Is there any way to do the following:

Scenario: You have users on your forum that want to reply to threads using email. Is there a way in which all threads created or replied to, get sent to those users who specifically asked for email functionality? Something that could be set through the CP?

The reason I ask is there is another board I am in the midst of setting up and we have users that don't have time to wade through all the forums to find what they need.