Thunderbird is a free, open source, cross-platform e-mail, news and instant messaging client. The project strategy is modeled after Mozilla Firefox.
It supports POP (storing mail locally on your hard drive so that it can be accessed without an internet connection), and IMAP. It features excellent mail filter capabilities and management.
Thunderbird has strong support for using multiple accounts and identities, including automated signature features. More Info »
Chat functionality is built in, so you can enjoy real-time conversation with your contacts, right from your favorite messaging application, with multiple supported networks. Thunderbird makes it easy to search through both past conversations and received emails.
Thunderbird can also function as an RSS/Atom feed reader, and has a large selection of add-ons that extend it with calendar support, PGP support, integration with online services and much more.
Available also as portable app: http://portableapps.com/apps/internet/thunderbird_portable
Thunderbird is listed in
Apps to keep your data private, Windows, My Essential Windows App List, A Software Engineer Work Essentials, Open Source Comms, Windows Apps, Linux Apps, MySoft, How to live without Google, My Software, Software I use on GNU/Linux, and Resources for small business
Links to official Thunderbird sites
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Built-in RSS reader Calendar integration Customizable Extensible by Plugins/Extensions Free API Import from Google Reader Integrated Chat Integrated Web Browser Multiple Account support Export to OPML Import feeds from OPML POP3 Support Tabbed interface
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Portable built-in-rss-reader calendar-integration customizable email-client extensible free-api google-reader-import imap instant-messaging integrated-chat integrated-web-browser multiple-account-support opml-export opml-import pop3 smime tabbed-interface