You can send emails to any Gmail user without knowing their email address. The only condition is that you should have a Google+ account.
Starting today, users will be able to see profile names of their Google+friends, in their Gmail address bar. But the sender won’t see the actual email address of the person being contacted unless that individual responds to the email. This feature will be available to all Gmail users with Google+ accounts within a couple of days.
“Emailing Google+ connections also takes advantage of Gmail’s new inbox’s categories. When someone in your circles emails you, the email will appear in the Primary category. But if you don’t have them in your circles, it will be filtered into the Social category (if enabled) and they’ll only be able start another conversation with you if you respond or add them to your circles.” David Nachum, Product Manager of Google said in a blog post.
Gmail users will have the ability to opt out of this feature, limit its ability to suggest contacts to only your circles, Friends of Friends, Anyone on Google+ or No one. But the default setting will be “Anyone”, you can change it by going to the settings tab in Gmail.
Facebook is already allowing its users to message anyone who uses the social network. Now Google too following its rival’s way.