External recipient is not receiving e-mails from our domain

Hi there,
I hope that any of you experts can help me.
One of my users is having a perplex issue. She is trying to send e-mails from our corporate account to an outside recipient (another corporate account). The recipient says that he is not receiving our coworker's e-mails.
Our coworker is not receiving any bounce backs or any errors from our mail server. It appears that her e-mails disappeared on CIBER-space. My coworker is able to send e-mails out of our domain to other domains without problems.

The external recipient said that they are not blocking our domain or our coworker's e-mail address.

We are using Exchange 2007 with Outlook 2007.

At this point, I don't know what to do. By the way, I sent the external recipient a test to his Verizon account and he got my test.

Does anybody have any ideas on what should I be doing?

Thanks

Colombia1Asked:
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dianthonymCommented:
Is it just the one user or the entire domain?  Try sending an email to him from at least 1 additional user in your corporate environment (preferably 2) and see if he gets those.  If he does than its most likely a spam issue on the recipient end.  If he receives no email from anyone there it could be an issue on your end.  Also try checking to make sure that you are not on any external blacklists.  http://dnsstuff.com has a nice tool for this.
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Colombia1Author Commented:
I sent him an e-mail from my corporate account also and it appears that de did not get it either. Unfortunately, I did not get any bounce backs. So, I don't know what the problem could be on my end. By the way, we are not in any external blacklists, I already checked that.
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Colombia1Author Commented:
Thanks for your post and I'm sorry it took me longer to give you the points. The caused of the problem was that a third party company was  blocking a range of IP addresses from our ISP provider, unfortunately our range of IP addresses was in the same area as the ones blocked. our mail server was not blocked, but we were affected by the whole blocking thing. I spoke to our ISP provider and they were able to fix their problem.
Thanks for your help.
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