Evolution, a linux app, is designed to be an outlook alternative, ideal for groupwisre services, and exchange servers. (comes with all good linux distros). Novell seem to be managing the code for the linux version now, as well as an OSX port Here
I have never tried the OSX port myself but the linux port is fantastic.
A windows port can be found here
Seriously if you want to use outlook features and functions on other platforms then look no further than evolution. However Outlook PST file support is limited unless you want to do some hideous converting between Mozilla and outlook express, DONT DO IT! Use a mail-box instead of a PST, transfer (drag and drop), all your PST data to your mailbox for an easy transition, and better mobility and access to your email.
If you need some help with the server side of things (I am an exchange admin), the PM me or ask here. I will give the OSX port a go soon, normally I just use web access. I am i holland at the moment, and I am choking up my buddies bandwidth