Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
HEY - Email at its best, new from Basecamp.
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
InstAddr - Disposable email address application.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.