Message Not Delivered: Message is too long
The message could not be delivered because it was longer than the recipient'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.
5.2.3: Official Definition
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
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.