Geary - Geary is an email application built for GNOME 3.
HEY - Email at its best, new from Basecamp.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
InstAddr - Disposable email address application.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.