Microsoft's Bill Gates once predicted the problem of email spam would be solved by 2006. Ha! Ha! Ha! That's like calling something "raccoon-proof."

An excerpt from the NYT story:

Antispam companies fought the scourge successfully, for a time, with a blend of three filtering strategies. Their software scanned each e-mail and looked at whom the message was coming from, what words it contained and which Web sites it linked to. The new breed of spam — call it Spam 2.0 — poses a serious challenge to each of those three approaches.

Spammers have effectively foiled the first strategy — analyzing the reputation of the sender — by conscripting vast networks of computers belonging to users who unknowingly downloaded viruses and other rogue programs. The infected computers begin sending out spam without the knowledge of their owners. Secure Computing, an antispam company in San Jose, Calif., reports that 250,000 new computers are captured and added to these spam “botnets” each day.

The sudden appearance of new sources of spam makes it more difficult for companies to rely on blacklists of known junk e-mail distributors. Also, by using other people’s computers to scatter their e-mail across the Internet, spammers vastly increase the number of messages they can send out, without having to pay for the data traffic they generate.

“Because they are stealing other people’s computers to send out the bad stuff, their marginal costs are zero,” said Daniel Drucker, a vice president at the antispam company Postini. “The scary part is that the economics are now tilted in their favor.”

The use of botnets to send spam would not matter as much if e-mail filters could still make effective use of the second spam-fighting strategy: analyzing the content of an incoming message. Traditional antispam software examines the words in a text message and, using statistical techniques, determines if the words are more likely to make up a legitimate message or a piece of spam.

The explosion of image spam this year has largely thwarted that approach. Spammers have used images in their messages for years, in most cases to offer a peek at a pornographic Web site, or to illustrate the effectiveness of their miracle drugs. But as more of their text-based messages started being blocked, spammers searched for new methods and realized that putting their words inside the image could frustrate text filtering. The use of other people’s computers to send their bandwidth-hogging e-mail made the tactic practical.

“They moved their message into our blind spot,” said Paul Judge, chief technology officer of Secure Computing.