Message Not Delivered: Unknown issue with mailbox
The message could not be delivered due to a problem of some sort with the recipient's email inbox.
Check the recipient's email address for misspellings or missing letters. If an error is found, correct it in contacts list or address book for next time.
If necessary, contact the recipient another way (e.g., phone or text message) to confirm their email address.
5.2.0: 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.
The mailbox exists, but something about the destination mailbox has caused the sending of this DSN.