paranoy is a simple, lightweight, open-source, fast but overall safe mail client. all its features are privacy and safety-oriented. it has a ncurses interface and a graphical one based on gtk.
it saves mail and configuration files totally encrypted with strong cryptography standard. it supports pop3, imap(TODO) and smtp connections over ssl, mail filtering and threading , attachments , multibox, OpenPGP standard , anonymous remailers and nymservers.
it's hightly configurable, easy to use and install, it's written in C and it compiles on Linux, all *bsd systems and win32.
note that paranoy isn't yet usable!