Linux Gazette: Experiments with SMTPSep 19, 1999, 15:20 (0 Talkback[s])
(Other stories by Jan Stumpel)
"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."