Why? If I'm writing a mail receiver, and I'm told there is some unique ID generated by the sender in a loosely specified way, the first thing I'm doing is ignoring that value forever. One lesson surely most everyone learns in CS is that unique identifiers are maybe unique to the system generating them, but to rely on foreign generated IDs being unique globally is a terrible idea that will break within the minute.
So at that point the ID has no value to me except being obliged to carry it around with the message, so maybe the originating system can at some point make sense of it. But then there is obviously no reason to ever reject mail without it, it's an ID valid for the sender and the sender didn't care to include one, great, we save on storage.
So at that point the ID has no value to me except being obliged to carry it around with the message, so maybe the originating system can at some point make sense of it. But then there is obviously no reason to ever reject mail without it, it's an ID valid for the sender and the sender didn't care to include one, great, we save on storage.