Delayed Message: Message is too long
The email message has not yet been delivered because it was longer than the recipient's email address's email system allows.
Check that the message contained only the intended text and attachments. (Some systems count attachments as part of the message length.) Try to divide the text and/or attachments into two or more separate emails.
if this error persists even when sending a short message with no attachments, contact the recipient another way (e.g., phone or text message) and suggest that they contact their email provider for help.
4.2.3: Official Definition
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.
A per-mailbox administrative message length limit has been exceeded. This status code should be used when the per-mailbox message length limit is less than the general system limit. This code should be used as a permanent failure.