The 'Best Practices Rules' are a set of rules designed to check if connecting servers are behaving according to SMTP Best Practices Policies. Drawn from various sources, these rules are based on the observation that most improperly configured servers that do not comply by such policies are statistically much more likely to be senders of spam than not.
For a comprehensive list of the rules and how they are applied, please refer to: http://www.linuxmagic.com/best_practices