Email Clients
1. Thunderbird
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Thunderbird is an e-mail client with a tabbed interface, excellent search capabilities and e-mail archiving. Use it with Lightning for calendar and to-do list capabilities. This mail client can run on Windows, Linux, and OS X.
Download : https://www.thunderbird.net/en-GB/
2. Evolution
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Evolution is the official personal information manager and work-group information management tool for GNOME. It combines e-mail, calendar, address book, and task list management functions. This mail client can run on Linux, BSD, and GNOME.
3. Opera Mail
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Opera Mail is one of those clients that surprise a lot of people. Most have heard of the Opera browser, few know about the mail client. That is a shame as Opera Mail is quite solid, supports POP, IMAP (no Exchange support), newsgroups, RSS, and Atom feeds. Opera Mail has a nice list of features: Threaded views, spam protection, allows you to browse websites within tabs, and has a very simple (and lightning fast) user interface. Opera Mail is free and available for Windows, Mac, and Linux.
Other open source Email Client:
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Mail Pile, Zimbra