Why do they Spam Us?

O’Reilly Radar > Spamonomics 101
Spammers have to get out a high enough volume of spam that the small sliver of profit is greater than the cost of sending it.

These economics drive the patterns of spam we receive. Traditionally porn advertisements have the highest click-through rate, followed by pharmaceutical advertisements, though penny stock spam is gaining popularity. And the spam messages that aren’t advertisements, scams, or virus attacks, but just random strings of text? Ken Simpson comments, “Those messages are sent by spammers to poison the spam filters. When someone receives a message full of gibberish and reports it as spam, the spam filters tune themselves to recognize gibberish as spam—which reduces their overall accuracy.”