The Open Source alternative to Outlook

Perhaps your like me and your work email goes through Microsoft Exchange which rather forces you into Outlook as the default Windows client unless your IT department has the where-with-all to provide IMAP support. When it comes to Outlook personally, I fit well within the 20% of the 80/20 rule. While it's far more than I need at the same time it's lacking things like basic NNTP support. I'm not a manager and I don't need a "cockpit" view for managing communications, people and resources. What I prefer is the following in one app:

• Email (IMAP and POP)
• NNTP newsgroup access
• RSS access
• Calendaring for the few meetings I have each week and the spurious all hands meetings

The first three come with Mozilla's Thunderbird 2.0 and the last bullet item you can get with two add-ons, namely Lightning and Provider for Google Calendar. These last two allow me to easily synchronize my work (read Outlook) calendar with Google which means I can access my calendar from any browser as well as receive notifications on my cell phone for meetings, all for free.