Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Geary - Geary is an email application built for GNOME 3.
Roundcube - Web-based IMAP email client
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.