Sylpheed
A well-behaved email client is a Good Thing (TM).
Sylpheed is a well behaved email client. I'm a Unix person at heart and it just feels right. I mean, it doesn't do silly things like displaying HTML, and it does all the right things, like letting me see what client something was sent with as a matter of course and putting things in threads (where they obviously belong). It's just incredibly polite about the whole email thing. It handles reply quote highlighting without adding HTML to the message. It just does email really, really well. I'm a fan.
I sure hope the rest of the world is paying attention to this program, because you know what? if it compiles on OpenBSD with no complaints, then it'll probably compile and run under cygwin. Mozilla, Outlook Express, eat your hearts out.