A new Gmail setting lets you choose whether you want people to be
able to send an email to your Google+ profile — even if they
don’t have your email address in their contacts. […]
By default, the setting will allow anyone on Google+ to send you
an email through Gmail. To be clear, Gmail users will be able to
type anyone’s name into the recipient field and even if you don’t
know that person’s email address. Google will auto-suggest the
names of Google+ users, though your actual email address will not
be revealed unless you reply to the email.
This has to be a mistake. Surely Google will change this from opt-out to opt-in.