This plugin is really neat. It’s not as much as a security plugin but it will help you protect your content until the reader will subscribe to your email campaign, which is really great.
Opt-In Content Locker is a plugin that allows you to lock important content on your WordPress website and display it for subscribed users only. All you need to do is to wrap protected content with shortcodes
If people want to view this content, they must submit their name/e-mail. After submission, all the locked content becomes visible.