Thunderbird is described as 'free, open source, cross-platform e-mail, news and instant messaging client. The project strategy is modeled after Mozilla Firefox' and is one of the leading apps in the News & Books category. There are more than 50 alternatives to Thunderbird for a variety of platforms, including Windows, Mac, Linux, Android and iPhone. The best alternative is K-9 Mail, which is both free and Open Source. Other great apps like Thunderbird are eM Client (Freemium), Evolution (Free, Open Source), Feedly (Freemium) and Claws Mail (Free, Open Source).
A whole new way of using email. Reimagined for the modern age, it transforms the way you communicate, share media, interact with notifications, manage your inbox, stay productive or track updates from your favorite publishers and brands.
Kube is a modern communication and collaboration client built with QtQuick on top of a high performance, low resource usage core. It provides online and offline access to all your mail, contacts, calendars, notes, todo's and more.
Interlink Mail & News is a free e-mail client based on open source community code built on the Unified XUL Platform. It is available for Microsoft Windows and Linux (with other operating systems in development).
Interlink Mail & News vs Thunderbird opinions
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Based on the same code as the original Thunderbird.