This is emm/sm, an electronic mail manager for Unix. Emm is very similar (by design) to the version of MM I remember fondly from the DEC-20, but fits smoothly into the Unix environment.
There are several additions to the MM-20 functionality, including command line editing and history, the ability to send and read multimedia mail (as much as you can with a command-line-based mailer that can run in a terminal window), command macros (aliases for commands), and a built-in version of the ce editor with additional features specifically for sending mail (for example, separate editing windows for the message headers and body). There is a rich set of variables available for customization.
Most of the good MM-20 features are present, including the `?' help facility, word completion, and the extensive internal help system. Some of the MM-20 features that are not implemented (e.g. `except') can be done in other ways. One big difference is that completion is invoked with the TAB key, since the command line editor emulates emacs and uses the escape key for extended key bindings.
My Master's Project report (from 1993), which describes emm in more detail, is also available.