Brad Templeton says it's the25th Anniversary of Spam:

That first spam was sent by a salesman for DEC - Digital Equipment Corporation. Today, you may not know DEC, since it was bought by Compaq and is now a unit of HP, but in those days it was the leading minicomputer maker, and its computers provided the platform for the development of Unix, C and much of the internet, to cite just a few minor events.

By 1978 the Arpanet (as the internet was then known) had already provided network E-mail to a large number of folks at universities, government institutions and universities for over 6 years. E-mail was the biggest source of traffic on the Arpanet. A few years prior, Dave Farber had created "MsgGroup," the first network mailing list. (Though Plato and other timesharing systems had laid the foundations for online community and conferencing some years before that.)

The DEC salesman, 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. It 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. The spammer didn't do a very good job. He simply typed addresses into his mail program, or possibly included them from a file. The mail program would only take 320 addresses. The rest got simply shoved into the top of the body of the message.

...via Dan Gilmor