Have someone who has a problem where they can’t receive email set it up.
I suppose the easiest way would be to create one email address for your own use, and make it so cryptic that no one would ever guess it. Like R1Fh7feN3Z@example.tld, for example.
If you truly want to make it impossible to receive mail, I think you could probably do it in /etc/postfix/main.conf with
inet_interfaces = loopback-only
But I’ve never actually done it. Usually I don’t go out of my way to intentionally break functionality. I do it well enough without trying.