ROI has an integration with SpamAssassin. This button is for e-mail spam filtering based on content-matching rules. The max possible score is unknown, as SpamAssasin is an open source tool and they continuously change the check to keep them up-to-date. ROI recommend your score be 0, but always make sure to stay below a spam score of 5. 

Find your spam score directly above Step 1. 

A few more technical details. SpamAssassin also assigns negative points if it thinks the email is not spam (which is called "ham"). Secondly, failing just one test won't mean that the email will be treated as spam. There's more information on the official website. Wikipedia has a good article as well. Additionally, this is a great resource of tips to avoid your "Ham" getting caught as "Spam."

Contact with additional questions.