Message Not Delivered: Message content not accepted by recipient server
The message could not be delivered because it included content or attachments that needed to be converted and the conversion was not allowed.
Fixes
If the message had attachments, try sending it again without them and then try sending each attachment in a separate email. If this error occurs when there are no attachments, check the text of the message for unusual content or characters.
If this error persists, contact the email provider for help.
5.6.2: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
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.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.6.2:
The content of the message must be converted before it can be delivered and such conversion is not permitted. Such prohibitions may be the expression of the sender in the message itself or the policy of the sending host.