Therefore, I would suggest using something based on the MIME
parameters,
such as
Annotations: classification=top_secret;
archived-at="http://foo.example/123456";;
x-spamassassin-score=5
it's strikes me that there's a serious problem with message headers.
it's _far_ to easy to define new header fields for half-baked ideas.
the last thing we need are more message headers for which the source of
the information is unspecified or ambiguous, so that it becomes
difficult to separate the original message from the cruft added to it
at various places and times.
and I'm inclined to think that annotations added to a message after
delivery really don't belong in the message header. we want to put
things there because it's easy to do so, not because it makes good
sense.