Spam now makes up more than 80% of mail message traffic. The first recognizable e-mail marketing message was sent on 3 May, 1978 to 400 people on behalf of DEC - a now-defunct computer-maker. You can see the first spam message here (archived version) including interview with Thuerk, the sender. From the BBC news page:
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Statistics gathered by the FBI suggest that 75% of net scams snare people through junk e-mail. In 2007 these cons netted criminals more than $239m
Statistics suggest that more than 80%-85% of all e-mail is spam or junk and more than 100 billion spam messages are sent every day.
The majority of these messages are being sent via hijacked home computers that have been compromised by a computer virus.