Geary - Geary is an email application built for GNOME 3.
Roundcube - Web-based IMAP email client
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.