I'm now utilizing “second stage” filtering, using the primary Spamhaus blacklist, the SBL. For me, it's an experiment. I just wanted to see how well it works and what kind of mail it catches. I know that a large number of email addresses are now behind this kind of filtering – at least one domain registrar (who hosts mail for a zillion different domains) has been using this type of filtering for at least the past few months. So I wanted to see what kind of senders are getting tripped up in this kind of filtering, and how well it works as a spam-blocking methodology.