About 90 per cent of all email sent worldwide is spam, according to a Cisco report released on Monday.
The report, from computer networking company Cisco, says that accounts for nearly 200 billion messages daily.
The United States is the biggest source of spam, accounting for 17.2 percent of the messages. Turkey and Russia ranked second and third, accounting for 9.2 percent and 8 percent of spam respectively, according to Cisco.
This year, botnets were used to inject an array of legitimate websites with an IFrames malicious code that reroutes visitors to websites that download computer viruses into their machines, according to Cisco.
"Using malware to infect someone's computers is an incredibly common mechanism and harnessing them all together is a way they do their click fraud, spam emails, and data stealing," the report said.
As computer security vendors such as Cisco get better at protecting machines from hackers and users grow wary of clicking on unsolicited Web links or email attachments, online criminals are turning botnets on Web-based email accounts.
Hackers are "reputation hijacking" by using botnets to figure out weak passwords protecting Web-based email accounts, according to Peterson.
Weak passwords consist of family names, birthdays, home addresses, or other terms considered relatively easy to deduce.
Once access is gained to legitimate email accounts, a plethora of spam messages are sent in the owners' names.