Delayed Message: Email address no longer accepts or forwards messages
The email message has not yet been delivered because the recipient's email address no longer accepts email messages (even though it did in the past).
Fixes
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.
4.1.6: 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.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.1.6:
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.