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is best if you want to have a rule only run when a particular user runs SA. Bear in mind that this is the user that executes SA, not the user who the mail is addressed To:. If you run SA from a sendmail filtermilter, the only user_prefs that will be used is the one for the user sendmail runs as (usually root). Even if you do have a site-wide configuration, user_prefs may be useful to you by allowing you to add rules to another user account's user_prefs and test them using the command line prior to adding them to your local.cf.
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