Delayed Message: Sending address is bad or can't accept replies
The email message has not yet been delivered because the sending email system either does not exist or cannot accept return email.
Fixes
Review the sender's email address to identify any misspellings or missing letters in the domain part of the email address. In the settings or options of the sender's email program, ensure that the sender's email address is entered correctly.
If the problem persists, contact the sender's email support team to see if they can help troubleshoot further.
4.1.8: 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.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.1.8:
The sender's system specified in the address does not exist or is incapable of accepting return mail. For domain names, this means the address portion to the right of the "@" is invalid for mail.