Multiple friends have forwarded a long this very detailed CNet News article on email filterer Postini (now owned by Google). Lots of good detail there, and it's definitely worth reading.
From my perspective of helping senders, Postini has always been a bit of a disappointment to me. Some of their filtering choices seemed pretty random and we often used to run into issues where somebody was filtered by Postini, but not by anybody else. And the mail being filtered would be order confirmations, details about your upcoming flight reservation, etc. When they were acquired by Google, I was hopeful that some of Google's good sense would trickle down on to Postini. Gmail has great spam filtering, and they do it while rejecting much less amounts of mail than other ISPs and webmail providers tend to. It's hard to say if my wish came true, standing here on the outside looking in.