There are indeed such situations. Two situations AFAICR, and neither of them apply to when you connect to someone else's MX. Gmail rejects the vast majority of

There are indeed such situations. Two situations AFAICR, and neither of them apply to when you connect to someone else's MX. Gmail rejects the vast majority of compliant messages, I think they've stated in public that they reject >99.9% of messages, and hearsay has it that the percentage for with minor errors like this is even higher. There are good reasons why a message might be unreadable. For example, message-id is often used by the threading algorithms in MUAs and IMAP servers, and many don't test whether their threading code handles ID-less messages. I use one that deduplicates by ID, what do you think it does when the ID is empty or missing? I don't know, I haven't tested and I'm not going to.