451 4.4.0 DNS query failed. The error was: DNS query failed with error ErrorRetry Solution

I encountered this issue when initially setting up an Exchange lab. I had attempted to send an email to the local domain on the inner network and the transport server was failing to forward this onto the inner server - returning the following error:

451 4.4.0 DNS query failed. The error was: DNS query failed with error ErrorRetry

I checked my DNS settings and performed a quick nslookup to verify that the correct MX record was in place for the domain it was failing to resolve - although everything looked OK*. I also found out that the transport server can also use it's own set of DNS servers rather than the NIC's default one(s). To verify this I used the following powershell cmdlet:

get-transportserver | FL *DNS*

As we can see above there are no explicit internal DNS servers set and the InternalDNSProtocolOption is set to ANY - so it will pickup the default NIC DNS servers. Finally I found the solution - I was using a public DNS for secondary DNS server - and for some reason the transport server was not using the primary DNS server - in a typical environment a forwarder would be put on place on an internal DNS server instead of explicitly setting one on the NIC but as this was a lab it was omitted! Simply removing the public DNS entry on the NIC did the trick and retry the queue operation. I could of course also explicitly set the appropriate DNS server on the transport server configuration.