...Unless you update your privacy settings. In Google's latest push to get everyone and their dog using Google+, the update will let strangers send you messages regardless of whether they are even in your Google+ "Circles" or "Extended Circles" unless a user digs into their privacy options.
The new "email from Google+" option won't hand over your email address to the Google Plus-er until you send an email in reply, so at least you shouldn't see your address harvested by nasty sorts and weirdos straight off the bat. However, it'll likely lead to many an unsolicited email coming through to your inbox and, as the messages are being sent through Google's own social networking service, it's unlikely to be immediately picked up as spam either.
The update will be rolling out to all users over the next few weeks, so be sure to hit up your privacy settings and change the options from allowing "Anyone" on Google+ to contact you on Gmail to "No one", or at least just those in your Circles. [Google]