Cone (COnsole Newsreader And Emailer) is a text-based email client. It seamlessly handles multiple POP3 and IMAP accounts and local mail folders. It is also a simple newsreader. It is designed to be foolproof enough to be used by inexperienced users, but also offers advanced features for power users.
SquirrelMail is a standards-based Webmail package. It includes built-in pure PHP support for the IMAP and SMTP protocols, and all pages are rendered in pure HTML 4.0 for maximum compatibility across browsers. It has very few requirements, and is very easy to configure and install. It has all the functionality you would want from an email client, including strong MIME support, address books, and folder manipulation.
Mutt is a text-based MIME mail client. It is highly configurable, and is well-suited to the mail power user with advanced features like key bindings, keyboard macros, mail threading, regular expression searches, and a powerful pattern matching language for selecting groups of messages.
Pine® - a Program for Internet News & Email - is a tool for reading, sending, and managing electronic messages. Pine was developed by UW Technology at the University of Washington. Though originally designed for inexperienced email users, Pine has evolved to support many advanced features, and an ever-growing number of configuration and personal-preference options. Pine is available for Unix as well as for personal computers running a Microsoft operating system (PC-Pine)...
Claws Mail is a GTK+ based, lightweight, and fast e-mail client and newsreader. Supports POP3, APOP, IMAP, SMTP, SMTP AUTH, NNTP, LDAP, multiple accounts, spell-checking, addressbook, SSL, GPG, filtering, i18n, & more.
Sylpheed is a flexible and helpful e-mail client which provides intuitive user-interface and is also designed for keyboard-oriented operation, so Sylpheed can be widely used from beginners to power users.