Various sources estimate that junk mail is responsible for 40% to 95% of all SMTP traffic. There is no shortage of vendors offering anti-spam products, but all those products fail to address the root cause of the problem: Internet messaging allows anyone to send as many messages as s/he wants.
full story: http://www.hexview.com/sdp/node/38
full story: http://www.hexview.com/sdp/node/38