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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.


Prabhat Nigam

Microsoft MVP | Exchange Server




7 Responses to “Exchange 2013: 451 4.0.0 DNS query failed”

  1. selva Says:

    While restarting “Microsoft Exchange Transport Service” is there any way to miss queued emails? please confirm.

  2. Prabhat Nigam Says:

    No you will not miss the emails.

  3. eliseu Says:

    Thank you. I was having issue with mails getting stucked in queue for ~12 hours. Investigation errors and warnings brought me to this post and I was able to fix my issue. Thank you again Prabhat 🙂

  4. Faisal Says:

    I’m just unbelievably grateful for you. You are a SAVIOR !!!

  5. Fabio Says:

    Best post ever.
    Solution so simple.
    Best one

  6. Jag Says:

    Thank you, my issues got fixed after removing External DNS(set as an alternative), you saved lots of time.

  7. Paul Says:

    Wow, all I can say is THANK YOU BROTHER!! You just ended an agonizing 96 hour blitzkrieg of troubleshooting for me and my friend and mentor Rob. This knowledge is gold. Thank you so much for sharing, I am forever grateful.

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