Browse to http://localhost/groupoutlook/servicemain.asmx on the server machine, and then replace the localhost with the servers internal / external IP to troubleshoot connectivity from other machines too.

This error generally occurs when IIS is not correctly 'serving' ASP pages.

To resolve this:

run the aspnet_regiis.exe program, available in the WINNT\Microsoft.NET\Framework\latestversion directory (by this we mean the highest version number). Specifically, run the program as:

aspnet_regiis -i

GroupOutlook needs IIS (Internet Information Services) to run the GroupOutlook web service.  This can be installed on:

Windows XP Pro

Windows Vista Home

Windows Server 2000

Windows Server 2003

Windows Server 2008

GroupOutlook will work on Outlook 2000. 2003 and 2007. 

Not all users need to run the same version of Outlook, in fact one of the strengths of GroupOutlook is that synchronization can happen between users running different versions.