Some time ago, in my battle against spam, I started using greylisting. Greylisting, for those not familiar with it, takes advantage of a standard fallback in email delivery systems. If a server is running but temporarily unable to accept incoming messages, it can send an error code saying so. RFC compliant servers are supposed to retry after a timeout period. Why is this useful? Because most spammers don't use compliant servers or don't have the time to retry.