The feature is possible because of improvements that keep Gmail contacts automatically up to date using Google+. Gmail will now use Google+ connections as recipients when you are composing a new email.
Google does let you opt out, and every Gmail user will get an email from the company notifying them and pointing them to the privacy settings.
The default setting will be "anyone on Google+" – change it if you do not want strangers cluttering your inbox. Critics have rightly objected to Google making this an "opt-out" rather than "opt-in" feature.
Google is aware of the issues - people who send you messages in this way do not actually see your email address. Strangers can only email you once using the Google+ feature. If you do not reply or add them to your circles, they will not be able to email again.