Aileron is an email client written for the Window Maker window manager. It has been designed for speed, beauty, and above all, simplicity. It tries to emulate the very well designed NextStep email client, but deviates from it as necessary.
The designers of Althea felt that there wasn't a stable, easy to use, GUI, IMAP (Internet Message Access Protocol) e-mail client for X Windows. The design goal was a stable e-mail client with the richness of usability of Microsoft's Outlook, Qualcomm's Eudora, and Cyrusoft's Mulberry.
Arrow provides an elegant, powerful interface to electronic mail. This is not just another single window mail reader. Instead, it displays each mailbox and message in a separate window, thereby allowing one to simultaneously open as many mailboxes and read and compose as many messages as one wishes. This paradigm also allows one to use drag-and-drop to transfer text between messages and to organize one's mail by rearranging messages within a mailbox and moving messages between mailboxes.
GNUMail.app is a fully featured mail application running on Linux (or FreeBSD, OpenBSD, etc) and Apple Mac OS X. It uses the GNUstep development framework or Apple Cocoa, which is based on the OpenStep specification provided by NeXT, Inc.. GNUMail.app is licensed under the GPL, and the full sources of the application are available.
Ishmail is a graphical mail reader for Unix systems. In addition to features you would expect in any decent GUI mail client, Ishmail: supports UNIX (mbox), mh and MMDF folder formats, shares folders and aliases with programs such as pine, elm, Z-mail, and sendmail, has internal IMAP and POP support, and PGP support.
Manitou-Mail is an open source database-driven mail user-agent and associated utilities. It provides a framework and programs for storing, exposing, and managing mail as organized data in a PostgreSQL database. It can be viewed as a versatile alternative to an imap-aware client using imap server storage.
mxMail is a program written for Linux using X11R6/Motif2.0. It is designed for reading and sending of E-Mail. It operates around a UNIX file structure, and uses 'sendmail' to send e-mail messages.
Postler aims to be easy, simple, clean, beautiful, and automagic. It handles IMAP beautifully, and provides the user with smart, sensible defaults.
Scribe is a small and fast email client with an intergrated contact database and calendar. It supports all the major internet mail protocols and uses international standards where possible. Scribe doesn't required installing or uninstalled and can be used from a removable drive without reconfiguration. It comes with a highly accurate bayesian spam filter and translations to many different languages.
Trojita is a fast Qt IMAP e-mail client.
Ultimate Mail Tool, X11 e-mail reader with MIME support. It runs on HP, SGI ,SUN and Linux platforms and has multilanguage support, a flexible configuration with an easy to use Point and Click interface.
XCmail is a MIME and multi POP3 server capable mailtool for X11 using the Xclasses layout library. XCmail was designed completely object orientated and by this may be improved easily. The main purpose of XCmail is to read and write mails which may have any kind of data added (attached). For this XCmail can handle MIME types and has so called "helpers" to show different types. And XCmail offers "encoders" which encode and decode binary data into ASCII to allow the transport as mail over the Internet. XCmail also offers "encoders" which encode and decode binary data into ASCII to allow the transport via mail over the Internet.
xP provides an intuitive, high quality, graphical (X11) user interface to the University of Washington's PINEŽ Messaging System. xP aims to provide an interface that is welcoming and easy to use for the new user, while still retaining the look, feel and key commands of PINE for the experienced user.
YPMail.app is a NeXT Mail.app lookalike for Window Maker. It has been written in C using the WINGs library from Window Maker. It requires the c-client library which is available as part of the imap package (imap-4.*.tar.Z)