Gmail users, Google has decided to make your Gmail username public through Google+. By default it’s turned on, so you need to manually opt out.
Sign into your Gmail account, and click the gear in the top right.
Click on Settings
You should then see the same screen as my attached screenshot.
One of the options you’ll see is: Email via Google+:
Click on the drop down that currently reads: Anyone on Google+, and change it to No one.
Then scroll to the bottom of the screen, and hit Save Changes.
Note: If you don’t see this option yet, check back daily until you do. Google is still rolling out to it’s users.