IF you can rate-limit them across the whole Internet, If you limit 2 million machines to 20 msgs/day per mail server you are back up to your 10 Billion msgs/day mark. This is where DCC or other distributed checksum systems come into play.
If you rate-limit 2 million compromised machines to 20 msgs/day each, there's only 400 million spams. Total.
-Matt
