ClamAV is generally excellent. May be you don't have an active key installed. Check your general server logs and you'll find a line saying 'couldn't find key'. As for Spam, spam assasin works but you have to use the RBL system. If using multiple RBLs in plesk, click on 'server' then 'mail', enable the RBL + service option and add your lists. Separate them using a semicolon if using more than one.
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