How? If you send an email with this on to me@protonmail.com and I download the message to my IMAP client how does google magically reach out and delete it from my hard drive? Is the email HTML only that only displays the text when the user is online and that text is fetched from the Google server? Let us say it is and I view the email, how does Google stop me from cutting and pasting that email using my thunderbird, et.al. IMAP client?
You view the message on a Google server through a browser. The message body is never actually sent to the recipient's address.
"When someone sends a confidential mode message, Gmail removes the message body and any attachments from the recipient's copy of the message. These are replaced with a link to the content. Gmail clients make the linked content appear as if it's part of the message. Third-party mail clients display a link in place of the content."
Source: am googler, have used.