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

mail user agent

    (MUA) The program that allows the user to compose
   and read electronic mail messages.  The MUA provides the
   interface between the user and the Message Transfer Agent.
   Outgoing mail is eventually handed over to an MTA for delivery
   while the incoming messages are picked up from where the MTA
   left it (although MUA's running on single-user machines may
   pick up mail using POP).

   Popular MUAs for Unix include elm, mush, pine, and
   RMAIL.

   FAQ

(http://cis.ohio-state.edu/hypertext/faq/usenet/mail/setup/unix/part2/faq-doc-3.html).

   (1996-03-21)