The Spamhaus Project has debuted a new whitelisting service that is designed to be the inverse of the way that most approved-sender lists work. The service comprises two separate parts: an IP address whitelist and a domain whitelist. The idea is that the whitelist will enable mail servers to separate incoming mail and identify messages from known good senders, known bad senders and unknown senders. The company recommends that users implement the whitlist in front of existing mail filters to make it most effective.
Source: https://threatpost.com/spamhaus-debuts-new-whitelist-service-092710/74516/