I'm not going to keep this one to myself, since someone not on my site isn't clicking my ads, anyway. If you have a blog or even a big ol' company website that has some Adsense ads on it (the Google ads), then you need to add a custom ads.txt file to your website. I did not receive an email about it. I just showed up to check earnings, which I don't really bother to do very often, and there it was: a dozen websites without the right code.