Recently, I'm receiving some bounce email from my recipient saying "
Delivery has failed to these recipients or groups:
email@address of my recipient
The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk.
this only happens in some of my recipient. So We check our exchange server if what's happening there, by checking, there was an error "451.4.4.0 Primary target IP address responded with "421.4.2.1. Unable to connect." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts or delivery failed to all alternate hosts."
What does it mean. What shall I do. please help. Thanks.