As far as I remember, you can use a MAPI->VIM converter (called MAPIVIM.dll I think) that will resolve MAPI calls and use the Notes mail system. A lot of installations will have this already installed: every system I've installed with Notes running has been able to accept MAPI calls, but I've probably been lucky, or they've used the latest version of Notes or whatever. (Actually, I think it's bundled with Word).

You might find more information at:-
http://www.lotus-developer.com/
http://www.groupsoft.com/products/Mapivim/mapivim.htm