Our company developed a product called clearmail that is white list based. Something you can not do with rules.
check us out:
http://www.corriganinc.com/clearmail
DNS Blacklist features of Domino 6 in conjunction with a white list has eliminated the spam issue for us and our clients and we do not worry about false positives because E-Mail is not getting blocked, just re-routed to a spam folder.