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

mail gateway

    A machine that connects two or more electronic
   mail systems (including dissimilar mail systems) and
   transfers messages between them.  Sometimes the mapping and
   translation can be quite complex, and it generally requires a
   store and forward scheme whereby the message is received
   from one system completely before it is transmitted to the
   next system, after suitable translations.

   (1996-02-26)