I have several domain names that I'm not currently using. I previously had three of these domains pointing to the same email account. I hadn't checked it in a while, but when I went in last week it had managed to collect 60,000 emails. I found one that was sent there by mistake, and have since found out that two or three more might also have been sent there by mistake. But that's still a hell of a lot of spam!
As a test, I set up a secondary Gmail account and set my unused domains to redirect their mail to that account. That was Friday night. I just checked the Gmail account and there are 320 pieces of spam sitting in my inbox. Which sounded like a lot, until I looked in the "Spam" folder and found out that Google nabbed 4,390 more emails that it automatically recognized as spam.
Wow. Out of 4,710 pieces of spam, only 320 (~7%) slipped past Gmail's spam filter. Kick ass! And my guess is that if I begin marking the ones that got through as spam, it will help Google's algorithms recognize even more in the future. Groovy.