Ximian Evolution provides integrated mail, addressbook and calendaring functionality to users of the GNOME desktop. It can connect to Microsoft exchange servers, uses SpamAssassin for filtering spam, provides virtual folders, advanced mail searching and much more.
Sylpheed is an e-mail client and news reader based on GTK+ GUI toolkit, and runs on X Window System. Fast and intuitive. The appearance and interface are similar to some popular e-mail clients for Windows, such as Outlook Express, Becky!, and Datula, but the interface is also designed to emulate the Emacs-based mailers. Supports SMTP, POP3, IMAP4 and NNTP. It handles multiple mail accounts, threaded display, junk mail filtering, GPG integration etc.
ELMO is a text based, feature-rich, highly configurable and above all a fast mail client with a very clean and usable interface. You could call it a Mail User Agent, but it offers much more than traditional MUA's. It supports POP3 and SMTP and includes Bayesian filtering.