We have a simple Exchange 2010 single server setup. Everything has been running fine for years. We have one domain that we are attempting to send an email to that fails. All other emails are flowing fine. The error in the queue is:
451 4.4.0 Primary target IP address responded with: '421 4.2.1 Unable to connect' Attempred failover to alternate host, but did not succeed. Either there are no alternate host, or delivery failed to alternate hosts.
The last successful message to this domain was about 8 weeks ago. I can do an nslookup on the MX records and see them there. I have done an mxtoolbox.com backlist check on my domain and that is coming back fine.