Google is taking extensive precautions, perhaps anticipating that this feature could be abused or that it would at the very least upset Google's users--who have been very touchy in the past when the company has interfered with their identities and their privacy online. If the sender is not in one of your circles on Google+, then your true email address will be hidden from the sender. If you do not send some kind of response, that sender will be blocked from sending you future messages via the service.
Additionally, messages from anyone who is not in one of your circles will be shunted to the "Social" tab in Gmail, along with other generic notifications from social networks.
You can reverse this change using the control panel in Gmail. Click on the gear in the top right corner of the screen, select "Settings" from the drop-down tab, and scroll down to the item labeled "Email via Google+."
Using the menu, change the setting from the default, which is "Anyone on Google+." ("Anyone on Google+" refers to some 343 million active users, according to one recent estimate.)
These changes are similar to Facebook's messaging function, which places messages from strangers in a segregated folder, but with an important difference. Facebook is still a social-media messaging service, not an email service, and the company's attempt to encourage its subscribers to begin using email addresses on the facebook.com domain has largely failed. Google, by contrast, is taking advantage of its direct access to its users' addresses -- and people still tend to feel that their email inboxes are more private, or should be, than their social-media inboxes, which they can always ignore.