If you want more aggressive filtering and can accept the increased risk of false positives, consider some of the other less-conservative blackhole lists such as the ones run by SPEWS or the various lists of blocks of dynamic IP addresses., Like SpamFilter ISP you may also consider using the reject_unknown_hostname option mentioned in the “HELO restrictions” section, but you can expect a small, measurable increase in false positives.

The ruleset described above should be sufficient on its own to eliminate the vast majority of junk email, so your time would probably be better spent implementing and adjusting it before testing other measures.