In the second half of 2009, the number of spam messages sent per day skyrocketed from 600 million to three billion. Researchers at M86 Security found that 78 percent of all spam was sent by computers that were part of one of the five largest botnets. The major botnets were the main culprits, using the millions of compromised machines as spam-spewing zombies. Much of this spam involved scams tied to financial fraud, specifically credit-card theft or attacks that involved loading a password-stealing Trojan onto the victim s machine.
Source: https://threatpost.com/malicious-spam-jumps-3-billion-messages-day-021610/73539/