Excellent point.

Our users don't know about the possibility to subscribe.

They don't check the forum because they expect a notification.

How to best proceed?

orschiro, you've responded to my message before I've deleted it...

It was deleted because I did not want to raise a debates here.

Anyway to summarized my deleted message - you may be subject to the anti-spam law if you force notifications.   

I'm sorry but there is no such an option. But you can use this feature to make them automatically subscribed to topics they created or replied:

This option located in Dashboard > Forums > Settings > Features admin page, name "Topic subscription option on post editor - checked/enabled".

This is close what you want but more specific to user.