Exactly. This minutiae is all so weird. Email as a formal specification does not work, and the industry as a whole has accepted that for decades now. It's not possible to filter spam from valid traffic without applying a truckload of heuristics and leveraging an ever growing set of auxiliary signals (SPF, DKIM, yada yada).

To wit: basically everything in this world is a "SHOULD", at best. The rules are a conversation.