NDR only happens for two particular domains. Error 451 4.4.0 DNS query failed. Error 421 4.2.1

Hello,
We have issue with NDR for two particular domains. All other domains have no issues:
Error 451 4.4.0 DNS query failed
Error 421 4.2.1

I followed the following two articles to change the setting to use External DNS lookups but it didn’t help…
http://www.networkadminsecrets.com/2010/04/421-and-451-exchange-2007-errors.html
http://www.noelpulis.com/fix-exhcange-2010-error-451-440-error-dns-query-failed/

THank you.
claudiamcseAsked:
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arnoldConnect With a Mentor Commented:
Check whether your 2010 configuration enabled anti-virus/spam mechanisms.

Are the domains with which you are having issues your own? Double check your internal DNS or the DNS on which the 2010 relies.

The difficulty to diagnose the issue, one has to know info.

Try mxtoolbox.com
Lookup the domain.
Then try the smtp test.
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arnoldCommented:
The error deals with an inability to resolve a domain.
Use betterwhis.com to lookup the domain's record to make sure its registration has not lapsed.

The information also includes the servers responsible for the domain
nslookup -q=NS domainname.com. <nameserver responsible for the domain>
If you get an error I.e. Data returned points to root servers or get no such domain, the issue is with the hosting name servers.

There is nothing you can do to solve it.  The domain owner has to address/resolve the issue.
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claudiamcseAuthor Commented:
But the users could send before to that domain two weeks before we implemented 2010 server.....so, the domain was good. Also, we have the same error for few domains. Please see the screenshoterror

Also I noticed this in the Exchange Send SMTP log:
69.43.161.123:25,*,,"Failed to connect. Error Code: 10061, Error Message: No connection could be made because the target machine actively refused it 69.43.161.1234:25"
      Note: I changed the real IP for privacy reasons, but just wanted to say that external IP of the 2010 is different than the IP mentioned in the Send connector log…..Not sure why it is showing different IP in the Send connector log than the IP should be

Please advice

Than you so much.

Also the message never leaves Exchange 2010 server.... it goes from 2003 and to 2010....
Is there a work around for this? As I said, the mentioned above links that I posted didn't work
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FutureTechSysDOTcomCommented:
Agreed with Arnold especially MXToolbox.  

On the screenshot you posted, 6th line down where it gives "Error 554 Spam Email" - if I am reading that correctly, it is rejecting YOUR email as junk, which means there is something it doesn't like about it.  

I'd check those two domains as well as your own with mxtoolbox.  If you see yellow or red, fix it :)
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Skyler KincaidNetwork/Systems EngineerCommented:
Another quick check is you can try to ping those domains just to see if the server is able to resolve the request to an IP or not.
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claudiamcseAuthor Commented:
Thank you
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