Been trying to resolve an issue for several days, that is currently only affecting a single external domain. Users reported the problem when email was failing to reach this external recipient. We are a single exchange 2010 environment. Looking in the exchange mail queue i can see the messages queuing up and reported the message 451 4.4.0 dns query failed. Again, this is for only one external domain currently. It screams DNS, but i have confirmed with ISP (AT&T) that our reverse dns is configured correctly. From the mail server, when i do an nslookup on this external domain, it fails.