In honor of the Safer Internet Day, Google on Tuesday announced in a blog post that it has added a lock icon in Gmail's web interface to denote whether or not your emails are encrypted. Additionally, a question mark icon on a sender's avatar indicates messages that are not authenticated.
Gmail has supported encrypted connections between a user's machine and its servers for some time now, but this doesn't provide the full protection if a sender's email client or email service does not support encryption in transit using TLS.
The new lock icon makes it easier to see if a message you received from someone is encrypted or not, and whether or not it can be authenticated.