Trevor,

The Invite by email button simply tells Windows to create a new email composition using the default email client. Once we send this command to Windows we have no control over which email client is used for the invitation.

I just tested 3 separate applications on Windows Vista that all create a new email composition using MAPI and saw the same thing happening. With Windows Mail set as the default email client and Outlook installed the emails from one application would be created using Windows Mail, while the other two would create new emails in Outlook. Strange behavior indeed, but I don't know if theres anything that can be done about it.

Now if the installed email client(s) do not support MAPI or the MAPI layer is corrupted then no new email composition will appear. Which email client does the other user have installed?