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If you are an "end user" and do not have update access to /etc/mail/spamassassin, then your configuration parameters should go into your $HOME/.spamassassin/user_prefs file. Assuming your mail host's configuration allows for individual configurations, this file is read and then merged with (and supercedessupersedes) site-specific and distribution parameters. (If your host's configuration does not allow for individual configurations, then chances are you do not have a $HOME/.spamassassin directory.)
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