MUMail is a Java applet designed to let you read your mail from a web page. It enables you to check your mail when there is no mail reader available, or if you don't want to change the configuration of someone else's mail reader. All you need is a Java 1.1 compliant Web browser. MUMail uses POP3 to download mail and is MIME enabled.
Mutt is a small but very powerful 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.
Heirloom mailx (formerly known as "nail") is derived from Berkeley Mail and provides the functionality of the System V and POSIX mailx commands. Additional features include support for MIME, IMAP (including caching and disconnected use), POP3, SMTP, S/MIME, international character sets, maildir folders, message threading, powerful search methods, scoring, and a Bayesian junk mail filter. Mailx can be used as a mail batch language in nearly the same way as it is used interactively. It can thus act as a mailbox filter, can fetch mail from remote accounts, and can send files as attachments.
NeoMail is a web-based email client written in Perl. It provides access to a machine's local mail spools without requiring a POP3 server to be running, and without giving users shell access. It supports sending and receiving attachments, multiple folders, user themes and preferences, templates for interface generation, quotas, language translations, etc.
Netbiff is an advanced version of the traditional biff (a mail-checking program). Features include BSD mailbox and IMAP4 support, a backend plug-in system (for other protocols), arbitrary numbers of backends, pluggable interfaces (GTK and text-based), and arbitrary actions on biff.
nmh (new MH) is a powerful electronic mail handling system. It was originally based on version 6.8.3 of the MH message system developed by the RAND Corporation and the University of California. It is intended to be a (mostly) compatible drop-in replacement for MH. nmh consists of a collection of fairly simple single-purpose programs to send, receive, save, retrieve, and manipulate e-mail messages. Since nmh is a suite rather than a single monolithic program, you may freely intersperse nmh commands with other commands at your shell prompt, or write custom scripts which use these commands in flexible ways.
NOCC is a simple and fast Web-based e-mail reader which can handle POP3, SMTP, and IMAP servers. It is written with PHP5, and uses sessions, has low requirements (no database, frames), and is easy to install. It also features multi-language support, MIME attachment decoding/encoding, and the ability to display HTML messages.