Delayed Message: Message is too large
The email message has not yet been delivered because the recipient's email provider has a message size limit and the message is larger than what is allowed.
Fixes
Check the body of the message for large amounts of text or for any large attachments. Try adjusting the text or reducing the size of attachments and resending.
Otherwise, contact the sender's email support team to see if they can help troubleshoot further.
4.3.4: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 4.X.X:
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.3.4:
The message is larger than per-message size limit. This limit may either be for physical or administrative reasons. This is useful only as a permanent error.