The most common reason why emails are filtered is due to a low historical open rate associated with that sender (typically tied to the domain that is sending) for this reason you should stop sending to the group of contacts marked “unengaged”. By sending to these folks continuously over time, it drags your overall open rate downward and increased filtering (to the spam folder or other folders like “quarantine”) will result. For a plan to address this issue check out my other post on Graymail.