If you're using imap, try thunderbird 3. plus install the zindus plugin.
It gives you the following that you don't get (easily) in Outlook;
1. Automatic IMAP folder sync (downloads mail so your messages can be read offline - this is automatic in TB3 and very smooth) Not like Outlook, where you have to mark messages for download...
2. Once you have the zindus plugin, your address book is syncronized with zimbra (web client), and you automatically get GAL on in thunderbird. -- AND unlike Outlook, as you type, TBird looks up in your address book plus GAL (not the nk2 file like Outlook which is so irritating - since you cannot actually edit the nk2 file and remove old or bad addresses etc)
3. If your laptop dies or is stolen, all your e-mail folders are on the server! Plus your current address book from Tbird
4. If you need to use another machine temporarily, (while traveling or ??) all your address book contacts are there!
5. No 2GB file limit on e-mail data file (yes, I know later versions of outlook can create and use a larger dat fie, but by default, outlook 2003 STILL creates a 2gb limited file (@)!
6. Then install the lightning plugin and you get Zimbra calendar sync. Works like it's supposed to for me even though it's still beta...
The only thing you lose is pda sync for older crappy pdas that must sync through outlook - so toss 'em and get a new one already!  | 
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