Tim Bray has an excellent essay entitled On Email in which he reviews the many e-mail clients out there. Good quick overview of the many options available to people. If you must stay with Windows, you probably should look at some of the alternatives available to LookOut .. err .. Outlook.
The one addition to this is Ximian's Evolution. It is a Linux mail client (much like Gnus or Pine which Tim mentions). It started off as a clone of Outlook functionality, but has surpassed that while still being easy for former Outlook users to switch to. Some of the pieces it wins on are some of its filtering abilities (which I don't know too well, since I use procmail on the server side), GPG integration, and it runs on Linux which means you aren't susceptible to all the various Windows viruses out there.
The next version of Evolution (2.0) will really push the envelope of the mail client to a whole new level. Things like APIs, GUI cleanups, code cleanups, etc are going to make it one of the best on the planet. And, since its open source, you can always fix/change things you don't like :)
