Source : Free On-Line Dictionary of Computing

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)