Arrow
- It displays each mailbox and message in a separate window, thereby allowing one to simultaneously open as many mailboxes and view and compose as many messages as one wishes.
- http://www.newplanetsoftware.com/arrow/

Sylpheed
- GTK+ based e-mail client for XWindows.
- http://sylpheed.sraoss.jp/

Postilion
- A clone of the NeXT Mail.app for the X Windows system.
- http://www.postilion.org/

GNUMail.app - A Mail.app clone for GNUstep.
- GNUMail.app is a clone of NEXTSTEP's excellent Mail.app application. It uses the GNUstep framework and runs on many Unix versions and under MacOS-X.
- http://www.collaboration-world.com/gnumail

XCmail
- XCmail is a MIME and POP3 capable mailtool for X11 using the Xclasses layout library.
- http://www.js-home.org/XCmail/

Balsa
- A GNOME email client, supports POP3 and IMAP4.
- http://balsa.gnome.org/

TradeClient
- A full-featured e-mail client with support for SMTP, POP3, basic IMAP, and LDAP support for a global address book.
- http://sourceforge.net/projects/tradeclient/

The Althea IMAP Client for X Windows
- Althea is an IMAP client for X Windows that uses the GTK widget set. It allows an almost unlimited number of servers, and has a very small memory footprint.
- http://althea.sourceforge.net

TkMail
- v2.0 is an X windows interface to reading mail built using Tcl/Tk and Perl.
- http://www.slac.stanford.edu/~raines/tkmail.html

Spruce
- Mail client written with the Gtk+ widget set. Supports POP3 and IMAP.
- http://spruce.sourceforge.net/

Ratatosk
- Tcl/Tk mail client, also known as TkRat.
- http://www.dtek.chalmers.se/~maf/ratatosk/

Pronto
- Themeable, GTK-based mail client written in Perl. Supports POP3, display of HTML mail.
- http://www.muhri.net/pronto/

Exmh
- Exmh is an X user interface for MH mail using Tcl/Tk.
- http://www.beedub.com/exmh/