now anyone can send email to anyone on google+ even if you dont know their gmail address. you have to opt out if you dont want to let strangers sending you mails using your g+ account
http://news.cnet.com/8301-1023_3-57616983-93/google-begins-to-merge-google-gmail-contacts/Google said the new feature would not expose the email addresses of any Google+ users to strangers. Emails from strangers on Google+ will be routed to a special section within the recipients mailbox that is separate from messages from friends and other contacts. If the recipient does not reply to the message, Gmail will block any future messages from that person. Google plans to send an email to all Google+ users during the next two days alerting them to the change and explaining how to change their settings.