Email not sending...

We have a few domains that we are not able to send to. One of which is Comcast. I tracked the messages and everything is going along correctly except after "SMTP:Message Routed and Queued for Remote Delivery" the messages stop. It should go 2 more steps. (1. SMTP:Started Outbound Transfer of Message 2. Message transferred to abc.com)
Any ideas on why the messages would stop here? I looked in the que and the messages are just sitting there. I tried to do a "Force Connection" and it attempts to connect again but fails. They were able to email to these addresses before but now can't.
aandoAsked:
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abhaighCommented:
If your exchange box now has a new external ip address, and you are using DNS to send out with, then yes, it does change where the mail is coming from

change  your reverse dns to point to the new external ip for your exchange box and you should be alright
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MrLonandBCommented:
Are you delivering your outgoing mail via DNS or a Smart Host? If DNS, suggest creating an SMTP Connector that uses your ISP as a Smart Host. Add the troubled domains to the Addresses that are to be delivered to the Smart Host.
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aandoAuthor Commented:
Everything was working up until last night fine without the smart host. We would like to be able to find out what caused it not to work before going the smart host route. Emails were going through to these domains and then just quit. Thanks for the suggestion though.
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aandoAuthor Commented:
Appears that I may have found the problem. I didn't mention before but we recently purchased a certificate for our exchange server. This cert put the exchange on its own external IP address. Does this automatically change where the SMTP comes from (as far as IP goes)? After reading the original header of the message I noticed that the external IP address had changed origination. The addresses that were blockinig email were doing so because the reverse DNS was at the gatway of the firewall. With the new certificate it looks like the exchange changed the SMTP address. Does it look like I'm on track to you guys? I'm just kind of thinking outside my box here and think this is the problem... Thanks for the help!
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MrLonandBCommented:
If you go to whatismyip.com from the IE browser on the server, does it return the correct address? Not sure what you might have done, but if the address is not correct, you might need to do a NAT at your firewall to have it go out on the correct address.

BTW...When you identified the server name for your certificate, did you give it the public FQDN or the local name. I'm sure you know, but just a reminder that the certificate needs to be issued to the FQDN and not the local name.
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