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Receiving a notification based on when a new post is created forum wide would be useless in practical terms. There are thousands of new posts created every day, and considering that, a new post forum wide notification would lead to turning off the feature pretty quickly.
Now if you delimit your interest to a given board or at thread level (post in a given thread) then begins to make more sense. For example, If you want to receive notifications from a thread that you like, then you can click the “notify” link on the top right of the screen. The Watchlist will allow you to be notified when a new thread is added to a given board.
There’s a recent thread on the matter if you want to read about different options:
Notification for new Post on the interested topics.