V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SMTP
       Simple Mail Transfer Protocol (RFC 821, TCP/IP)


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

Simple Mail Transfer Protocol
SMTP

    (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 fred@doc.ic.ac.uk
   	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)