The best MUA I know of is the vm package for emacs. And, this is a very qualified "best" -- if your mail box is several megabytes, and you want reasonable performance, you'll need a lot of memory to run emacs in. Also, you'll need to put something like: (fset 'rmail 'vm) in ~/.emacs or /usr/lib/emacs/site-lisp/site-start.el to prevent using a non mbox format.
There are other options, both within emacs (gnus is an option, for example) and without. Out of curiosity, why is unix mailbox format so important? -- Raul
