Suppose I reply to a message whose Message-ID field has a left id that
conforms to obs-left-id, but not left-id. How should I copy the msg-id
then? Strip the non-comforming parts and risk to break older mailers'
threading or copy everything from the "<" through to the ">" verbatim
from the parent, risking to thus create a message in violation of
rfc2822?
I'd say the latter. It's certainly incorrect to create a completely
new message-id (correct or not) and say that you've replied to it.
Say you were replying to a message that had a newly-invalid From address
(valid under 822 and invalid under 2822). Would you create a new address
as the destination of the reply?
Keith