SPEWS claims that many of the addresses are used by home computer owners who are unaware that their machines have been hijacked and are being used to churn out spam.
Taken together these exploited PCs form a network of bots (short for robot), and are used to increase the speed and breadth of spam attacks.
SPEWS, set up by a group of anti-spam activists, contains a list of known spammers and spam operations, which is used by ISPs and other companies to identify and block incoming spam.
According to the BBC, over 900,000 customer addresses owned by Telewest's Blueyonder broadband service have now been placed on the list.
The cable company is now working to help those customers clean their computers, and to strengthen its network security systems, says the report.
