The Free On-line Dictionary of Computing (30 December 2018):

MCI Mail

    The first commercial Internet electronic mail
   service, launched by MCI in about 1981.  Vint Cerf was the
   chief engineer.

   Reading mail was free but you had to pay to send.  Users
   discovered you could communicate for free by sharing an
   account.  One user would save a message as a draft and the
   other would read it and replace it with his response.

   (2004-08-25)