An Israeli firm that specialises in bouncing spam in aggressive fashion, has had its own “do-not-spam” user list targetted by an angry spammer.

Blue Security launched its “do-not-intrude registry” last July, which critics said amounted to vigilantism because it traced and sent opt-out email to the senders of unwanted email.

Now the spammers have struck back by emailing threats to a portion of Blue Security’s 450,000 members, demanding they stop using the service.

Normally, the list the spammers are told to steer clear of for fear of being deluged with returns is encrypted – the spammers use an identical algorithm to determine a permitted list of names. The implication is that this has somehow been reverse engineered, allowing them access to some of the user list.

"We've been using passive anti-spam methods for the last four to five years and it's not working," company CEO Eern Reshef was reported as saying when launching the service nearly a year ago.

Now he is equally defiant. "Spammers are feeling the pressure. This incident is only a futile attempt by a degenerate spammer to fight back through intimidation and extortion," he was quoted as saying.

The company has maintained that spammers are still falling into line with the scheme, but the fact that its own list is now under attack undermines its argument. The best defence against spammers remains layered filtering, or simple invisibility.

No doubt, those who would like to turn money from delivering email using a guaranteed service, will also take heart from events.

Blue Security’s company website and Blue Frog anti-spam site were down on Thursday.