Comment 138 for bug 849416

There are a few issues here. The first is that when using mulitpart/alternative, one uses the preference order of the sender in the order that the receiver understands. Thus, in your example, you act on text/calendar if you understand text/calendar, and otherwise move up the list to text/html.

The challenge here is that I don't believe the standard matches the user's expectations, particularly when lightning isn't installed. In particular, the user may wish to view the appointment *and* "double click" on it to hand it off to a handler for Outlook or iCal.