We have built an internal messaging, and notification system that authors can use to reach out to their readers, and re-engaging them at any point in time.It's as simple as clicking the "Message All Subscribers" button on the People page of the Guide Editor.

If you prefer to use a third-party bulk email app (Mailchimp, etc), you can export your entire readership list as a CSV file, and then import it into your application of choice.