[Spoom]

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."

From https://support.google.com/a/answer/7684332

[myrandomcomment]

Thank you for explaining. Also I guess I will be able to write an auto reply message that say “Sorry, I refused to received messages of this nature.”