I'm having an issue with one domain. Mail is not being delivered to them from our server. Every test message sits in teh Queue with the "451 Primary target ip address responded with: 421 4.2.1 Unable to connect. Attempted failover to alternate host., but that did not suceed. Either there are no alternate hosts, or delivery failed to all alternate hosts."
I've tested the domain and done everything I can think of to get it to work. It's jmfinancial.net. I've seen a delay on the SMTP test, but nothing concrete. It's driving me nuts and of course it's a friend of our CEO. He's blaming our system because he can "get email fine from other people". I'm almost positive it's on his end, but I have no proof and I'm out of ideas. We can send and receive email fine to all other domains. We're not blacklisted and all of my Domain/MX checks return everything ok on our end. Any help would be greatly appreciated! We're running exchange 2010.