There may yet be a grey area for very sophisticated or experimental MLMs
(like "Hmm... SpamAssassin medium score; maybe let it through but don't
sign"), but then they don't need a BCP; we need them to publish the
results of the experiment ;-)
Quite right, and as always, the ASRG stands ready if someone wants to do
some, you know, research.
The only thing that leaves are non-participant MLMs and there really isn't
much to be done with them.
We have one concrete failure scenario, in which someone who publishes
dkim=discardable sends mail to a MLM that as usual breaks the signature, a
subscriber's mail system carefully follows the ADSP and rejects that mail,
causing the subscriber to be bounced off the list. (This really happened,
on an IETF list.) The advice is obvious a) put a shim in front of your
MLM to reject discardable mail and b) the usual advice not to use ADSP at
all, but it definitely needs publishing.
I'm surprised we haven't seen that problem on this list, since we have at
least one subscriber whose domain publishes dkim=discardable. I keep
having to fish his mail out of the spam folder.
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