Message Not Delivered: Email servers are not responding appropriately
The message could not be delivered because special characters (non ASCII) in the email address are not permitted by one of either the sender's email program or the recipient's email program.
Check and the email address(es) of the recipient(s) for unusual characters. If an error is found, correct it in contacts list or address book for next time.
Otherwise, contact the email provider for help.
5.6.8: 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.
This indicates that a reply containing a UTF-8 string is required to show the mailbox name, but that form of response is not permitted by the SMTP client.