Refusing signups from those domains is probably a bit extreme, though.
What about a warning at signup time if a "discardable" ADSP record is
found for the registrant's domain?
That doesn't help. In the IETF's scenario, domain A sent mail which
was marked discardable to the list, and recipients at domain B were
rejecting it and the people at B got bounced off the list. I'd have
to squint awfully hard to come up with an argument that it is wrong to
reject unsigned discardable mail at SMTP time.
Since ADSP causes problems for innocent bystanders, I think it's
reasonable to decline A's mail in the first place. This is doubly
true since the ADSP RFC rather specifically says that you shouldn't
mark a domain discardable if its users send mail to lists.
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