Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
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.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.