I'm getting "Server Error in '/' Application". What does this mean?

If you see the following in the error details:

Parser Error Message: It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS.

This happens because you've not configured WebMail Pro virtual directory as an application in your IIS. To do this, run your IIS Manager (Start / Settings / Control Panel / Administrative Tools / Internet Information Services) and open WebMail Pro virtual directory properties, then click Create button in Application Settings section of Virtual Directory tab:

Once you configured the virtual directory as an application, you'll see the following:

• Windows 2000/XP (IIS5):

Windows 2003/2008/Vista (IIS6, IIS7):

If you see the following in the error details:

The WebResource.axd handler must be registered in the configuration to process this request.

that's a well-known problem of IIS 7 if you have ColdFusion installed on your server. To get rid of this error, you need to remove default ColdFusion handler mapping entries from WebMail Pro application through your IIS 7 Manager. Once that's done, ASP.NET starts working properly.