The war against spam is mostly lost. People don’t care about the security of their PCs (if they even know what that means). Bot nets are here to stay. But the bots need crawlers that harvest mail addresses and scientists at the University of Indiana have found out that these come from a relatively small number of IP addresses. Blocking these would effectively cut off the spammers – from getting new addresses.
Until they train their bot nets to crawl.
Link: Blick in die Spammer-Trickkiste (German)