A lot of apps already offer the option to send messages which will then by itself disappear, think of Snapchat and ProtonMail. Also Gmail seems to be soon an option to get for self-destructive messages.
The site TechCrunch writes about the still unannounced ‘confidential’ mode. By this option while composing an e-mail, a message is sent after a specified period of time not more to open. You can also a one-time passcode to generate a sms is sent.
It may not work the function is only between Gmail users themselves, because when you open such a mail re-logged in with a Google account. That way could, for example, Outlook users are excluded. Moreover, Outlook has recently to create a similar option for Office 365 subscribers.
Gmail will soon be getting a major makeover. Allows this confidentële mode. Google is not yet on the rumors, but keep in may, an event where new products and services will be unveiled. Who knows, we hear more.