Below is a good overview from the 25th anniversary.  Spam remains a major problem with email today and folks should always be careful in avoiding taking any actions other than deleting it.

http://www.templetons.com/brad/spam/spam25.html

QUOTE: In fact, the earliest documented junk e-mailing I've uncovered was sent May 3, 1978 -- 25 years ago this month. (It was written May 1 but sent on May 3.) And in a surprising coincidence (*), just a month ago marked the 10th anniversary of March 31, 1993, the first time a USENET posting got named a spam

The DEC marketer, Gary Thuerk, identified only as "THUERK at DEC-MARLBORO" (There were no dots or dot-coms in those days, and the at-sign was often spelled out) decided to send a notice to everybody on the ARPANET on the west coast. In those days there was a printed directory of everybody on the Arpanet which they used as source for the list. The message trumpeted an open house to show off new models of the Dec-20 computer, a foray into larger, almost mainframe-sized systems.

This was a spam, though the term would not be used to refer to it for another 15 years. Thuerk had his technical associate, early DEC employee Carl Gartley, send the message from his account after several edits. Alas, at first he didn't do it right. The Tops-20 mail program would only take 320 addresses, so all the other addresses overflowed into the body of the message. When they found that some customers hadn't got it, they re-sent to the rest.

More on the History and Types of SPAM
http://en.wikipedia.org/wiki/E-mail_spam