Roundcube - Web-based IMAP email client
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
HEY - Email at its best, new from Basecamp.
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.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Geary - Geary is an email application built for GNOME 3.