I'm not professional sysadmin and I don't have professional experience in this, but I have read this article some time ago and it makes sense to me.
http://www.techrepublic.com/blog/it-...ight-spammers/
Quote:
Giorgio Maone (NoScript): Blacklisting has always been the weakest form of protection in security, on principle. A much larger address space just makes this more evident. But, it's hardly news. For example, Mark Ranum, father of the firewall explains in this old editorial.
I believe statistics method to recognize spam, e.g. Bayesian filters, are the only really scalable solution, for now at least.
[..]
Cameron Schmauch (EdgeWave): In my opinion blacklists and whitelists have been obsolete for several years. This is not so much because it's hard playing whack-a-mole with the spammers, but rather that the lists usually aren't maintained in such a way as to aggressively prevent False Positives (FP).
EdgeWave (Powered by Red Condor) hasn't relied on third-party blacklists for anything other than supplemental information. We do employ methods for sussing out IP blocks that are operated by spammers. Blocking on IP can be very effective and efficient, but it should not be the mainstay technology if keeping FPs to a minimum is your top priority.
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Bayesian filters for emails and captchas (or other problems to prove client is a human and not a bot) for www.