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.
Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. In style it is similar to Smail 3, but its facilities are much more extensive. Exim is in use at many sites around the world.
IndiMail is a messaging platform built using qmail and other messaging software. IndiMail has the ability to distribute the users in a domain across multiple hosts. IndiMail has multiple queues (called a queue collection), which can achiever very high concurrencies. The system IDs and configuration files used by IndiMail are all configurable through a set of environment variables, allowing you to customize IndiMail the way you want...
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.
Sendmail is a Mail Transfer Agent, which is the program that moves mail from one machine to another. Sendmail implements a general internetwork mail routing facility, featuring aliasing and forwarding, automatic routing to network gateways, and flexible configuration.
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.
Synovel CollabSuite provides email, calendaring, and instant messaging for small and mid-sized companies. It uses open standards for communications (IMAP, POP, and SMTP for email, CalDav for calendaring, and Jabber for IM). It consists of an integrated server which runs on Linux, a Web-based administration panel, a desktop client suite which runs on Windows and Linux, and Ajax-based Web mail access.