Message Not Delivered: Email address no longer accepts or forwards messages
The message could not be delivered because the recipient's email address no longer accepts email messages (even though it did in the past).
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.
5.1.6: 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 address provided was at one time valid, but mail is no longer being accepted for that address. This code is only useful for permanent failures.