In admin > settings you can search for “mailing list” and you’ll find default email mailing list mode. Enabling this turns on mailing list mode as the default for all new users. (If you need to enable mailing list mode for all existing members you can contact us at team@discourse.org since we host your site).
You’ll also see an option called default email mailing list mode frequency with a send an email for every new post except my own option, which you might want to set so users don’t receive emails for their own posts.