I want to set a standard email-program (including .eml files, MAILTO and Mapi) for all users on a Windows Terminal Server 2008. I was told, that "Group Policy Properties" could do something like this.
It depends on the program. For a generic procedure to integrate programs into Terminal server, you can look here.
The unofficial method is to logon as the Server admin, go into Internet Options and set the Email client there. This changes the registry setting at
HKEY_LOCAL_MACHINE\Software\Clients\Mail and the changes are inherited by all users.