damn_registrars's Journal: Why filtering is the WRONG answer for spam

In a word:

Profit

Spam filtering does nothing to cut into spammers' profits. It does nothing to remove the incentive from spam. Most spam comes from compromised mail systems, so even if a filter rejects and returns the spam, nobody cares.

More importantly, the filter doesn't do anything to prevent future spam. Spammers techniques evolve, just like the filters themselves.

As I've said before, the spam solution is to remove the profitability of spam, by making it more difficult for spammers to get customers to their spamvertised domains. If the profit is gone, the spam will go with it.