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The auto-whitelist, or AWL, tracks scores for your regular correspondents in a small on-disk database. It In 3.0, it is enabled by default and turned off with the configuration setting use_auto_whitelist 0. In earlier versions, it is enabled using the -a flag to spamassassin or spamd.
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auto_whitelist_factor can be tweaked in the configuration, and you may find this useful when starting off. The contents of the database can be examined using the program 'tools/check_whitelist'.
How do I whitelist someone?
Not with the auto-whitelist! The auto-whitelist is an averaging system. Instead, you want to use the manual whitelist. See ManualWhitelist.