachment, or several, sent to a mailbox with a low size limit.
• Base64 encoding of attachments, which makes the message larger than the files themselves.
• A per-user or per-group limit set by the recipient's administrator.

How to fix it

1. 01Check the limit if the bounce states it, and compare it with the size of your message.
2. 02Share the file as a link to cloud storage instead of attaching it.
3. 03Compress large attachments, or split them over several messages.
4. 04Recipient admins: raise the mailbox's message size limit if large files are expected.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• RFC 3463, section 3.3: Mailbox Statusrfc-editor.org
• IANA, SMTP Enhanced Status Codes registry (Enumerated Status Codes)iana.org