Source : Free On-Line Dictionary of Computing

Simple Mail Transfer Protocol
     
         (SMTP) A {protocol} defined in {STD} 10, {RFC}
        821, used to transfer {electronic mail} between computers,
        usually over {Ethernet}.  It is a server to server protocol,
        so other protocols are used to access the messages.  The SMTP
        dialog usually happens in the background under the control of
        the {message transfer agent}, e.g. {sendmail} but it is
        possible to interact with an SMTP server using {telnet} to
        connect to the normal SMTP {port}, 25.  E.g.
     
        	telnet mhs-relay.ac.uk 25
     
        You should normally start by identifying the local {host}:
     
        	HELO wombat.doc.ic.ac.uk
     
        You can then issue commands to verify an address or expand an
        alias:
     
        	VRFY [email protected]
        	VRFY postmaster
     
        or expand a {mailing list}:
     
        	EXPN c-help
     
        You can even send a message:
     
        	MAIL From:
        	RCPT To:
        	DATA
        	What is the point?
        	.
        	QUIT
     
        This is useful if you want to find out exactly what is
        happening to your message at a certain point.
     
        See also {Post Office Protocol}, {RFC 822}, {sendmail}.
     
        (1995-10-17)