Gmail has introduced a new feature using which anyone can sending email to you using your Google+ profile. And this feature by default is enabled to make it easy for people you know to reach you. Your email address will not be displayed to the Google+ user until you reply them. In case you want to turn off this feature then use the option as available as part of Gmail settings.
Click the Settings icon and select Settings from the menu list.
In the General Settings screen, navigate to Email via Google+ option. The default value for this option is set to Anyone on Google+.
Using this drop down, you can control people who can mail you. And to disable this feature select No one from the drop down list. Then click the Save Changes button available at the bottom of the page to apply and save the changes.