Have you ever started typing an email to someone only to realize halfway through the draft that you haven’t actually exchanged email addresses? If you are nodding your head ‘yes’ and already have a Google profile, then you’re in luck, because now it’s easier for people using Gmail and Google to connect over email. As an extension of some earlier improvements that keep Gmail contacts automatically up to date using Google , Gmail will suggest your Google connections as recipients when you are composing a new email. (via Official Gmail Blog: Reach the people you know more easily)

Google will sort of link email with your G+ contacts to help when you don’t really know someone’s email. If you don’t like that, you can opt out.