Three Web sites that provide spam blocking lists have shut down as a result of crippling Internet attacks in what experts on Thursday said is an escalation in the war between spammers and opponents of unsolicited e-mails.
Anti-spam experts said that they think spammers are behind the attacks, although they have no way of proving it.
The technological war comes as Congress considers a federal anti-spam law and California adopts what is widely considered to be the toughest law in the country.
The California law, signed on Tuesday, allows people to sue spammers for US$1,000 per unsolicited e-mail and up to US$1 million for a spam campaign.
"This definitely marks an escalation in the spam wars," Andrew Barrett, executive director of The Spamcon Foundation, a spam watchdog group, said of the recent Internet attacks on lists used to block spam.
"Before, it was a guerrilla war ... This is the first time we've seen [spammers] employ such brazen tactics," he said.
Anti-spam advocates maintain hundreds of spam block or "black hole" lists, which are Web sites with lists of the numerical Internet protocol addresses of specific computers or e-mail servers that are unsecure or are known sources of spam.
Network administrators and Internet service providers consult the lists and block e-mails coming from those computers as part of their spam filtering techniques.
Two of those spam block lists have shut down after being attacked by denial-of-service attacks, in which compromised computers are used to send so much traffic to a Web site that it is temporarily taken down. The operator of another list shut down fearing a pending attack.
"There seems to be a methodical well-planned attempt to use pre-assembled networks of zombie machines to create sustained denial of service attacks on servers where these block lists run," said Barrett.