[Status: Error, Address:<...>, ResponseCode 421, Host not reachable.]
(Email address deleted.  The original message had the actual email address.)

Is there any way to determine the current status of the email? 
Does zoho send me another message saying the email was finally delivered? 
Does Zoho send another error message if the 4 days of retry fail?

Also, how often does Zoho try to resend the original email?