Does anyone know of an email client that can serve as an equal or more powerful alternative to MS Outlook?

MS Office 2007 will be released in January along with Vista. As part of this release, Office 2007 suite will be altered to force compatibility conflict with prior Office versions by changing extensions of certain files. For example, Word docs will carry a .docx extension. Because my daily life is dependent on Outlook, and because I don't see the point in paying $300+ for the upgrade when there are more affordable (even free) alternatives, I'm trying to find a software solution for severing dependency and use of Office without losing the Outlook functionality.

First, OpenOffice is free, but it doesn't offer an email client. Second, Corel WordPerfect Office X3 at $200 entails WordPerfect Mail, which offers such features as email, calendars and contact management. Uh-oh. Doesn't seem like WordPerfect Mail offers a 'Task' feature, which could be a problem, as I manage a lot of my work life in this area. Even as a stand-alone buy at $29, WordPerfect Mail may not be worth it if the app can't import and support 'Tasks' (of course, I could just spent the 2 hours to manually move everything over). I even tried looking at Matt's beloved Mutt, but it certainly doesn't offer what I need as far as calendar and contacts.

One solution is to download OpenOffice for free and then buy Outlook separately...but we all know MS won't sell Outlook separately. There are plenty of variations on this mix-and-match theme, I guess.

Anybody have ideas for an Outlook alternative?