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.
From a practical point of view, I think this is debatable.
It's certainly possible to create a new MIME type for each envisaged piece of information, but practically message headers have a privileged role.
1) They are easy to find without parsing a complex MIME structure.
2) They are often available in cases where the actual body isn't directly available.
3) They are intended for meta information about the message rather
than message content per se, so it could be argued that they are the only
correct place to add annotations which describe aspects of the message
rather than new content.