GNUMail.app is a clone of NeXT's excellent Mail.app application.
It uses the GNUstep development framework, 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.

GNUMail.app was written mostly from scratch though it does use some
code from elm and mpack/munpack.

LICENSE: GPL2 or later

WWW: http://www.collaboration-world.com/gnumail/