..., the broader standard of "unwelcome" is more widely applicable than the narrow standard of "illegal."
This is where we arrive at "Acceptable Use", which is why it is required. But these policies need to be propogated and enforced at smaller points of intervention.
That's vaporware at the moment. Until it's realized, senders must follow a universal standard for determining whether their traffic will be welcomed by receivers and intermediate systems whose AUP's aren't published in a mechanised form and with whom the sender has no direct relationship, or contract, or terms of service. And unlike a direct relationship where it's safe to simply enumerate the things which mustn't be done and then assert that, subject to revision of that list, everything else is OK; in the indirect, transitive case where the recipient is distant and their policy isn't known, it's only safe to err on the side of extreme politeness: send what you know to be welcome, and hold onto the rest.