Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
Roundcube - Web-based IMAP email client
Alpine - Alpine is a fast, easy to use email client that is suitable for both the inexperienced email user...
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.