The best alternative to Microsoft Outlook is Thunderbird, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Microsoft Outlook and six of them are so hopefully you can find a suitable replacement. Other interesting alternatives to Microsoft Outlook are Evolution, Betterbird, Claws Mail and The Bat.
Cross-platform email client supporting multiple identities, chat, and strong filtering. Extend functionality with add-ons like RSS/Atom feed readers and PGP support.
Evolution is a personal information management application that provides integrated mail, calendaring, address book and task list functionality.
Betterbird, a soft fork of Mozilla Thunderbird, improves user experience with advanced features, bug fixes, security patches, and multi-row descriptions similar to Outlook. Compatible with multiple platforms and can be used alongside Thunderbird, sharing the same profile.
Email and news client on GTK+ foundation featuring quick performance, extensible plugins, secure protocols, advanced filtering, strong folder and contact management, lightweight design, powerful search, customizable interface, multi-language support, and encryption.
The Bat! is a secure email client software, built to protect your correspondence against third-parties.
Koma-Mail is an email client which is comfortable and has very handy functions. It does not need an installation, so that you can run Koma-Mail from your flash drive but, of course, also on your desktop.
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Despite its age, Thunderbird remains the most fully featured email client out there. Various add-ons can extend its functionality, it (now) has a built in calendar and, unlike Outlook, it's free, open source and respects your privacy.