What is the usefulness of a header field containing a URI which only
fetches another copy of the same message?
a) so you can copy the URI to anywhere else you would like to put a
reference to that message.
b) so you can delete the message but keep the URI.
c) so you can use that URI as a starting point to get to other messages
that appeared in the same context. the trick is how to identify what
that context is (is it the mailing list in which the message appeared?
which mailing list?) and how to navigate around that context.