SQmaiL is a mail reader for GNOME that stores all its mail in a MySQL database. This makes it blazingly fast for very large (10000 message) mail databases. It supports hierarchical vfolders based on SQL queries, so you can have your mail automatically categorized based on arbitrarily complex SQL expressions. It has excellent MIME support on both incoming and outgoing mail. Mail can be fetched from local spool files and POP servers. Full picons support and partial xface support is available, to give your mail that personal touch. The user interface is customizable with Glade, and the program itself is easily extensible with Python. Some command line utilities allowing access to the mail database without X are available.
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.
Sylpheed is a simple, lightweight but featureful, and easy-to-use email client. It has a well-polished and comfortable user interface. It has many features such as multiple accounts, thread display, MIME, powerful filtering, various search interface, spell checking, and junk mail control. It supports common email protocols such as POP3, IMAP4rev1, SMTP, and security features such as GnuPG and SSL/TLSv1. It is also fully internationalized and multilingualized.
tkbiff allows arbitrary commands to be executed upon mail reception. If you like programs such as xbiff and xbiff++ but wish they were more flexible, then you'll like tkbiff. Unlike other biffs, tkbiff is fully customizable. tkbiff also doesn't waste your valuable screen space with icons; instead, it shows you the mail itself. It supports UNIX, Mac, and Windows, IMAP, POP, and UNIX-style mail files, and SSL and APOP.
TkRat is a graphical Mail User Agent (MUA) which handles MIME. It is mainly written in C, but the user interface is done in Tcl/Tk. The program currently understands UNIX mailboxes, POP, IMAP, and mh folders. Messages are sent via SMTP or any user configured program (sendmail, for example). It has support for disconnected IMAP operations.
TWIG is a Web-based groupware suite written in PHP, compatible with both PHP3 and PHP4. Its features include IMAP and POP3 email, Usenet newsgroups, contact management, scheduling, shared notes and bookmarks, a todo list, and meeting announcements. Any piece of information can be shared with others via group oriented access controls. By using only plain HTML, it is a simple, cross-platform, fast, and browser-independent way to access or share almost any kind of information, without the complexity or costs of other intranet/groupware packages. It is fully internationalized, with 24 translations.
VAMP (Very Attractive Mail Program) is an extremely easy-to-configure, usable, Web-based email client written in PHP3 (and compatible with PHP4). It is for use with POP3 servers, and it also supports "Virtual Domains", encryption (under Linux/PHP3), attachments, contact list, a signature database, an easy-to-use setup tool, and more.
VM is a mail reader written in Emacs LISP for GNU Emacs and XEmacs. It can retrieve mail from local spool files or remotely via POP and IMAP, display and send MIME messages, thread messages, auto-sort messages into folders, and manipulate "virtual folders" of messages matching certain criteria. In Emacs versions with X support it provides an interface with toolbars, menus, and mouse-based message selection.