Message Not Delivered: Unknown issue
The email message could not be delivered because of an unknown issue.
Check the recipient's email for misspellings or missing letters. If you find an error, correct it in your contacts list or address book for next time.
If necessary, contact your recipient another way (e.g., phone or text message) to confirm their email address.
5.0.0: 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.
Other undefined status is the only undefined error code. It should be used for all errors for which only the class of the error is known.