HawkOpen classifies each request and gives the verdict a confidence of “possible” or “likely”, never “certain”. When there is too little information, the category is unknown rather than human activity.
The strongest sign that a person engaged with your message is something they did themselves, such as a reply.
The same goes for a read receipt or a “read” status: it shows that a program reported the message as opened, which can also happen in a preview pane or when the message is marked as read, and the recipient can refuse to send one.