For Google+ users, by default it is possible to directly email anyone in their circles even when the email addresses of these contacts are not known. Here is how to change this so that emails can be received only from desired Google+ contactsÂ (instead of the default):
Login to Gmail and click the gear icon on top right. Then, select “Settings”.
Then, from the “General” tab, scroll down to “Email via Google+” section and change the settings from the default of “Anyone on Google+” to what is preferred.
Scroll to bottom and hit “Save Changes” for this to take effect.