Ironport and senderbase.org (Score 1)

A much more feasible option is to use the Ironport appliance to replace your public MX. Ironport does DNS lookups for each inbound connection to get a reputation score for the connecting IP from senderbase.org. Senderbase monitors nearly 30% of the world's e-mail and gives each IP address sending mail a reputation score. If the score is too low (you can select how sensitive you want to be) then the Ironport never even sends back an ACK to the connecting SMTP client, making it look like you arent even there while still allowing reputable servers to send mail your way.