Ten Ways to Avoid Spam: http://www.viruslist.com/en/spam/info?chapter=153350654
Spam - What exactly is it?: http://www.viruslist.com/en/spam/info?chapter=153350526
Evolution of Spam: http://www.viruslist.com/en/spam/info?chapter=153350530
Barracuda Spam Blocking Techniques: barracuda_spam_blocking_techniques.pdf