Message Not Delivered: Sending address is bad or can't accept replies
The message could not be delivered because the sender's email system either does not exist or cannot accept return email.
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.
5.1.8: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
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.
[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.
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