Mozilla Thunderbird is a free, cross-platform e-mail and news client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a smaller and faster web browser. Just as Firefox aims to redefine the web browser, Thunderbird is a refinement of the mail and news interface. 

Simple to use, powerful, and customizable, Thunderbird is a full-featured email application. Thunderbird supports IMAP and POP mail protocols, as well as HTML mail formatting. Easily import your existing email accounts and messages. Built-in RSS capabilities, powerful quick search, spell check as you type, global inbox, deleting attachments and advanced message filtering round out Thunderbird's modern feature set.

Useful Links

Thunderbird Home Page

Thunnderbird Documentation  

Thunderbird Addons