In recent days, e-mail users who had registered for Blue Security's "Do Not Intrude" list have instead been the target of a spam campaign and received extortion e-mail messages threatening to continue the campaigns unless the users remove their name from the Do Not Intrude registry, according to CEO Eran Reshef.

The message claims that Blue Security is "not playing fair," and that members can only avoid spam messages by removing their name from the Blue Security list.

"You are receiving this email because you are a member of Blue Security," the message reads, in part. "Due to the tactics used by Blue Security, you will end up receiving this message, or other nonsensical spams 20-40 times more than you would normally."

Blue Security launched its "Do Not Intrude" list in November, 2005. The company claims to have 500,000 registered e-mail addresses in its database, and business relationships with other antispam firms.

The Do Not Intrude program allows individuals to register an e-mail address with Blue Security and tracks spam messages with desktop client software known as "Blue Frog." When spam e-mail is sent to a Do Not Intrude Member, Blue Security traces the message to its origin, and then bombards the Web site behind the campaign, known as the "sponsor," with requests to remove the e-mail message from his or her distribution lists. Millions of e-mail messages translate into millions of "opt out" requests, bogging down the spammers' servers.