I’m trying again to use a email client, and need the integration with calendar features, then here it comes the lightining ! =) http://www.mozilla.org/projects/calendar/lightning/index.html.

Additional links :

How to setup the integration with google calendar
https://addons.mozilla.org/en-US/sunbird/addon/4631
http://bfish.xaedalus.net/?p=239

How to import Lotus notes calendar
http://not-everything-that-counts.blogspot.com/2008/04/how-to-import-your-lotus-notes-calendar.html