Delayed Message: Email servers are not responding appropriately
The email message has not yet been 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.
Fixes
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.
4.6.8: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 4.X.X:
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
[RFC6531] (Standards track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.6.8:
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.