Electronic mail generally consists of three basic pieces:

    1) The link level transport - which could be
       UUCP, TCP/IP, or a host of others.  We'll call
       this the "transport medium" (TM)

    2) the "Mail Transport Agent" (MTA) which is responsible for
       transporting mail from source to destination, possibly
       transforming protocols, addresses, and routing the mail.

       The MTA often has several components:
	    - Routing mechanisms
	    - Local delivery agent (LDA)
	    - Remote delivery agent
       Many MTA's have all of these components, but some
       do not.  In other cases, it is possible to replace
       certain components for increased functionality.

    3) The "User Agent" (UA) is the user interface -
       the software that the user uses to read his mail,
       sort things around in folders, and send mail.
       Sometimes called "Mail User Agent" (MUA).