I'ts under Edit Options, in the Messaging and Notification section, called:
Default Thread Subscription Mode
When 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.
Default Thread Subscription Mode:
You need to choose something here other than: Do not subscribe
But I thought there was another option elsewhere as well, and I can't find it now. If I do, I'll let you know. You can also message Sheik or Fred Zed, they should be able to help.