Mark, I recommend reviewing Bruce Barnes' anti-spam settings documentation below. It contains a lot of great information pertaining to SmarterMail anti-spam settings. Many people have seen significant improvements in catching spam by adhering to it.
EDIT: MessageSniffer and SpamAssassin are also good 3rd party addons/apps that will help with catching spam.
http://portal.smartertools.com/community/a444/smartermail-antispam-settings-document-updated.aspx