Security software vendor Symantec has found that the volume of spam sent from botnets accounted for 95 per cent of total spam sent in August.
This has increased from 84 per cent of spam sent in April.
The company also said that the Rustock botnet was responsible for sending 41 per cent of the spam in August, even though the number of computers infected by the botnet have been reduced significantly.
Symantec said that computers infected by the botnet fell from 2.5 million in April to 1.3 million in August, but that the volume of spam sent by the botnet increased from 32 per cent in April to 41 per cent.
According to Symantec, the reason behind the high levels of spam by the botnet is that it has stopped using TLS encryption.
“It is likely that because TLS slow connections due to the additional encryption processing required to send a spam email, the botnet controllers realized that this tactic impeded their spam-sending capabilities,” Paul Wood, MessageLabs Intelligence Senior Analyst, said in a statement.
The spam metrics are a part of the MessageLabs Intelligence Report released by Symantec.