There are very good technical reasons for why SMTP has an envelope that is separate from the mail headers. DMARC ignores those completely and insists that the envelope matches the mail body and headers.
Net result: DMARC completely breaks mail forwarding and mailing lists. The fun part for anybody running mailing lists (like me) is that there are no alternatives - to appease the DMARC cretins you'd have to violate real mail standards or make your lists broken by disabling useful features of proven efficiency (like VERP) and exposing all your subscribers to bounces and other crap.
In the physical world DMARC would be like prohibiting blank envelopes and requiring that all letters must have the full letter head printed on the front of the envelope.
"this letter contains a new credit card - please steal me!"
Clearly only cretins would think that's a good idea. The list of cretins who insist that DMARC being a great idea includes yahoo, aol and hotmail.
I hope yahoo's dumb move does serve to 'improve protection' - by prompting people to go to somebody more sensible for their mail hosting.
