You've probably noticed that when someone replies to your thread, you're being sent an email notification. This is a tutorial to explain how to turn off these notifications.
You receive notifications of topics set as Favorites. Topics you have started are set as Favorites by default, but you can change this on your Profile page.:
You can also do this for an individual topic by clicking the Subscribe button at the bottom of the post. It stays grey to indicate that you're not subscribed to that topic.
For (un)subscribing to a forum, please see here
I hope this helps you in keeping your inbox clear. Should you have any further questions then please don't hesitate with asking.
All the best,