A couple of month ago we installed a spam grey listing system on to a couple of our servers to reduce the server load caused by floods of spam emails. The way it works is to identify any email that has a spam score over a certain threshhold and null route the source ip address for a time period. This time period ranges from 1 hr to 2 weeks depending on the type of email and the spam score.
After a few initial teething troubles (eg grey listing a few of our own hosting customers) the system is running smoothly and seems to have made a dramatic difference in the volume of spam emails we process. A less expected consequnce is an across board 10% reduction in web site visits but without any noticable reduction in conversions or revenue. It seems that in blocking the sources of spam we are also blocking some of the less desirable visitors such as scrapers and referrer spam scripts. This makes sense as the grey list seems to be largely blocking zombies and blackhat servers.