The Spamhaus Project has told a US court that it plans to appeal against a recent ruling that threatened the volunteer organisation with millions of dollars in legal fines and a possible shutdown of its database of known spammers.

Spamhaus, based in the UK, has a team of 25 investigators and claims to block between eight billion and ten billion email messages per day. Its database is used by several major security vendors, including Microsoft.

The filing marks the group's return to a legal fight against an email marketing company called e360 Insight LLC that Spamhaus had tried its best to ignore.

Spamhaus had not shown up in court to dispute the charges against it, and the organisation has made it clear that it does not accept US jurisdiction over its activities. However, this position was apparently reconsidered after a proposed court order was published earlier this month calling on Icann, the Internet Corporation for Assigned Names and Numbers, and Spamhaus's registrar, Tucows, to pull the project's domain name.

"Spamhaus is concerned by how far a US court will go before asking itself if it has jurisdiction, and is intending to contest the ruling in order to stamp out further attempts by spammers to abuse the US court system in this way," the organisation said in a statement.

The order, written by the plaintiffs in the case, does not have the force of law, but if the judge decides to accept it, observers said that it could kick off a power struggle between Icann, which is responsible for the internet's domain-name system, and the US courts.

E360 Insight could not be reached immediately for comment. In a statement on its website, the email marketer cited Spamhaus's earlier decision to walk away from the case as evidence that it "could not produce its justification for the listing of e360 on its blacklists".