FairUCE: The Smart E-Mail Proxy

FairUCE, which stands for Fair use of Unsolicited Commercial Email, is an SMTP proxy, running between multiple instances of Postfix, that verifies e-mail by attempting to verify the sender through lookups, a user customised challenge/response. It claims to be able to stop a vast majority of spam without the need for content filters, and virtually eliminates spoofed addresses, phishing, and even many viruses with a few cached DNS look-ups and a couple of if/then statements