The change is subtle and will only affect users who use their web browser to access Gmail. Those who use legacy mail clients like Outlook and Thunderbird will not receive this warning.
When a user receives a message from - or, is in the process of composing a message to - a party that doesn't support encrypted connections for email, they may see an open lock icon in the message.
This icon is advising the user that the party they're conversing with isn't using encrypted email transfers and thus the information that's being exchanged is at risk.
Encrypted email exchanges are already performed for Gmail and Google Apps users.
A second security feature involves displaying a question mark for the user's avatar in Gmail when the sender can't be confirmed:
Generally speaking, a user should exercise caution when responding or acting upon email from an unauthenticated sender. It could potentially mean that the senders mail account was hacked, or, they're a spammer.
These notices are advising the user that the exchange is with a party outside of the Google ecosystem who haven't implemented forms of mail control called DKIM/TLS/SPF to encrypt email and authenticate senders/receivers. If you're wondering, all of my mail setups for clients include these settings so that their mail is as secure as possible.
There's nothing users need to do take advantage of these new features. They will be rolled out in the coming weeks. Any and all questions from my clients can be directed to me and I'll be happy to assist you.