In order to elicit feedback on new feature ideas, and get early and frequent feedback from the tester community, we will be releasing several alpha and beta releases of Thunderbird. These releases are not for production use, but for developers and testers to give us early feedback. To emphasize this point, these alpha releases are called Shredder -- they do not represent the final state of any Thunderbird release, can contain significant bugs, and should be handled with appropriate care.
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Thunderbird shredder comes with excellent gtk integration, which makes the interface much cleaner than with thunderbird 2 when used on gnome.
Add the lightning extension to manage calendars, and you have an impressive piece of software to manage your email, contacts, and appointments.
Very cool and simple e-mail client right on your desktop but configuring it may be a little hard, :P!