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On your computer, go to Gmail and click Compose.
Click "Turn on confidential mode" in the bottom right. For emails already in confidential mode, click "Edit" at the bottom.
Set an expiration date and passcode. These settings impact both the message text and any attachments.
If you choose "No SMS passcode" lets Gmail app users open it directly. Non-Gmail users receive an emailed passcode for access.
If you choose "SMS passcode," recipients will get a passcode by text message. Make sure you enter the recipient's phone number, not your own.
Click Save.