[Exchange] Users can Not recieve mail from outside.

The mail environment is Exchange 2007.Outlook 2007 Problem; Users have No problem sending email to other users inside- internally in the Company, - The problem is users can not receive emails from outside users. 1. How to troubleshoot this problem. 2. Is this an Exchange problem. 3. What is the answer - to - Why can't users receive outside mail and how can we solve this problem. Thanks Joseph