Message Not Delivered: Unknown issue with the message content
The message could not be delivered for unknown reasons related to its content or attachments.
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.
5.6.0: 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.
Something about the content of a message caused it to be considered undeliverable and the problem cannot be well expressed with any of the other provided detail codes.