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My organization is having trouble with our Server 2008 R2 server running Exchange 2013. Our email client we are using is Outlook 2007. The issue is that Exchange isn't receiving external email. Our exchange users can send and receive mail from each other, and send mail to internet addresses, but they aren't receiving mail from the internet. After checking the Transport logs, it seems that the Exchange server isn't receiving the emails from the internet at all. I am at a loss at what the problem could be. How can I get Exchange 2013 to receive email from the internet?