I am running Exchange 2007: I have a problem with sending email to a customer with the messages getting stuck in the que: The error message says: 451 4.4.0 Primary Target IP address responded 421 4.2.1 unable to connect. Attempted failover to alternate host, but that did not suceed. Either there are no aternate hosts or delivery failed to the all alternate host.
Does this message indicate a problem on my end or the customer's end? All other mail from my mail server is going out and in with no other problems.
Thanks for the help.