As of yesterday, any mail that arrives to your Gmail account that is not compatible with the TSL encryption will appear a padlock icon next to it. If you click on the icon, you’ll get a message warning you that it may not be a good idea to respond to the email with confidential information.
Gmail will also alert you of the users it can’t verify.
These methods are intended to protect you from harmful emails. If Gmail doesn’t recognize the sender as an authentic and verified sender, it will also replace the profile picture with a question mark. This is to say that if you receive an email supposedly from your bank; if the padlock icon appears in the top right corner, and you have the question mark as the profile image, it would best to not trust the source and delete the email immediately.
Source: The Verge