Yup. I used it for a few years. It also had a Tk-based GUI. Every email message is a single file. "Folders" are directories. So you can use your usual text-processing commandline stuff: grep, sed, awk, etc.

http://rand-mh.sourceforge.net/book/mh/shomes.html#WeOBeYRe

Using MIME or GPG required a little configuration (mostly preset properly) which hooked external programs to preprocess before sending or displaying the message.