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

mail exploder

    Part of an electronic mail delivery system which
   allows a message to be delivered to a list of addresses.  Mail
   exploders are used to implement mailing lists.  Users send
   messages to a single address and the mail exploder takes care
   of delivery to the individual mailboxes in the list.

   (1996-02-26)