I still use mutt. It's not great, but it's better than anything else I've tried. I ssh to my mail server and run mutt in a screen. My mail server has imap and webmail set up too, so it's fairly convenient no matter where I am.

I've wanted Mutt to be properly scriptable; it doesn't have much good in the way of configuration. I've been casually puttering around ideas for a long time about writing my own email client that's much more customizable. Maybe use a database backend?

Edit: looking at Sup, maybe it's what I want.