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Holguer

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Exchange 2010 error 451 4.4.0 DNS Query Failed

Hello I am having problems sending to a couple domains. We have recently upgraded to an exchange 2010 server and everything was working fine. I was able to send email to example1.com with out any issues and all of a sudden we can not. The messages get stuck in the queue with error 451 4.4.0 DNS Query Failed. Can some help shed some light on this error?

Thanks
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Dustin99352

Is the server using two NICs or something?  It's possible you could be querying a different DNS server then you suspect.  Make sure that your exchange environment is querying your Domain Controller and not some external DNS or something.

Thanks
try your basic nslookup test
nslookup
set type=mx
example2.com

verify that dns is locating the mx record for the domain

do your windows dns servers use your isp dns servers as forwarders?
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It only has one nic card set with the internal dns
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ASKER

I just reran the nslookup to example2.com see the proper mx records. DNS appears to be working
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Holguer

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Try to create exclusive Send Connectors for the domains to which server is failing to
send mail to and add the MX Records’ IP Addresse as the Smart Host for respective
domains on the Send Connectors and check if you are able to send mails.
MILIKAD

To support the earlier comment check the below link
http://support.microsoft.com/kb/972744 
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Thank you,
@Holguer

I added my ISP's external DNS servers and it work. I looked in the Queue viewer and I dont see any emails.

Thank you,

E