At least in the back-of-the-envelope scheme I suggested, the receiver
doesn't need to distinguish mailing lists: The originator's system finds
out where the mail is going, gets some information from the destination,
and signs that and sends it with the message. The mailing list sends
that along to the recipients. When my (one of the recipient's) server
looks at that info, it determines that the originator sent the message
directly to the mailing list, and I can tell that the mailing list sent
it to me.
I still think it has security holes but it's worth arguing about, so
I'll follow up on ietf-822, which seems relevant and hasn't been very
busy.
R's,
John