The search for January's Google+ opt-out setting of the month appears to have been found, with users of Gmail soon gaining the ability to email any Google+ contact that allows emails from the entire Google+ network.
Google said in a blog post that it sees the latest encroachment of Google+ on Gmail as an extension of the existing Gmail ability to add email contacts to Google+ circles. The company said that the email address of a user would not be exposed to another unless some form of email correspondence was entered into, whether that be sending or replying to an email from a Google+ contact.
For Gmail users who make use of Gmail's new tabbed interface, emails from existing contacts in Google+ circles will appear in the "Primary" tab, and emails from unknown contacts would appear in the "Social" tab, should it be enabled. A user who sends an unsolicited email to an unknown contact would only be able to send another if that email is responded to, or the user is added to a Google+ circle.
Fortunately for users wishing to be left alone, Google has allowed for the receiving of email from Google+ contacts to be switched off, or limited to contacts within a user's circles or extended circles. The default setting appears to be allowing anyone on Google+ to email you.
Users will know when the new feature has been switched on, as an email will be sent to the user.