Totally free and open-source, but lacking some key functions.
What It Is
The upstart from the Firefox browser folks. Their applications are open source, meaning anyone can modify them. As a result, there's a big community of developers eager to create add-on applications.
Free. It doesn't get any better than that.
Customizable tool bar and integrated news reader. Can create "saved search folders" that list messages according to criteria you set up. Lots of add-ons, including SunBird, for calendaring, and Enigmail, for encrypting and decrypting e-mails.
Thunderbird's junk mail filter requires you to first train it. Then it puts spam in a separate folder or deletes it. Doesn't allow scripts to run automatically, which can keep viruses and worms from spreading. No phishing filter, but expect one in version 1.1.
Can't beat the price. Plus, Thunderbird runs on any operating system. And all those add-ons designed by developers allow you to piece together the specific functions you need for your business. Finally, Thunderbird is so new it's relatively immune to the kinds of attacks that have plagued Outlook.
Still new, still growing. Doesn't have all the functions of the more mature e-mail clients. Plus, its open-source nature leaves its future somewhat unclear.