Four-fifths of the spam clogging ISPs' mail servers and affecting businesses emanates from so-called zombie home PCs infected with spam Trojans, according to security firm Sandvine.

The company found that Trojans, typically installed surreptitiously by worms or spyware, exploit vulnerabilities to bypass normal email routing and drop spam messages directly into end user machines.

Many of the most well-publicised worm attacks in recent months were launched expressly to install spam Trojans on unsuspecting end users' machines, waiting to be used at a later date as a spam delivery relay, Sandvine warned.

The report also found that the behaviour of spam Trojans taxes ISPs' infrastructures and, in the case of smaller ISPs, creates perceptions that some networks are generating more than their fair share of spam.

"Subscribers' in-boxes are bombarded daily and, while spam filters can provide an effective treatment, the scale and scope of the problem means that additional remedies are needed," said Marc Morin, co-founder and chief technology officer at Sandvine, in a statement.

"As a complement to existing mail server and client-based tools, service providers need to arm themselves with network-based anti-spam defences."