According to Google's support website, confidential emails have a sender-defined expiration date (from one day to five years), but access to the email can also be revoked at any time. Recipients who are using a Gmail account will get the passcode via SMS, while those who are on a third-party email service can get it either via SMS or email.
Here, find the mail that you sent with confidential mode and open. Passcodes can either be through a separate email (users not using Gmail) or through an SMS (just do not forget to enter their mobile # when you turn it on).
But now the feature has made its way to mobile with both Android and iOS apps receiving an update.
Tap on the three-dot-menu in the top right corner and then tap on Confidential mode. Additionally, recipients can't forward, copy, print or download these emails.
For users of the Gmail web client, Google's support page notes that confidential mode isn't available in the classic version of Gmail. This is done by going into your Sent folder, opening the email in question, and tapping the "Remove Access" button that appears at the bottom.
You will then have to choose a passcode which can be an OTP on the recipient's phone number, or you can skip it.
The Electronic Frontier Foundation (EFF) digital privacy advocacy group thinks that this new mode isn't secure at all, and might even lend users a false sense of security. Ensure you enter the recipients phone number not of your own.
Keep in mind that once the email is in Confidential Mode, any attachments, documents, or pictures will also be locked.