You need to develop on a machine that has Outlook 2000 installed if
you want to use the designer to set up the registry entries for the
addin and if you want to use early binding. Otherwise you must use
late binding (all objects declared as Object), set up your own
registry entries and make very, very sure not to use any properties or
methods or events that aren't available in Outlook 2000 that are
available in Outlook 2003.