Gmail has started labeling mail that is sent without encryption with a broken lock icon .
Email encryption in transit (TLS)
Gmail supports encryption in transit using Transport Layer Security (TLS), and will automatically encrypt your incoming and outgoing emails if it can. Some other email services don't support TLS, and therefore messages exchanged with these services will not be TLS encrypted.
In Gmail on your computer, you can check that a message you’ve received was sent over TLS by clicking the small down arrow at the top-left of the email and reading the message details.
If you see a red open padlock iconon a message you’ve received, or on one you're about to send, it means that the message may not be encrypted.