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

Message Handling System

    (MHS) The standard defined by ITU-T
   as X.400 and by ISO as Message-Oriented Text Interchange
   Standard (MOTIS).  MHS is the X.400 family of services and
   protocols that provides the functions for global electronic
   mail transfer among local mail systems and MTAs.

   It is used by CompuServe, among others.

   (1996-09-25)