Spam blacklist targets Telewest
The Spam Prevention Early Warning System blacklisted the addresses because many of the machines using them have been hijacked by spammers.
The army of remotely-controllable machines have probably been recruited by viruses and worms.
Telewest said it knew about the problem and was working with customers to regain control of their home computers.
In late April, Spews announced that it had started blocking more than 900,000 net addresses used by Telewest's Blueyonder broadband service. Many were suspected of being used by spammers.
In a statement Telewest said: ""We are aware of the increase in e-mail volumes due to customers' PCs which have been infected by worms and viruses."
Telewest blamed recent virus outbreaks for the sudden rise in the number of hijacked home PCs.
"We are currently contacting affected customers to help them clean their PCs which, as you can imagine, is a time-consuming task," it said.