The DMARC proponents did engage mailman. Version 2.1.16 includes support
for a setting that makes the operation of the list DMARC-friendly, though
likely in a way some people will find unpalatable.
Please tell us more about that.
It turns it into a pure forwarder, no subject tags, message footers,
or anything else that might break a DKIM signature.
Of course, even if you do that, it still won't solve the problems that
DMARC causes. Consider, for example, a Yahoo mail user who prefers
the Gmail interface, so has set up Gmail to collect her Yahoo mail,
and sends her mail from Gmail using her existing Yahoo address. Gmail
does COI confirmation before letting you use your other outgoing
address, no forgery allowed. Yahoo provides POP and IMAP service, so
this is clearly something Yahoo anticipated.
As I said in my prior screed, the only way to un-break DMARC is to
whitelist the exceptions.
R's,
John