"They abuse their position not to stop spam but to exercise censorship without a court order," Kamphuis said.
Gilmore and Prince said the attack's perpetrators had taken advantage of weaknesses in the Internet's infrastructure to trick thousands of servers into routing a torrent of junk traffic to Spamhaus every second.
The trick, called "DNS reflection," works a little bit like mailing requests for information to thousands of different organizations with a target's return address written across the back of the envelopes. When all the organizations reply at once, they send a landslide of useless data to the unwitting addressee.
Both experts said the attack's sheer size has sent ripples of disruptions across the Internet as servers moved mountains of junk traffic back and forth across the Web.
"At a minimum there would have been slowness," Prince said, adding in a blog post that "if the Internet felt a bit more sluggish for you over the last few days in Europe, this may be part of the reason why."
At the London Internet Exchange, where service providers exchange traffic across the globe, spokesman Malcolm Hutty said his organization had seen "a minor degree of congestion in a small portion of the network."
But he said it was unlikely that any ordinary users had been affected by the attack.
Hanna said his site had so far managed to stay online, but warned that being knocked off the Internet could give spammers an opening to step up their mailings — which may mean more fake lottery announcements and pitches for penny stocks heading to people's inboxes.
Hanna denied claims that his organization had behaved arbitrarily, noting that his group would lose its credibility if it started flagging benign content as spam.
"We have 1.7 billion people who watch over our shoulder," he said. "If we start blocking emails that they want, they will obviously stop using us."
Gilmore of Akamai was also dismissive of the claim that Spamhaus was biased.
"Spamhaus' reputation is sterling," he said.