You should wear sunscreen to the beach. Its recommended as a good way to prevent sunburn. However, the beach police aren't going to come get you for not wearing it. You just might get a sunburn if you don't plan accordingly with other sun countermeasures.
So if I send an email that lacks a feature that MUST be there, will the email police come get me? At a certain point, looking for an analogy stops making sense I think.
I mean, kind of, yeah? If you're sending traffic that fails to meet the MUST requirements, you're probably going to be marked as malicious traffic and eventually wind up on spam/bot lists. If you fail to do the SHOULD things, you might find yourself in some odd edge cases of your peers and experience behavior you didn't expect.
