To avoid and edit war, I would like to invite third party experts to jump in and look at this article. There is a culture of people who are behind SPEWS, APEWS, UCEPROTECT, and backscatter.org who are openly hostile to callback verification. I personally use callback verification in my spam filtering business Junk Email Filter and it works very well. However, admittedly it needs to be done correctly as callback verification has some gotchas. That's why I added sections to instruct people on how to correctly implement this spam fighting method without the side effects that others are concerned about.   