Since the free service was launched on a trial basis last summer, Blue Security claimed to have signed up more than 500,000 subscribers -- including corporate customers -- and had gotten six out of the top 10 spammers to abide with their opt-out lists.

But the speed at which the company folded following the DoS attacks suggests that it was ill-prepared to deal with the consequences of attacking spammers directly, said Todd Underwood, chief operations and security officer at Renesys, a Manchester, N.H.-based Internet monitoring company.

"Spammers have a history of launching massive attacks against those who try to stop them," Underwood said. While others have taken precautions against such attacks by building more redundancy into their networks, Blue Security appears to have been "woefully under-prepared for the things that happen when you attack spammers," he said.

More importantly, the incident is an alarming reminder of how the vast botnets available to spammers for sending bulk e-mail can be easily used to launch devastating attacks, said John Levine, chair of the Internet Anti-Spam Research Group.

The attacks against Blue Security caused serious disruptions for five service providers who were hosting the company's servers. It also interrupted service at Tucows Inc., a major provider of core DNS services based in Toronto. Thousands of blogs being hosted by San Francisco-based Six Apart were also knocked offline after PharmaMaster launched a DoS attack against a server hosting a Blue Security blog. In an interview following the attacks, company CEO Eran Reshef said the attacks appear to have been carried out with the help of someone within a large Internet Service Provider.