The History and Future of SMTP: SMTP's Adaptations to a Hostile Internet
http://www.freesoftwaremagazine.com/free_issues/issue_02/focus_spam_smtp/

Strauser, Kirk
Free Software Magazine
2005

Abstract:

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.