In order to fight spam, you have to know and understand spam.
Lets take a look at some of the reasoning behind spam.
Spam is somewhat equivalent to paper junk mail. The difference is that third-class mailers pay a distribution fee. With spam, either you or ISP's pay the fee. These payments are in the form of reduced bandwidth, wasted disk space, overwhelmed server resources, lost productivity and money. A more direct example, something that some of you can relate to, is checking your email on the road.