USPS returning mail addressed "Heirs of (so and so)". Is it a coincidence or rule?

I haven’t paid enough attention to notice if it is every single mail piece(small business marketing) I mail out that is addressed this way but I suspect it is. Anyone know the rules for the recipient? I know I can mail to “homeowner” or “current resident” as I get these all the time, so could "Heirs of… " be against some rules or something?