Just wanted to mention that there are some really nice hybrid commercial/foss messaging systems out there and I was curious how Thunderbird witn Lightening plugin and Sunbird work with them. Particularly Zimbra Collaboration Suite, Zarafa, Scalix, and a few others you can find listed here: http://en.wikipedia.org/wiki/Category:Free_groupware
For myself, I've been using Zimbra at work and for my personal email for over 4 years and have been through many updates to get to the current release of 6.0.5 OSE... including the calendaring functionality which allows for multiple personal calendars, resources (calendars for scheduling rooms, equipment, etc), and sharing via iCal (example: https://mail.montanalinux.org/home/dowdle/MontanaLinux) and https (https://email@example.com...). Note that the SSL certs on that domain is self-signed. The Zimbra Web Client is fantastic with the full Web 2.0 / AJAX experience. It seems some people love Zimbra (like I do) and some people hate it... but potential interoperability with Lightening and Sunbird sounds interesting.