Currently there is a "subscribe" button when you post a reply to a topic. I tested this logged in as User A by replying to a post made by User B and selecting "subscribe". I then logged in as User B and posted another reply. No email was sent to User A. Not sure how the feature should work, but here is my suggestion:

Allow a user to subscribe to a topic, without having to "Reply"

Email all users who have subscribed to the topic, anytime a new post is made.

Allow each user to Unsubscribe to a post at any time. User should be able to see a list of all posts they are subscribed to.

Don't bother with "Send email notifications" checkbox, if users are subscribed, they should be notified, if not they won't. Not clear on what this checkbox would be for.

This will probably be a very important feature for my users. Thanks!