“SMTP, as you probably know, stands for ‘Simple Mail
Transfer Protocol’. It is the method by means of which mail is
exchanged between computers on the Internet. Basic
communication between computers (exchange of single packets and of
streams of information) is provided for by TCP/IP. SMTP is a
protocol ‘on top of’ TCP/IP for exchanging messages between
computers. Let’s do a few experiments to see how SMTP works.”
“…SMTP is really a ‘simple’ protocol. The commands are
described in the Internet standard RFC821. Some ‘extended’ commands
were added later, in other RFC’s, for instance RFC1869. Systems
which recognize the extended commands are said to support ‘Extended
SMTP’, or ESMTP. Such systems announce this in their ‘welcoming
line’, as Exim 3.03 did above. The differences between SMTP and
ESMTP are not great.”