Hi. I have a site which used to be 4.6. There was a great, simple module (that has not been updated to 4.7) called Mail - which let me create an instance of a new content type, Mail, and send it to all users depending on role. This was exactly what I needed to be able to tell everybody on the site that something was new. As I said, it has not been updated, and there is no equivalent way to do this on 4.7 (Mime/mail plus Send, etc, etc - I have tried them all, and none of the ways work.) I am looking into what it would take to update the Mail module to 4.7, but in the meantime ...
It seems to me that I might be able to use the Subscriptions-4.7.0 module to do this: I want to compose a message (some sort of node - it could be a page, or a story, or a new content type that I only use for this purpose) and then send that node, or if I can't send the node, then a URL that points to the node, to the entire list of registered (that is, authenticated) users. Note that there are already about 15,000 users and none of them have signed up for anything - but I do have their email addresses in the users database, and they have opted-in. I would want all users, new ones who 'auto-subscribe' and also old users (I want to auto-subscribe all users with a given role) to get the message.
I would not necessarily want all 15000 users to get emails every time something new is posted to the site, so it seems that the best way to do this would be create a new content type just for this use.