To add, change the frequency of or to switch email notifications off all together you can select the Member Options as seen in the image on the right.
Once you are in the Member Options menu, you are presented with a list of options. Select 'Edit Forum Subscription' from the list as seen in the image on the right.
You will then see a list of all the forums that you are subscribed to and a drop down menu on the right hand side. In that menu you can change the frequency or cancel the email subscriptions. Once you have made your selection hit the update button at the end of the list of forums.