“SMTP is an abbreviation for ‘Simple Mail Transfer Protocol,’
and is the standard internet protocol for sending email from one
system to another. Although the word ‘simple’ belies the inherent
complexity of the protocol, SMTP has proved to be a remarkably
robust, useful, and successful standard. The design decisions that
made it so useful, though, have given spammers and infectious code
an easy way to spread their unwanted messages. Its recent evolution
reflects the tug-of-war between those unsavory players and the
administrators who want to protect their systems and their
users…”