Google says that your email address won't visible to a Google+ connection unless you send that person an email, and likewise, that person's email address won't be visible to you unless they send you an email.
Emailing Google+ connections also takes advantage of Gmail's new inbox's categories.
When someone in your circles emails you, the email will appear in the Primary category, one of Gmail's new inbox's categories. But if you don't have them in your circles, it will be filtered into the Social category (if enabled) and they'll only be able start another conversation with you if you respond or add them to your circles.
Google said the company planned to send an email to all Google+ users during the next two days alerting them to the change and explaining how to change their settings.
One exception to the new feature is celebrities on Google+, who are followed by a large number of fans. The Gmail accounts of such public figures will not automatically receive emails from other Google+ users.