After working this issue for just about six months I am finally at the stage where I can blog about this. Windows Vista changed the COM Activation model to prevent activiation of COM components registered in HKEY_CURRENT_USER when the COM client is running with elevated privileges. When a MAPI Form is downloaded from the Exchange Organizational Forms Library for the first time Outlook registers it temporarily in HKEY_CURRENT_USER so that the form can be launched. Upon launching the form, it's the form's responsibility to actually register itself just as any Local COM Server would. The problem occurs that when Outlook registers the component in HKEY_CURRENT_USER and Outlook is running with elevated privileges (i.e. High IL) then COM prevents the creation of the COM component. The result is Outlook displays the error "The form you selected could not be displayed. The form required to view this message cannot be displayed. Contact your administrator." I have confirmed this issue with Outlook 2003 SP2 and Outlook 2007. The workaround to this issue is to either always run Outlook with medium IL, which is the default when UAC is enabled or provide an installer to your users which registers your Form Server in HKEY_LOCAL_MACHINE.