Google has introduced a so called confidential mode to their email which supposedly allows an expiration date to be set on the email as well as preventing forwarding, copying, printing and downloading as explained here. They do this by sending you a link to the message instead of the actual message.
https://www.computerworld.com/articl...tial-mode.html
Well in my opinion, #@#$ that. I never used custom sieve code before, but this seems to do the job. I put it between sections 5 and 6 of the code shown when you click to edit the custom code.
if exists "X-Gm-Locker"
{reject "Google confidential mode emails are automatically deleted at this email address."; }