If you use Thunderbird, you might want to take a look at a recent review of the current 2.0 Beta. Designed to be a Web 2.0 client, the featureset is fairly rich and gives you pretty much everything you could demand out of a typical mail client. The verdict so far seems to be that not a whole lot has changed from 1.5, and there were some complaints that have stuck around, such as Thunderbird defaulting to using a single SMTP server even if you define multiple accounts. Regardless, it's a thourough review for a good email client, and worth taking a look at. You can download Thunderbird and its beta releases at Mozilla's site.