Andrew Daviel writes:
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.
Unfortunately, the footer boilerplate is non-standard.
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.
Only because it already has, except for the RFC, and
http://people.dsv.su.se/~jpalme/ietf/ietf-mail-attributes.htmlmentions the definition of "X-No-Archive: Yes".
In practice, I believe most of the standard (Mailman's Pipermail[1], mail-archive.com, GMane) and large-scale proprietary archiving engines (Google and Yahoo! Groups) already do.
I wondered if this had been done already somehow, or considered and shot down, and how others handle the problem.
I'm not sure why Palme's I-D wasn't taken to RFC (Informational) status; probably it just was considered insufficiently authoritative since new headers are being invented all the time, eg, X-Spam and relatives aren't in it).
However, the real practical problem is that to be reliable, it needs to be a header field, and insertion of header fields is the province of mail clients, over which the IETF has no authority. (That is, the IETF can define a header field's semantics, but it is up to the user or the mail client to add it, and the IETF can't mandate addition of a user choice field the way it can mandate addition of From, Date, and Message-ID.) Although archivers recognize X-No-Archive, few users realize that there is a header in mail to which they can (in principle) add arbitrary fields, and even fewer know how to add the field with their mail-client-of-choice.
I think the best strategy is for mail clients and outgoing mail gateways that add confidentiality notices to also automatically add "X-No-Archive: Yes" to the header. And that's what I suggest you recommend to your posters. Not all of them will have the skills and access to do so, of course, but it will save the competent ones the occasional headache.
Footnotes: [1] I need to check Mailman 3's HyperKitty.