Rustock stopped using TLS, an encryption protocol, thus decreasing overhead and maximizing throughput. The controllers seem to have concluded that TLS gave them little or no discernible benefits and instead impeded their sending capacity. The botnet currently sends some 46 billion spam emails per day, which is up from about 43 billion when it had control of twice as many computers.
Overall, the global ratio of spam in email traffic this month increased 3.3% to 92.2%, or 1 in 1.08 messages. Hungary is the most spammed country at 96.3%. One out of every 327.6 emails carries a virus, and one out of 363.1 is a phishing attack. Interestingly, the automotive industry was the most spammed sector with a spam rate of 94.8%. You can read the full 15-page report here (PDF).