exchange 2010 451 4.4.0 dns query failed

Been trying to resolve an issue for several days, that is currently only affecting a single external domain. Users reported the problem when email was failing to reach this external recipient. We are a single exchange 2010 environment. Looking in the exchange mail queue i can see the messages queuing up and reported the message 451 4.4.0 dns query failed. Again, this is for only one external domain currently. It screams DNS, but i have confirmed with ISP (AT&T) that our reverse dns is configured correctly. From the mail server, when i do an nslookup on this external domain, it fails.
mray77Asked:
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mray77Author Commented:
As info...DNS on mail server is configured to perform an internal dns query, not external.
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Roshan_cCommented:
Hi,

Did you tried to mail that external domain user using any other mail services such as yahoo or google etc. as your employees are getting this error it has nothing to do with dns entry for your domain. check if any policies are set for sending mail. check restricted domain list is clear.

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mray77Author Commented:
Actually, nslookup works on our old DNS stuff server but not on our new DNS server; which is where our mail server points. Looks like an internal DNS issue.
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mray77Author Commented:
To answer the question, yes we can email this external domain from other accounts like gmail, etc...
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mray77Author Commented:
No forwarders were configured in DNS.
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Roshan_cCommented:
hi,

I am bit confused with above DNS thing. so you have two internal DNS servers. from above i can understand that old dns server is properly resolving the hostname and new DNS is not and your Exchange is pointing through the new DNS server.

can you send mail using old dns configured on exchange server

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Roshan_cCommented:
Hey,

good that you found there is problem in internal DNS, but when you questioned there was no such mention of things, as you could access external sites then exchange should also be able to talk using the same DNS server still ok if you are happy to find solution for any other problem
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mray77Author Commented:
No forwarders were configured in DNS.
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itsbmCommented:
queue full
Hello, i have this issue. My mail server is MS Exchange 2010. How to protect my mail server from yiyr.net, qiqi.net, lolx.net, jrfn.net, arzn.net.
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