There is, however, a way to opt-out.
To use this feature, as you compose an email, you begin typing a contact's name in the “To” field. Matching contacts display at the top of the list of possibilities, and your Google+ connections appear below. Notably, unless your email address is not visible to your Google+ connections until you send them an email. Their email addresses are not visible to you until and unless they respond.
However, as we said you can be emailed by someone outside of your circles. Those emails will be filtered into the Social category of your inbox (if it is enabled). Only after you respond or add them to your circles can the Google+ member start another conversation with you. If you don't have those new Gmail tabs enabled -- there are many who still use the “Priority Inbox” interface -- you might see your inbox crowded with messages.
For those who might prefer to opt out of the feature.
You can control who can email you this way by changing your settings in Gmail.
- Open Gmail.
- Click the gear in the top right.
- Select Settings.
- Scroll down to the Email via Google+ section in the "General" tab.
- Click the drop-down menu and choose Anyone on Google+, Extended circles, Circles or No one.
- Click Save Changes.