Whitelist

From Spam Wiki

A whitelist can be used to add exceptions to a blacklist policy. For example you can allow to recieve mail from some specified senders even if their messages contain blacklisted words.

Other use of whitelists is to create strict recieving policies: if an e-mail comes from a non-whitelisted address the mail server will block the message. This type of whitelisting can be used only at few cases although this is a very effective way of filtering spam.