Forget who asked about this recently. At the time I recommended Twig (its what I use). I'm currently evaluating SilkyMail:

http://www.cyrusoft.com/silkymail/

If you don't need the groupware features of Twig (I don't) SilkyMail makes a compelling argument. The basic user interface if quite a bit sweeter than Twig's.

And yes, its GPL.

-- J C Lawrence claw@kanga.nu ---------(*) http://www.kanga.nu/~claw/ The pressure to survive and rhetoric may make strange bedfellows