Message Not Delivered: Message content not accessible to recipient server
The message could not be delivered because it included content or attachments that needed to be converted but that was not completed.
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.
5.6.5: 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 conversion was required but was unsuccessful. This may be useful as a permanent or persistent temporary notification.