I think this might be what you're talking about. Go to User CP; Top left corner, then to edit options to change. See below
Default Thread Subscription ModeWhen you post a new thread, or reply to a topic, you can choose to automatically add that thread to your list of
subscribed threads, with the option to receive email notification of new replies to that thread. Do not subscribe No email notification Instant email notification Daily email notification Weekly email notification Default Thread Subscription Mode:
Hope this helps.