Gmail Now Alerts You to Potentially Unsafe Messages With Two New Icons
Starting today, you can see the new broken lock icon in Gmail messages. It can alert you if you are sending a message to an email service that does not support encryption, or if you received a message that was not encrypted.
While Gmail itself supports email authentication and encryption of email in transit, not all email services support it, which is why a new warning icon has been added. If you see this while composing your message, make sure your email does not contain sensitive information. (But if you send confidential information by e-mail you all the same should encrypt it yourself .)
Additionally, if you receive an email from a sender that cannot be authenticated, Gmail will replace the sender’s avatar with a question mark icon so you can be more careful when replying to the sender or clicking on links in the message.
TL; DR: These two new icons are security alerts, so be careful when you see them.
Improving Email Security For You | Official Gmail Blog