This is probably a case of beating a dead horse after everyone switched to automobiles, but nevertheless:
We had an occasion where some private information got forwarded to a public mailing list and then got found by a search engine. Probably happens all the time. At least one of these messages had some boilerplate on the bottom saying "may be confidential, do not forward" etc.
Computers are better than people at reading boilerplate. It would be nice if there was some way to flag a message "do not archive", or even "do not forward" so that a user would have to do something to override that. I found "X-No-Archive" in wikipedia that was apparently honored by google when archiving usenet.
In an ideal world hundreds of people would immediately agree that this was an excellent idea, get an RFC written, and everyone would implement it by next year. Not going to happen, I know.
I wondered if this had been done already somehow, or considered and shot down, and how others handle the problem.