Worldwide spam volumes are being driven even higher by an increase in online pharmacy messages, according to the latest research.

Security firm Marshal8E6 said in a new report that spam is now responsible for 90 per cent of all email traffic, and that 75 per cent of that spam is promoting online pharmaceuticals.

The researchers found that much of the spam traffic is driven by botnets. A recent report from MessageLabs reported similar findings.

Marshal8E6 highlighted the Rustock botnet as among the worst offenders, noting that the infected machines are known for distributing particularly convincing spam messages.

"Rustock typically uses HTML templates from legitimate newsletters and inserts, or blends in its own images and URL links," said Bradley Anstis, director of technology strategy at Marshal8e6.

"This helps give Rustock spam the appearance of professional, legitimate email which tricks recipients into clicking on the links or buying the advertised products."

Other trends noticed by researchers are the increased targeting of social networking sites, and a rise in image spam.

The security assessment from Marshal8e6 points to a relentless rise in spam volumes during 2009. Initially devastated by the shutdown of hosting service McColo in November 2008, spammers rebounded in the early months of the year and later weathered the loss of hosting firm 3FN to keep overall spam levels high.