This sharp increase can be largely attributed to the Srizbi botnet, which is currently responsible for 46% of all spam sent, said Marshal.
Malicious spam jumped from 3% of total spam traffic at the start of June to 9.9% the following week.
Malicious spam is spam that is not designed to sell a product or service, but is intended specifically to infect recipients' computers with malware.
It typically involves a social engineering ploy to lure recipients into thinking it is harmless or related to something of interest, such as free pornography or an invitation to view a greeting card from a friend.
It usually includes a URL link to a website hosting malware. Often the malware is falsely presented as a video or game that the recipient is tricked into activating.
Once the malware is loaded onto users' machines it can be used to turn their PCs into active participants of the botnet to spread further spam and to steal their identities for criminal activities.
Phil Hay, lead threat analyst with Marshal's Trace security team, said, "The Srizbi botnet is behind much of the increase in malicious spam. Srizbi's criminal controllers are currently on a major expansion drive. The more computers infected by Srizbi bots the more money they can make".