Your Creator Profile just keeps getting more powerful. You can now choose to limit email broadcasts you publish as newsletter posts to subscribers only. This is perfect for creators who want to give new readers a sneak peek of their content, but require subscription for full access. For any readers who are already a subscriber, they'll be able to login to start reading.
To set this up, go to Grow > Creator Profile, and select "Posts". You'll see the option to choose the visibility settings of your posts.
For more information on how to set this up, check out this Knowledge Base article.