Thunderbird is a fast, flexible e-mail client. You can quickly search through your e-mails, worry less about spam thanks to the spam filters, and even customize how you view Thunderbird. Thunderbird works with most Firefox add-ons as well, adding more content to your e-mail client. Thunderbird makes the e-mail experience fun again.

Download: Mozilla Thunderbird 3.1.2 | Other Systems and Languages

Changes: Release Notes

Homepage: Mozilla Thunderbird

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You might want to update those links.....

mozilla.com is now mozillamessaging.com for Thunderbird

and the latest release notes are on .../3.1.2/... (not .../3.1/...) as in http://www.mozillamessaging.com/en-US/thunderbird/3.1.2/releasenotes/

Smart folders have been fixed. (The upgrade from 3.06 to 3.1 broke them.)

There's a few other POP3 & IMAP fixes too.

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In related news Mozilla contacts.

http://mozillalabs.com/messaging/2010/08/04/thunderbird-contacts/