Although there are some email features, fundamentally, Nabble provides a web-based service.
To give emphasis to an important post the conventional method on a forum is to "pin" the post to the top of the page. This feature is provided by Nabble.
Nabble can be set up as a mail list, when all posts appear in people's mail box, but there is no mechanism for sending only certain posts as emails. This is because the expectation in any forum, blog, etc is that users visit it on the web and have no email relationship with the site visited. Consequentially, there is no mechanism within Nabble for bulk emailing users of a forum - unless your forum has a specific sub-forum for important announcements and you oblige your users to subscribe to it before you grant them access to the rest of your forum.
Having said that, here are methods of extracting user addresses from Nabble, which you could use to generate an email but these are almost certainly illegal in your country as they would involve the use of personal data for purposes not specifically authorised the those providing the addresses. Further as once registered on a Nabble forum there is no mechanism for de-registering, there is no way for the user to unsubscribe from from any mail that you send using that method of harvesting the addresses.
In short, people must subscribe to get emails - unless you are determined to break the law.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.