Several folks at BBN worked on MMDF as well, often as part of the CSNET project which BBN ran.*  I always thought the five control-A's were a failure of imagination (never going to send binary?).  MMDF's biggest failure was synchronous sending -- if you made a typo in an email address the mail UA would bring it back to edit-mode -- which didn't work for store-and-forward mail configurations.

Steve Bellovin had a funny quip about CSNET and Usenet, comparing professors applying for NSF grants and grad students just setting up modems.