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.
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.