Source : Free On-Line Dictionary of Computing

Message Transfer Agent
     
         (MTA, Mail Transport Agent) The program
        responsible for delivering {e-mail} messages.  Upon receiving
        a message from a {Mail User Agent} or another MTA it stores it
        temporarily locally and analyses the recipients and either
        delivers it (local addressee) or forwards it to another MTA
        ({routing}).  In either case it may edit and/or add to the
        message {headers}.
     
        The most widely used MTA for {Unix} is {sendmail}, which
        communicates using {SMTP}.
     
        [Other OSes?]
     
        (1997-02-28)