And in any event, even if your assertion were true, so what? Our
working assumption is that receivers will use valid DKIM signatures to
whitelist mail from signers they like. How does it matter whether the
signer happened to be a related to a list?
If I'm a consumer of your list of domains that you're convinced sign
all their mail and I get an unsigned message from one of them, I'm
bound to be extra suspicious of that. "Oooh, it came from a mailing
list so I don't care" isn't the most likely response.
I can't speak for other hypothetical lists of domains who sign all
their mail, but the domains on my small but real drop list send no
mail to mailing lists.
I suppose it is hypothetically possible that there would be well run
lists that would leak the occasional forged message from some ESP
domains. But since that happens now basically never, I don't see any
reason to spend any effort on it. Maybe you'd look at the list's
signature and deliver it, maybe you'd look at the drop list and drop
it, leading to one filtering error in umpteen million messages. If
that's the worst problem you have with your mail filtering, you don't
have any problems.
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