One suggestion - you could create a course that contains a forum, set the Forum Subscription setting to Auto, create an Audience that contains all activer users, assign the Audience to the course as Enrolled Learning. Then everyone will be enrolled on the course and subscribed to the Forum, then when you make posts in the forum and check the "mail now" checkbox, the users will/should get emails on the next cron run?

By modifying Forum Administration -> Permissions you can remove posting rights from the Learner/Authenticated user role so they can see the news but not reply.