Exchange 2013: 451 4.0.0 DNS query failed
Today I got 2 issues which are interesting so I am sharing the experience.
This issue can occur for various reason. Feel free to look previous blogs with the same error.
On this 2 MBX+CAS DAG setup Internal and External mails got stuck in the queue with 2 errors.
451 4.4.0 DNS query failed. The error was: SMTPSEND.DNS.NonExistentDomain; nonexistent domain
451 4.4.0 DNS query failed. The error was: SMTPSEND.DNS.NonExistentDomain; nonexistent domain while resolving the outbound proxy frontend server fqdn
Event ID HAREDIRECTFAIL with “No Suitable Shadow servers”
After checking different DNS and AD domain controllers test and checks I got the fix.
In my exchange servers, I had given AD DNS servers and external DNS servers which were causing this issue
So I removed the alternate DNS Server from TCPIP IPv4 properties.
Restarted Microsoft Exchange Transport Service on all Exchange 2013 servers and Issue got fixed.
Microsoft MVP | Exchange Server