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Unable to Set Different E-Mail Programs for Different Users
http://www.vistaarticles.com/articles/174/1/Unable-to-Set-Different-E-Mail-Programs-for-Different-Users
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Published on 11/23/2006
 
You may experience any of the following symptoms: If an administrator changes the e-mail program on the Programs tab of Internet Options, the change applies to all users.
Unable to Set Different E-Mail Programs for Different Users

The e-mail program that is selected on the Programs tab of Internet Options is not stored on a per-user basis, and only administrators can change this setting. This setting specifies which program is run for a mailto: Uniform Resource Identifier (URI), programs that make Messaging Application Programming Interface (MAPI) calls, and the Send Page by E-Mail command in Internet Explorer.

Details: http://support.microsoft.com/kb/309292/en-us?spid=2073&sid=global