The monthly State of Spam & Phishing report has been tracking the decline of overall spam in recent months. The chart below shows the global spam volume dropping significantly since August.

An even more significant drop was observed during the Christmas holiday. The chart below shows the global spam volume for the month of December 2010 and first few days of January 2011.

What could have caused such a big drop? We have some potential answers. According to MessageLabs, there was a huge reduction in output from the Rustock botnet, which was by far the most dominant spam botnet in 2010. Since December 25, the Rustock botnet has basically disappeared as the amount of spam from it has fallen below 0.5% of worldwide spam. In addition to the decline in the Rustock botnet activity, MessageLabs also pointed out that two other major botnets disappeared off of the spam map. The Lethic botnet has been quiet since December 28, and the Xarvester botnet went silent on December 31. The chart below shows relative botnet spam volumes.

Did the people in charge of these botnets suddenly go on vacation? Currently there are no explanations on why these botnets stopped spamming. Visit the State of Spam homepage for up-to-date information as we wait to see when the volume will pick up again.