I want to try to keep this as non-spam as possible, but Symantec acquired a company about 5 years ago called TurnTide that did almost *exactly* that. Take the reputation of the sending address, and shape the TCP/IP packets to slow down the rate of mail into the system. Symantec touts a 70% reduction in mail volume and an 80% reduction in the amount of spam that hits a mail server. I've had it in production in one environment where the customer went from approximately 5 million messages/day to 500,000 messages/day.