Since the first spam e-mail sent on the Arpanet, there's been a steady rise in the complexity of online threats, and the industry's best efforts to respond. Here's a historical review.
When it began: The first spam e-mail was sent in 1978 over the ARPAnet, the Defense Department network that was precursor to today's Internet, by a Digital Equipment Corp. marketing executive named Gary Thuerk to flog a new computer. To read it and see some of the reactions, see this entry on Brad Templeton's website.
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What it is: Mass mailings (usually advertisements, though today they are as often criminal attacks) via an ever expanding array of channels—including e-mail, newsgroups, instant messaging, comment fields in blogs, cellphones and VOIP telephone systems—to a large group of recipients who have not requested them and have no ability to remove themselves from the mailing list (The CAN-SPAM Act made it illegal to send unsolicited email without offering a way to opt out from future mailings.) Over time, spam has grown more malevolent, as criminals have made it the carrier for a host of scams, from identity theft to fraud to malicious software designed to control the recipient's computer.
Variant: spim, the name for spam sent by instant messaging
Response: IP address blacklists, Bayesian content filters, content heuristics engines, and content fingerprinting schemes augmented by sender authentication.