Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
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...
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.