Greylisting does not throw anything out. It does the exact opposite. The mail server simply says, "I have a temporary problem, so I can't take this right now. Try again later" Mail servers are supposed to (and do) try again, at which time a greylisted message will be delivered. Accepting a message for delivery In Go Faith and then dropping it on the floor, is poor, lazy, cheap, RFC-breaking spam filtering.
What I mean is that greylisting is a trick mechanism where some mails, declared spam based on an unrelated and technical criteria, never reach the user's spam folder. This contradicts the 'spam should go to spam folder' point.
