Thunderbird is the most popular choice and supports add-ons. For comprehensive alternatives to Microsoft Outlook, Evolution is well-integrated into GNOME, whil

Thunderbird is the most popular choice and supports add-ons. For comprehensive alternatives to Microsoft Outlook, Evolution is well-integrated into GNOME, while KMail (part of Kontact) is well-integrated into KDE. Alternatively, Geary (GTK+ 3 based, adopted by GNOME), Pantheon Mail (Elementary's fork of Geary), Claws Mail (lightweight GTK+ 2 based), Sylpheed (lightweight GTK+ 2 based), Trojitá (lightweight Qt 5 based), Kube (Qt 5 based, integrates with Kolab Now), and Mutt (text-based) also work on Linux. All of these applications are free and open-source, and some of them also work on Windows and macOS. Thunderbird: https://www.thunderbird.net/en-US/ https://www.thunderbird.net/en-US/ Evolution: https://wiki.gnome.org/Apps/Evolution/ https://wiki.gnome.org/Apps/Evolution/ KMail: https://www.kde.org/applications/internet/kmail/ https://www.kde.org/applications/internet/kmail/ Geary: https://wiki.gnome.org/Apps/Geary https://wiki.gnome.org/Apps/Geary Pantheon Mail: https://github.com/elementary/mail https://github.com/elementary/mail Claws Mail: https://www.claws-mail.org/ https://www.claws-mail.org/ Sylpheed: http://sylpheed.sraoss.jp/en/ http://sylpheed.sraoss.jp/en/ Trojitá: http://trojita.flaska.net/ http://trojita.flaska.net/ Kube: https://kube-project.com/ https://kube-project.com/ Mutt: http://www.mutt.org/ http://www.mutt.org/