I believe that email notifications are sent to those users who choose receive notifications from this forum. In other words, to receive notifications from a forum thread a user must visit it first and enable notifications.
Myself I have to actively enter the forum to read new posts and sometimes I browse through the complete thread listing to find interesting threads.
To avoid bombarding users with emails. I would suggest sending to all users weekly emails with a summary of the most active forum threads of the week. Or just cherry pick some thread to promote users visiting this forum.
I suppose that the forum software provides stats about user usage and visits.