Once you've turned on confidential mode for a specific email, you can set an expiration date and passcode so that you can restrict access to the email either in the web interface or via SMS.
Gmail's confidential mode is an interesting feature for email that provides low-level protection for your messages.
Over on its support pages, Google explains exactly what Confidential Mode is and what you can do with it: "You can send messages and attachments with Gmail's confidential mode to help protect sensitive information from unauthorized access". Confidential mode is rolling out to mobile devices today, so be sure to keep an eye out for it.
The app will then present a list of options related to the new feature.
The feature allows you to specify an expiration date or manually revoke access to a message. Of course, the feature is only available in the latest iteration of Gmail and is now limited to mobile devices, meaning it won't be accessible for those using the "classic Gmail" layout in a browser. After that, tap on the three dot menu on the top-right of the screen, which will then show a "confidential mode" option.
It is worth noting once an email is sent any self-destruct date that was chosen can not be altered. If you choose to have a passcode, the recipient will have to enter the same to view the email content.
You can also enable mandatory passcode for accessing the email.
Even with all this, Gmail still allows users to take screenshots of confidential emails. I sent an email to myself with confidential mode, the attachment was a photo.