Please take a look at our software, Spam Dam for Lotus Notes, at
http://www.spamdam.net.
Spam Dam allows users to automatically build their own "whitelists" of accepted email addresses by using challenge/response to confirm that email is coming from legit people. Spam Dam requires no administrative effort and very little user intervention, and works with nearly 100% effectiveness.
There's a 30-day risk-free version available on the website, so please try it out (and send feedback!). Spam Dam works well with server and firewall solutions, too - the stuff that gets through those will be caught before reaching the Inbox. ND6 DNSBL filters and Spam Dam have cut spam in the Inbox down to virtually nothing.