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.