* GNUMail.app is a well-rounded email program
* Support for GnuPG message encryption and the exclusive use of plain text offer security
* GNUMail.app organizes threads well
* GNUMail.app doesn't support HTML
* You cannot set up smart folders in GNUMail.app, and comprehensive message search could be faster
* GNUMail.app has no internal spam filters
Linux Magazine says (cached by Google):
open source developers have created a GPL’d program named GNUMail that is not only similar to Mail in most ways, but actually surpasses it in a few important areas. It runs on Linux and other varieties of Unix, including Mac OS X.