Source : Free On-Line Dictionary of Computing

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://www.cis.ohio-state.edu/hypertext/faq/usenet/mail/setup/unix/part2/faq-doc-3.html)}.
     
        (1996-03-21)