About 40% of questions I get from SC's and SE's are about customer issues with Exchange Server connectivity. Check out this new FREE tool from the Exchange Team @ Micrososoft.  http://msexchangeteam.com/archive/2009/03/25/450908.aspx. Here an overview:

Overview

The Exchange Server Remote Connectivity Analyzer is a web site for IT Administrators to validate and diagnose end-to-end Exchange scenarios.  The site simulates multiple Exchange client access and inbound SMTP scenarios from outside the customer’s infrastructure and reports whether the test was successful.  If the test fails, we tell the IT Admin exactly where in the process it failed as well as provide troubleshooting tips on resolving the issue.  See the screenshot below.