Error :- 

451 4.4.0 DNS query failed

Cause :- 

Local DNS is unable to resolve the name

workaround :- 

–> Change the DNS from “local to 8.8.8.8

–> restarted the transport service and email flow is working fine

Solution works in my case :- 

–> Restarted the domain controller and exchange

–> revert back from 8.8.8.8 to local

–> check email flow and working fine