Domain administrators will have the option to control the availability of this feature through a new setting in the Admin console under Google Apps-> Gmail Settings -> End User Settings. If this feature is enabled in the Admin console, users can control whether people outside their circles can reach them with a new setting in Gmail. They will be able to choose from Anyone on G+, Extended Circles, Your Circles and No one. The default setting will be 'Anyone on Google+'.
Google Apps for Business, Education, and Government
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Over the next few days we'll be rolling out a new feature that will suggest users' Google+ connections as recipients, along with their traditional Gmail contacts, when composing a new email. Your email address isn't visible to a Google+ connection unless you send that person an email, and likewise, that person's email address isn't visible to you unless they send you an email. Importantly, if someone outside a your Google+ circles sends you an email this way, you can decide whether you want to receive more emails from that person.
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