Internet service providers (ISPs) across the globe rely on the Spamhaus Project, which claims it blocks up to 50 billion spam e-mails per day.
Judge Charles Kocoras of the U.S. District Court for the Northern District of Illinois threatened to shut down Spamhaus earlier this month for ignoring a US$11.7 million judgement against it for listing an e-mail company called E360Insight in its database of known spammers.
Spamhaus, based in London, has said that it ignored the judgement because it cannot be enforced in the UK.
Peter Stewart, Australia/New Zealand managing director of security e-mail provider TotalBlock, fears the court order will set a precedent for the closure of other blacklist organizations and without their protection global business communications could be severely disrupted.
"Blacklisting was never an efficient way of curbing spam, since far too many innocent e-mail users are wrongly listed and find it very difficult indeed to clear their names from blacklists," he said.
"Legal action against such lists was always on the cards, instigated either by legitimate e-mail users or spammers."
He said that if blacklists were closed down ISPs would likely continue to use existing blacklist information, but its effectiveness would diminish quickly as new spammers came online.
"The resulting flood of spam would leave business people ploughing through a host of unwanted e-mails for every legitimate message received, which would curtail commerce significantly," Stewart said.
"Crime would rise sharply too, as e-mails bearing phishing scams, spyware, keyloggers and other malware arrived unchecked in millions of in-boxes. Overall the closure of blacklists would have a dramatic effect on global business efficiency."
Stewart likened the effect to police stations going on strike, leaving skeleton staff to try to cope, unsuccessfully, as the criminal element gained in strength and confidence.
The Debian Project has been a user of Spamhaus for a number of years and will be impacted greatly should the service be shut down.
"We use the Spamhaus SBL+XBL list to block over 120,000 spam emails a day to our bug track server, and 10,000 spam emails a day to our list server," said Debian Project member Pascal Hakim. "It's one of the few blacklists we are able to use without complaints from users."
This amount of spam is a big number for Debian. "You would notice 130,000 spam messages in your inbox," he said.
Despite the court order, the Internet Corporation for Assigned Names and Numbers (ICANN), said in a statement it does not have the ability or authority to enforce the court order to suspend the Internet service of Spamhaus.
Both ICANN, the nonprofit organization set up to manage the domain name system of the Internet, and Toronto-based Tucows Inc, the Spamhaus.org registrar, are named in the order.
"Even if ICANN were properly brought before the court in this matter, which ICANN has not been, ICANN cannot comply with any order requiring it to suspend or place a client hold on Spamhaus.org or any specific domain name because ICANN does not have either the ability or the authority to do so," the organization said.
ICANN said that in most cases only the Internet registrar with whom the registrant has a contractual relationship can suspend an individual domain name. Tucows officials could not be reached for comment.
(Linda Rosencrance and Howard Dahdah contributed to this article.)