Gmail will inform you on receiving e-mail Without encryption
These days gmail is working on a different kind of notification system, that will alert you when you will receive unsafe or encrypted email.
According to google, there is no issue when someone will contact from gmail to gmail, but the emails that come from other servers may be encrypted.
This new alert system from Google is started for that kind of emails.
In a month or two all gmail users will be notified on that kind of emails.