Source: http://www.engadget.com/2014/01/09/gmail-email-google-plus-users/
Even though the rest of the tech world is preoccupied with a little trade show in Vegas, the folks in Mountain View are still working hard. Today, Google has rolled out a nifty new feature for Gmail and Google+ users: letting them email each other whether or not they've actually exchanged email addresses. That's right folks, now, when you are typing in email recipients, Gmail will make suggestions to add from your Gmail and G+ contacts, too. Naturally, users can decide which of their circles get access to their inbox, or whether to accept emails from social networkers at all. In practice, should a Google+ contact in one of your circles send you a message, it'll pop up in the Primary category of your inbox, while all others coming from G+ will wind up under the Social tab. Sound good? The feature will be rolling out worldwide soon, so look for an email from Big G telling you how to set it up in the next few days.
Filed under: Google
Source: Official Gmail blog