I'm pretty excited by the work being done by the Open Source Applications Foundation. I spent a few days consulting with them on their security architecture, and I was really impressed by the people there. If you poke around their Wiki, you can see all the thought that has gone into their design. This is meant to do everything that Outlook + Exchange can do, but without requiring a centrally-maintained server infrastructure.

Of course, like any kitchen-sink project, they'll eventually have to make hard decisions about what they will and won't put into version 1.0. Still, if I were thinking about starting a new mail client from scratch, I'd probably think twice, and maybe consider trying to hack into Mozilla, this OSAF thing, or some other existing project so I wouldn't have to reinvent the wheel.