Email as a communication device has been around for a long time. The first unsolicited message to a group of users may have been as far back as 1971. Tom Van Vleck, co-author of the CTSS MAIL command, reports a message sent on MIT's Compatible Time Sharing System (CTSS) that year. A system administrator named Peter Bos used CTSS MAIL to send everybody the anti-war message that read: "THERE IS NO WAY TO PEACE. PEACE IS THE WAY."
The first marketing email sent is often thought of by true historians of online nerdery to be a message sent by DEC (eventually part of Compaq, now HP) in 1978. DEC announced a new DEC-20 machine by sending an invite to all ARPANET addresses on the west coast, using the ARPANET directory, inviting people to receptions in California. The community chastised DEC at the time for breaking the ARPANET appropriate use policy, and a notice was sent out reminding others of the rule.
In 1994, the law firm Canter and Siegel out of Phoenix posted an advertisement for green card assistance to more than 7,000 newsgroups. The company had posted its message a few times before, but on April 12, it hired a mercenary programmer to write a simple script to post its ad to every single newsgroup (message board) on USENET, the world's largest online conferencing system.
Because of its online advertising strategy, Canter and Siegel was hounded, mail-bombed, and kicked off one service provider after another. Unfortunately, this strategy was labeled spamming.
Soon after, Web-based email applications launched. The advent of Hotmail and those other services quick to follow gave marketers direct access to massive numbers of email addresses to which they could send marketing messages. The effectiveness borne of the targetability quickly led to the spread in popularity of this form of marketing.