"You are seeing people move towards blended antivirus and antispam solutions - there are overlaps in spam and viruses in terms of looking for anomalies and certain behaviors, which has led to a surge in outsourcing for the two," Warrilow said.

"Spammers use botnets to infect users with spam tools and virus creators use spam to distribute their malware - they often use similar coding so it's obvious they're in each others' pockets," Ducklin said.

Spamming techniques include challenge-response, sender policy framework (SPF) or Microsoft's Sender ID, tagged message delivery agents (TMDAs) and Bayesian filters.