SpamAssassin manages an automatic whitelist (AWL). It actually functions as both an auto-blacklist and an auto-whitelist. Generally, an auto-blacklist is ineffective as spammers rarely use the same email address for any period of time. However, SpamAssassin tracks both the IP address of email sources and the email addresses used, adding to its effectiveness.
The auto-whitelist keeps a record of the SpamAssassin scores for emails from senders. Senders that only send ham emails receive a weighting towards ham by the auto-whitelist. If they later send an email marked as spam, then the SpamAssassin score for the new email will be adjusted downwards, due to their past behavior as a source of only ham emails. The converse is true of those who normally send spam.
The AWL works by adjusting the score of the email being processed towards the average of all previous emails received from that sender. The amount or strength of this adjustment can be altered using the auto_whitelist_factor...