Thunderbird is Mozilla's next generation e-mail client. The Mozilla Web site claims that "Thunderbird makes e-mailing safer, faster, and easier than ever before with the industry's best implementations of features such as intelligent spam filters, a built-in spell checker, extension support, and much more."
Thunderbird supports the IMAP and POP e-mail protocols, is available for Linux, UNIX, PC and Mac platforms and is distributed at no charge as an open source project. The current 0.7 release of Thunderbird is considered a "technology preview" rather than customer-ready software, but the reports I've gotten are that this release is quite stable and usable.
You don't mention which Outlook release you are using or what about Outlook makes you dissatisfied, so it's hard to say whether Mozilla addresses those concerns. In researching this answer I found an interesting blog discussion comparing IMAP mail clients, including multiple Outlook versions and Thunderbird, at this site. Whatever the case, there are strong capabilities and opinions on both sides of the Thunderbird versus Outlook debate.
This was first published in September 2004