Exchange 2010 some mail not sending

We have Exchange 2010 SP1. We had an issue a while back where some email domains were stuck in the Queue. I don't recall the reason, but I remember at that time reading somewhere to set-SendConnector ignorestartTTLS to True which resolved it. Just a couple of days ago started having domain stuck. The error is 451 4.4.0 DNS query failed.

The one thing I noticed is that on any of these domains if I run nslookup -q=mx domain name it fails. But the client says they are receiving mail from others and my other mail is working.

Any one have any ideas what could be wrong. Please help.

Thanks
swenger7Asked:
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Simon Butler (Sembee)Connect With a Mentor ConsultantCommented:
That would tend to suggest the DNS servers on your domain controllers are unable to resolve the domain properly. You would need to look at their configuration.

The external DNS configuration in Exchange is not a solution I like - I consider it a hack, and it doesn't deal with the underlying problem - that your domain controllers have DNS issues.

Simon.
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Simon Butler (Sembee)ConsultantCommented:
Exchange 2010 SP1 is end of life and no longer supported. You should upgrade to Exchange 2010 SP3 with the latest rollup (5 at the time of writing).
That is the first thing I would do - see if that clears the problem. Once you have done that, if you continue to have issues, run the best practises analyser from the toolbox and see what that flags. However until you put SP3 on the server I wouldn't do anything else.

Simon.
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Andy MInternal Systems ManagerCommented:
Try doing a telnet from your email server to the client's mx record, if this fails to connect try via the IP address of the client's mx record.

If both of those fail I suspect an internet routing issue between yourself and the client's mail server (it's more common than you think).

If the hostname fails but the IP works it would indicate an issue with the dns servers used on your mail server - you could try changing them to see if this makes a difference.
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swenger7Author Commented:
My mistake. I do have SP3 with rollup 2 installed. Didn't do rollup 3 yet.
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swenger7Connect With a Mentor Author Commented:
I found the following on google

http://wfcastle33.wordpress.com/2011/06/26/exchangednserror451/

I change the DNS on my exchange under Server Configuration - External DNS lookups to use my ISP Dns and all mail worked .

Any idea what this means. Do I have a problem with my internal DNS servers?
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swenger7Author Commented:
This resolved the issue but doesn't seem like this is the underlying result
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