It seems that everybody and his brother has solution to the growing spam problem these days. At out office, three out of every more mails is spam. We filter and delete by hand, but nothing is perfect.
The solution I'm proposing won't work for a small outfit like ours, but should work for operations like hotmail, with huge mailservers. You set up a couple of bogus e-mail accounts that you have spam bots harvest, respond to dictionary attacks etcetera. Make sure with these accounts you're on every spam list out there.
Then when a mail comes in, you check whether this mail has already been received by one of the bogus accounts. If so, it is spam. If not, probably not. You might have to hold the mail for a little bit, in order to give the bogus accounts some time to catch up.