Making Reflexion more stringent

I want to make our email SPAM filter become stronger.

 

Right now, we are using Content Filtering and any email having a score of 50 or less is considered to be free of SPAM and is delivered. Otherwise, messages between 51 and 100 are held in quarantine.

 

My question is, do I make the Content Filtering Threshold higher or lower to make it tougher for an email to make it through to the users inbox?