Greylisting works because nearly all the unwanted (“spam”) messages are not being sent from real email servers. Instead they come from compromised home PCs that are skimming addresses from local owners’ inboxes or contact databases, and spewing massive amounts of email. To avoid detection, these networks of PCs have a minimal software footprint — they are tiny programs. And they do not contain any inter-server messaging capability. They are not looking to hear back from target email servers