Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Roundcube - Web-based IMAP email client
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.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.