If you're a Gmail user you'll probably already know, thanks to the email Google sent around, that starting this week Gmail will suggest you Goole+ people as recipients without exchanging email addresses.
What emails will I recieve from other Google+ Users?
By default settings, other users can contact you Once until you reply to them, thereby sharing your email address.
How will I know which emails are not from my google contacts?
Emails from people outside your circles will be filtered into a new Social category in your Gmail inbox (if enabled).
Where can I get more information?
At current you can get more information from the Google support section on Emailing Google+ connections