Today I had problem with Edge server.
Edge Transport Server was not able to route incoming emails to Hub Transport.
I found this error in Queue Viewer on Edge Server: 451 4.4.0 DNS query failed.

I checked Host file and it included the Host records for the Hub Transport servers.

I tested connection to Hub Server by ran command ping and everything was ok the name was resolve correctly.

Name resolution was working fine.

I checked configuration of Internal DNS Lookups on the Edge Transport Server properties, proper DNS addresses were configured (one IP of the domain controllers running on the internal network should be place here).

I tested DNS port between Edge Transport Server and DNS Server by ran command:
telnet DNS_server 53
? connection was failed

It turned out that firewall between Edge Transport Server and DNS Server blocked traffic.