Message Not Delivered: Domain name of email address not valid
The message could not be delivered because the recipient's email provider says that the recipient's email address does not exist or cannot receive email.
Check the recipient's domain name in 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.
It is possible that the domain is temporarily inactive. If the spelling looks correct, contact provider and if necessary, contact the recipient another way (e.g., phone or text message).
5.1.2: 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 destination system specified in the address does not exist or is incapable of accepting mail. For Internet mail names, this means the address portion to the right of the "@" is invalid for mail. This code is only useful for permanent failures.