Exchange Error 451 4.4.0 primary target IP address responded with…
KB ID 0000252 Problem You see this error message on your Exchange outbound queues. 451 4.4.0 primary target IP address responded with “421.4.4.2 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. Solution Essentially this is a communication error, you can’t connect to the other end on TCP port 25 (SMTP)....