A new "feature" today allows anyone on Google+ to directly message you, unless you specifically opt-out. This means that advertisers, hated rivals, Somali warlords, serial killers or whomever can simply type your name into their 'To:' field and Google will try and send you the mail.

Fortunately, there is a way to stop this from happening. Go to your settings, and midway down the page, you'll see "Email via Google+:"

You'll notice that it's set to the default of 'Anyone on Google+'. Which is to say, a significant portion of the internet's denizens (after many iterations of Google's insistence that anyone using their services be required to have a Google+ account, whether you like it or not).

So what you should probably consider doing is setting that drop-down to 'No one'. Like this:

This sets the options back to the way they were – namely, the person emailing you needs to actually have your email address.

There are some other options here, 'Circles' and 'Extended Circles', which basically mean 'Friends' and 'Friends of Friends', but it's probably worth not bothering with that.