See my thread in this section about "Making xp_sendmail behave like Outlook." MSDN doesn't seem to have anything on @type, or IPM that ties in with what BOL says about IPM.Vendorclass.subclass, the best I could come up with was to use the sp_OA stored procedures to invoke a CDO object.
What is a CDO object? No idea! So I've got some reading ahead of me!