This isn’t the thing for me, as I use Evolution e-mail under Linux, but some of my clients use Thunderbird or are considering a new client. It looks like the new Thunderbird may be worth a serious look. See Thunderbird Ups the Email Ante from Linux Magazine.
Choosing an e-mail client is a pain, because if you make the wrong choice, it’s often a nightmare to export your files and accounts and then import them into a new account. Thus reviews such as this comparison of five cross-platform clients on nixCraft are very valuable.
(Note: Page author Henry Neufeld is compensated for sales made through links on this page.)