HEY - Email at its best, new from Basecamp.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
InstAddr - Disposable email address application.
The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Geary - Geary is an email application built for GNOME 3.