It strikes me that the RFC-5322 resent-headers, which don't seem to be
used much, might be a good starting point for such standardization - for
a start, by getting past the overloading of the From: header that causes
so many problems.
Keep in mind that anything a mailing list can do, a spammer who wants
to piggyback on the generally good reputations of mailing lists can
also do. Anything that says "I'm a mailing list" is only useful with
some sort of external validation that it really is a list. If you
have to do that anyway, you might as well use the list's DKIM
signature as the key which doesn't involve inventing any new
mechanism.
I think it's also pretty clear that any scheme that depends on the
DKIM signatures of incoming messages transiting mailing list software
won't work, either.
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