The email program I have been using for the past seven years is now free (for the moment): Courier Email from Rose City Software.
Special Offer: For a limited time, Courier Email can be registered for FREE for unlimited use. Your free registration for Version 3.5 will never expire.
This registration won’t expire, but they’re working on version 4 (in fact they’re looking for a C++ programmer to help them finish it.
Courier used to be Calypso and I still think it’s the best email program around. Of course I’m biased, since I’ve been using it so long, but it has too many useful features for me to walk away from it. Check it out.