The user-friendly, light weight and fast email client (with IMAP4rev1)
http://www.claws-mail.org/
http://www.claws-mail.org/features.php?section=general
After http://news.cnet.com/8301-1023_3-57467973-93/mozilla-calling-it-quits-on... can we have Claws now? 
https://portableapps.com/node/27682 seems defunct.
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thunderbird itself isn't dead, mozilla is going to keep it alive but more of a community effort instead of just themselves maintaining it, or at least as I understand.
your friendly neighbourhood moderator Zach Thibeau