Google Profile provides a contact form that let people send messages to your main email address, if you’ve enabled "Allow people to contact me through my profile (without showing my email address)" in your Google Profile. "We've set some limits on how often messages can be sent and received, and you can disable the feature at any time by editing your profile and deselecting the checkbox," explains Google.
At the bottom of the email, Google shows an explanation:
"This message was sent to you from your Google profile. The sender does not have your email address. If you no longer wish to receive messages from your Google profile, you may edit your settings at http://www.google.com/s2/profiles/me/editprofile".
Source:→ Google System