In Outllok Express, you go to Preferences and Programs to set which programs should get the "mailto" URLs. I looked around a little in the registry and found where this is defined. The attached file is registry entries which will add another "MyMail" program to this menu.

I guess you can see from the attached file how to call iexplore.exe http://yourserver.com/yournewmail.php3?to=%1

Marius (Escort Cab + Mark II)