> One would expect a given provider should, unless instructed explicitly otherwise, block outside emails from domains it controls mail for. It's not even somet

> One would expect a given provider should, unless instructed explicitly otherwise, block outside emails from domains it controls mail for. It's not even something you need to specifically craft much infrastructure for. There are public standards for "don't accept email from this domain unless the sender has a cert encrypted by one of these CAs" (Google runs their own CA, yes?) and "don't accept email from this domain except from these servers" and so on.