That raises several new issues with Martin's draft. They can be
summarized as "what is the utility of the proposed field; who
will use it, under what circumstances, and for what purpose(s)?".
Note that that is not a question about syntax, or about semantics,
or about validity, but about utility.
okay, let's talk about utility.
1. I can certainly see using this field as providing a way to pass
along a message to others (who might not have received a copy of the
message) without actually having to send them a copy. Instead of
forwarding the message to them, paste the URL into the message text.
2. Another thing that would be extremely useful is to be able to see a
message in context - in relation to other messages in the same
discussion. So if an archive points to a message in a read-only IMAP
mailbox, the context could be supplied by other messages in that
mailbox. Or if an archive points to a HTML document on an HTTP server,
the context could be supplied by "next in thread" style links. In this
case the Archived-at field serves as much to point to a collection
of messages as to a particular message within that collection.