Exchange Edge 2010: 451 4.4.0 DNS query failed

What causes 451 4.4.0 DNS query failed on Exchange Server 2010?

After installing Microsoft Exchange Server 2010 Edge and subscribing to it all my emails were hanging on the queue and not being delivered to the hub server. I had all my network cards set up with my DNS server so I wash´t expecting any issues but I kept seeing the 451 4.4.0 DNS query failed message.

My first approach to solving this was to understand what DNS query was the one failing. While looking at the different settings I stumbled when I saw the smart host being “- -“. First I thought it was a typo and then I assumed the DNS was failing as it could not resolve that address. It turns out “- -” actually means the subscribed hub address so it automatically provides the correct hub address for you. Still confused as to what 451 4.4.0 DNS query failed meant so the next stop was the DNS resolution settings on the Exchange Edge server. If you go to the properties of your Exchange Edge server you’ll see a tab that reads “Internal DNS”. Here the default setting was to use the network card’s DNS automatically. I assumed that would work as the network card did have my internal DNS configured (which resolves outside addresses as well). It happens that for some reason that was not working.

The solution

Go into the properties of the Edge server and for the Internal DNS instead of using my network cards simply type in the address of the internal DNS and voila! That fixed it. For some reason is using not your internal DNS server (if you happen to have multiple set up) so it cannot resolve the hub transport address correctly. As simple as that.

For more information and troubleshooting steps if adding your internal DNS Server does´t work check out: Troubleshooting Mail Queues That Are Increasing on Edge Transport Servers