What is Thunderbird?
Thunderbird is a free, open-source and cross-platform mail client for most operating systems including, but not limited to, Windows, Linux and Macintosh. It is based on the Mozilla codebase. It is a robust and easy to use client, similar to competing products like Outlook Express, but with some major advantages such as junk mail classification. Read more about the mail client at the Product Page.
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http://www.mozilla.com/support/thunderbird/faq#q1.1