there's an ARC addon for Mailman and we use that, the DMARC damage should drop considerably, without us having to change the way we use our lists.
It's not clear to me that this is true John. DMARC Validators will need to take ARC into consideration and we don't know what adoption will look like other than a handful of players at this point.
I certainly don't think that ARC will make all the problems go away, but it should mostly fix the problems at Gmail and Yahoo which are a large fraction of the mail traffic.
For smaller sites, if they do anything with DMARC at all, they'll still need to whitelist mailing lists, which of course they could do now.
R's, John