Gmail will allow you to e-mail Google+ users without knowing their e-mail address
Google has added a new controversial feature to Google+, which has privacy advocates concerned. Google+ and Gmail are becoming even more integrated because there is now a feature that auto-suggests Google+ contacts when Gmail users are typing in the names of e-mail recipients. This means that you can e-mail Google+ contacts without knowing their e-mail address and vice-versa. This feature will be rolling out in the next few days.
Gmail will automatically suggest Google+ users that the e-mail sender added to his Circle on Google+. Fortunately, the e-mail senders will not see be able to see the full e-mail address unless the contact replies to the message. Google+ users that are not in your Circles will have their messages appear in the Gmail “Social” tab instead of the “Primary” tab.
Users that fear getting spammed can opt out entirely so that their e-mail address will not appear in Gmail auto-suggest. The feature can also be limited so that only people that are added to their own Circles can send them an e-mail. It can also be extended to second degree Circles so that your friends and friends of friends can contact you. The final option is to opt-in and allow anyone on Google+ to contact you.
Gmail is not the only service that Google+ is heavily integrated within. It also works with Blogger, Apps, and YouTube.