Generic SMTP delivery response
Understand SMTP 5.2.3: Message too large for the mailbox
The message is larger than the administrative size limit applied to this mailbox.
A representative response is:
552 5.2.3 Message too large for the mailbox.
Retrying the same message will fail. Reduce its encoded size or obtain a higher limit before sending again.
What this response means
Standard 5.2.3 Message length exceeds administrative limit means this mailbox allows a smaller message than the one submitted. MIME and base64 encoding make the transmitted message larger than the source files.
Fix it
- Check the complete message size reported by the sender, not only the attachment size.
- Remove unnecessary inline images, quoted history, and attachments.
- Compress files only when the recipient allows the resulting format, or send an access-controlled file-sharing link.
- If you administer the mailbox, compare mailbox-specific and organization-wide limits before changing policy.
- Generate a smaller new message and test it; do not repeatedly resend the original.
Splitting one large attachment across arbitrary messages can confuse recipients and may violate security policy.
Who can fix it
The sender can reduce the message. The receiving administrator can change the mailbox limit if policy and capacity allow.
Related troubleshooting
Use 5.3.4 guidance when the entire receiving system, rather than one mailbox, reports the size limit.
Frequently asked questions
What does SMTP 5.2.3: Message too large for the mailbox mean?
The complete encoded message is larger than the receiving mailbox allows, even if the original attachment appears smaller.
Can MailFixIt prove this entire bounce from public DNS?
No. The current checker can evaluate selected public DNS configuration, but it cannot recreate recipient state, credentials, reputation, message content, or a historical provider SMTP decision.
Related email diagnostics
Primary sources: IANA SMTP Enhanced Status Codes.