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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
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Simon Butler (Sembee)
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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.
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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swenger7

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My mistake. I do have SP3 with rollup 2 installed. Didn't do rollup 3 yet.
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This resolved the issue but doesn't seem like this is the underlying result