A couple of weeks ago, an American ISP called McColo was shut down after accusations that it was helping spammers send out junk mail including online fraud and child pornography.

It was shut down and suddenly the amount of spam on the Internet slumped by a whopping two-thirds.

But now it's creeping back up to its old levels again, although security experts are disagreeing as to how much spam there is out there.

Says the Beeb, around 450,000 infected computers have been spotted trying to connect to the largest of the networks McColo hosted, and these allegedly account for around half of the world's spam.

But the actual figure for the level of spam is contentious.

Messaging security firm IronPort Systems claims that spam remains at a level below half that prior to the McColo shut-down, whereas MessageLabs says that the level is about two-thirds.

What they do seem to agree on, however, is that spam is back on the rise.