The recent global decline in spam could be short lived, researchers have suggested.
According to security firm NetWitness, the Rustock botnet, which mysteriously shut down over the holiday period, has emerged from its Christmas hibernation to resume normal service.
Speaking to the BBC, Alex Cox of NetWitness said that the botnet restarted early on 10 January, and that the reason behind the lull is still unclear.
"As best we can tell, they took a holiday," he suggested. "The people running Rustock are running a business - albeit an illegitimate one - so maybe they needed time off too."
But senior analyst at Symantec Paul Wood suggests that the shut down was most likely due to the botnet owners renting it out to new spammers. He said that the spamming business is mostly automated, and so doesn’t require people sat at computers to operate.
"Previously Rustock was primarily sending out spam related to a group known as the Canadian Pharmacy. The spam we're seeing today is for Pharmacy Express," he told the BBC.
Since reactivation, the Rustock botnet has sent out an estimated 70 billion spam e-mails, accounting for around 30 per cent of all spam worldwide, The Guardian reports.