When connecting your email client to your Gmail account, Google runs a "caching" service that helps make sure you don't get messages from senders it determines are bad and that emails are "ordered properly" - according to its discretion.

You can run your own "caching service" and connect your email client to that. In this case, that means a fully alternative IMAP provider other than Gmail, because Google's "caching service" is so overwrought at this point that it is barely recognizable as the open protocol it started on.

Sure, someone can "speak valid email" to you via your Gmail account, but that doesn't mean you'll necessarily see it.

"Caching". A harmless performance improvement that definitely doesn't destroy open protocols in order to benefit the DAU and bottom line of centralizers (and besides, you can always just run your own!)