Gmail Confidential Mode with Disappearing e-mails.

To begin with tap on Compose, write the mail. You'll see a small lock and time icon at the bottom of the Compose bar.

A new pop up will appear where you could set an expiration time for the mail along with a Require Passcode option.

 If you proceed with selecting the SMS Passcode, you'll be shown with a window below, where you should enter the recipient phone number.

Once finished with that your mail appears to be like this.

The recipient will receive the mail where he got to enter the passcode, that'd be send to his phone number as given by the sender, to view the message.

If you opt Remember this passcode for next 5 minutes, you won't have to enter a passcode to access the mail for the next couple of minutes. Else whenever you wanna view the mail you'll have to submit one for the same.

And finally here's the message.

There's also this option to Remove and Renew Access once you sent a mail. The Remove access will revoke the recipient from accessing the mail. 

You could also Renew his authority by Renew Access. But you can't add additional recipients once a mail is send.