If you're trying Gmail Confidential Mode beta, help me do an experiment: What happens if you a) set an expiration date on a message, and b) revoke a message, when someone has received those messages a) elsewhere than Gmail, and b) IMAP'd down from Gmail.

https://gsuiteupdates.googleblog.com/2019/03/keep-data-secure-with-gmail-confidential-mode-beta.html