I am not an exchange guru but I know enough to be dangerous.
I havs an SBS2011 with Exchange 2010 for my business (5 users). Mail is working for the most part however there are a few domains that we can not send mail to. The messages sit in the queue until they finaly fail. The error we are getting is:
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.
95% of our messages get delivered just fine, with the exception of a few domains where we get the above error. We are not on any SPAM blacklists. I don't know where to begin.