Thunderbird - Reclaim Your Inbox

There's also Thunderbird, the Mozilla Project's email client. Version 0.9 was recently released, still a development release but very stable and could easily replace Outlook Express on your machine. Maybe not Outlook since there is no calendar, unless you want to try the Mozilla Calendar extension or even Sunbird (a standalone version which I haven't tried yet).
And if you like all of them, I guess you could download the Mozilla Suite which has them all in one application, plus a couple of extra things. Some people say it's bloat-ware now, I'm not so sure but I don't need the extra features all that much so I just use Firefox (and Thunderbird at work, ain't they grand using open source software in a comercial environment. How cost effective of them ^_^)