Jeff Breidenbach writes:

I checked

Thank you!

and these two lists are responsible. One uses 'No' and the other uses 'no' for every message. I can't speak to the intention. There are an impressive number of headers on each message including DKIM but I don't see a clue as to the list server software.

I took a quick look at the archives for one of them. From the disclaimer in the footer it seems pretty clear that they intend to make all of those posts public, which isn't surprising given that they use your service. :-)

However, since it's on all posts and they also add a disclaimer to each post, I conclude that it's something in their mailing list system (whether it's actually coded in the MTA or the MLM, who could guess). It's possible that their MLM suppresses "X-No-Archive: yes" posts, so they never reach mail-archive.com, but again, we'd have to ask them. More likely, they didn't catch the nuance that there's only one value for X-No-Archive, and that is "yes" -- otherwise don't insert the field at all.

So I think we can conclude that (1) individuals who intentionally use "X-No-Archive: no" are *extremely* rare, and (2) the example of these lists doesn't provide any guidance to Mailman since Mailman provides no facility for generating X-No-Archive itself.

Maybe it should? Ie, for lists with "private" archives, maybe it would be a good idea to add "X-No-Archive: yes" to the header to advise chained lists not to archive?