What is "gnumail"

GNUMail (aka GNUMail.app) is a free and open source, cross-platform mail client for GNUstep and Cocoa. It is the official mail client of GNUstep and is also used in Étoilé. It was inspired by NeXTMail ( NeXT's Mail.app), the predecessor of Apple Mail. GNUMail is based on the mail handling framework Pantomime.

Furthermore, GNUMail demonstrates that it is possible to develop cross platform programs for GNUstep and Cocoa.