"You are aiming for accuracy and speed; there is no 'one technique' that works for everything so a good product must include a number of techniques," Ducklin said.

"Spam filtering systems must use a variety of different mechanisms for analyzing and categorizing e-mails, [which] may include older filtering techniques, that deploys them in a way that gives accurate results quickly."

He said challenge-response filters, such as blocking, slow the process down by adding an unnecessary layer.

"Challenge and response filters work similar to greylisting in that they attempt to determine the legitimacy of a source by [issuing a challenge] to incoming mail; however, this has the same shortcomings where it fails to recognize legitimate mail sent through different servers."

Warrilow and Ducklin said similarities between virus and spam techniques have led to a merging of antivirus and antispam solutions.