Gmail's upcoming self-destructing emails will require the recipients to click on a link and log in with their #Google accounts to see the content, if they are accessing via IMAP/SMTP.
Don't be fooled: Google's purpose is not to give us more confidentiality. They want all of their users to access their mailboxes via the Gmail app or their web interface, so they are creating a burden to the recipients and calling it “security” to convince people to adopt it.
@gergely I did not say that it cannot be secure. What I meant is: They use security merely as a selling point for a feature whose purpose is to drive people away from third-party apps. Of course this is all about more control over their users and deeper data gathering for their ads.