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So after doing some digging around I found that our new ISP did not have a PTR (reverse DNS) listed for our domain. I have had them add it and they told me it should be in full effect in about 24 hours.
I think this could be it, but I'll have to wait and see I guess.
Yes...alot of mailservers will verify that the domain resolves to the IP before accepting mail....if it doesnt...it will bounce.
MLF
From your exchange server use telnet to connect to port 25 one of the email servers that is not accepting mail. Try and send an email, when it fails note the response. Paste the response into google, this will confirm if dns is the problem.
It does not sound like mail is being blocked by an anti spam solutions but you can verify here www.dnsstuff.com Go to ip tests, spam database lookup and enter your exchange servers external IP address.
Best of luck!
MISSION ACCOMPLISHED.
As soon as our new ISP completed adding the PTR entry and gave it a few hours to propagate everything starting moving through smoothly. No emails are left in the queue.
Your help was very much appreciated. A special thanks to Morganlefay.
Glad to hear its all sorted out..
we changed ISP, webhosting nd mailservers recently... it took about a week all in all for the dns to replicate throughout for everything.
Just because our records were updated....didnt mean everyone elses were
I likes special thanks.... ;)
MLF