Dear friends,
following the RFC 5322 all identification fields are optional. But they SHOULD have it. Of course, sometimes we need it.
RFC 5322: Internet Message Format http://tools.ietf.org/html/rfc5322
We have min 3 fields: message-id in-reply-to references
With that, we can create all ordering of mails based on a thread. I think, it would be never a problem to use a new (little changed) message-id, because if the listserver send the mail, he changed it. And consequently, he create a new message-id (but for himself and for tracking it can be a modification of the original message-id).
In the discussion on this list Mark and other people args, this would lead to a big confusion for the different mail clients. Is that true? I don't have the necessary oversight. But i think, you have it.
The easest way would be a configuratin point for the user to decide, that his sended mail get a new message-id, if the listserver changed it when he send the mail. And this he always do it.
many greetings, willi Panama City