Redwood is a very simple email client that only has the few functions that I personally find critical. At this point, it is more interesting as a wxRuby sample application than as an actual email client.
Here are slightly out-of-date screenshots of the main screen, composing a message, and viewing a message more or less full screen.
Why would anyone care about yet another email client? Two possible reasons:
I used an earlier version of Redwood as my only email app from December 2000 until about April 2001. This version is a complete rewrite from scratch, using wxruby, and built on top of nmh.
Redwood is released as GPL software.