Nolisting is a anti-spam feature that requires an extra IPv4 address. When an email is delivered, the first prioritized MX will not respond. The senders MX will then try the second MX (spammers will not), which will accept the email, so it filters a lot of spam.
More information and statistics: http://nolisting.org
Worth implementing default instead of greylisting or as a feature that can be enabled if someone has multiple IPs?
