Pine® is a tool for reading, sending, and managing electronic messages. Pine was designed by the Office of Computing & Communications at the University of Washington specifically with novice computer users in mind, but it can be tailored to accommodate the needs of "power users" as well. Pine is not being developed further, in favour of Alpine.
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.
XCmail is a MIME and multi POP3 capable mailtool for X11 using the Xclasses X11 layout library. Its main purpose is to read and write mail, allowing any kind of data to be attached. It can handle MIME types, has helpers to show different types, and can encode and decode binary data into ASCII to allow it to be transported as mail over the Internet. It has many built-in functions such as 2 vCards, unlimited POP3 servers, SMTP, SMTP after POP3, SMTP-AUTH (RFC2554), addressbook, encoders, filters, a special spam filter, spell checker, xface, and a new "virtual folder" concept. It also uses external programs to enhance its feature set. XCmail can be extended through a plugin interface.
The procmail and formail mail processing package can be used to create mail-servers, mailing lists, sort your incoming mail into separate folders/files (real convenient when subscribing to one or more mailing lists or for prioritising your mail), preprocess your mail, start any programs upon mail arrival (e.g. to generate different chimes on your workstation for different types of mail) or selectively forward certain incoming mail automatically to someone.
Majordomo is a program which automates the management of Internet mailing lists. Commands are sent to Majordomo via electronic mail to handle all aspects of list maintainance. Once a list is set up virtually all operations can be performed remotely, requiring no intervention by the postmaster of the list site.
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.
Deliver is a program for reliable and flexible local mail delivery. It's a perfect adjunct to your mail transport agent of choice (such as Sendmail or qmail). You can use it to kill spam, drop messages from specific senders, add a [LISTNAME] prefix to the subjects of messages that are delivered through mailing lists, and lots more.