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Blue Security's BlueFrog service responded to spam sent to its customers with an automatic opt-out reply. With some 450,000 customers, the volumes of opt-outs quickly flooded the servers of spamming groups everywhere, resulting in six of the 10 biggest groups agreeing to filter out spam sent to BlueFrog users entirely, according to

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SecurityFocus.

However, other spammers weren't so amenable, and began a campaign of denial of service attacks in retaliation.

Furthermore, Blue Frog users received threatening emails warning of more spam unless they stopped using the service.

Blue Security's initial response to the attacks was to move its homepage to a blog hosted at Six Apart's TypePad service. The attacks followed, taking down the TypePad and other services until Blue Security returned to its domain.

Yet the attacks have continued, making Blue Security's continued operation extremely difficult and leading to the decision to suspend antispam activity as of Tuesday.

The company said it couldn't be held responsible for the escalating cyberwar it was seeing and that it would explore other non-spam opportunities.