Email spam saw a massive decline at the end of last year. One company’s report shows a decrease of 150 billion spam messages per day from August to December. Although it’s difficult to say for sure what caused this decline, these are some of the best guesses.

Inactive botnets
Botnets are networks of infected computers which are the cause of the majority of the world’s spam. Just one of the botnets, Rustock, was responsible for nearly 50% of all global spam at its peak. Rustock, along with two of the other prominent botnets stopped churning out so much spam in December.

Typically, when botnets stop producing spam it is due to a temporary disruption in the network, but at least one of the three major botnets still seems to be intact. It could be that the people running the networks are just deciding to stop spamming, for whatever reason.

Anti-spam campaigns
Spammers are primarily driven by profit. When they stop seeing profit as a result of their efforts, they are likely to give up and move on to something else.

It seems that a number of recent anti-spam campaigns have caused several spammers to lose profitability and shut down. One of the biggest spammers, Spamit, closed in September due to increased attention caused by these campaigns.

Other outlets
One very credible explanation could be that spam isn’t declining so much as it is just being transferred to other forms of media. Twitter and Facebook, for example, have recently become huge outlets for spam.

Although all of these theories provide possible explanations, there is no way of knowing for sure what is causing this decline in spam. If you’re interested in reading more on the topic, check out this BBC News article.