SpamAssassin is an extensible email filter which is used to identify spam

spamassassin adds a header line that shows if the mail has been determined spam or not. This way, you can decide what to do with the mail within the scope of your own filtering rules in your MUA (Mail User Agent, your mail program) or your LDA (Local Delivery Agent). 
 See the files in the documentation directory /usr/share/doc/packages/spamassassin/ for more information on how to use the filter.