The problem is that it happens to me more and more often that especially with my business contacts in larger companies my e-mail is filtered out / ends up in their junk folders / disappears for other, unspecified reasons. Simple public black lists do not show anything. I am more the developer and not the infrastructure type, but the company policies seem to contain more and more features of downgrading private domains. And in my opinion this is supported by some kind of dirty game of global IT players at technical and other levels.