moderated Re: automatic deletion, why?
wunder,No, that is explicitly wrong. DMARC is an anti-spoofing mechanism. It has nothing to say about whether the messages are spam or not.
DMARC is an anti-spam system ...
You've got it backwards. DMARC is a request by the sender that anyone receiving a message claiming to be from them (email header From) should make some extra tests and if the message fails the tests the receiver is asked to follow the policy. Some receivers do, some don't.
Hence Yahoo Mail's DMARC policy does not control mail inbound to Yahoo Mail. But Yahoo Mail does implement DMARC checking on inbound messages and they appear to respect the policy settings of others (including their own - for inbound messages that originated at Yahoo Mail).
Correct. They effectively "whitelisted" messages from Yahoo Groups by marking the yahoogroups.com domain as an allowed sender under Yahoo Mail's policies and/or including yahoogroups outbound servers in Yahoo Mail's SPF policy.
No, the member is not unsubscribed for a bounce. An unsubscribe would occur only if Yahoo Mail also sent an abuse report to Groups.io - and I'm not sure it is possible or valid to send such a report for a message the service rejected at the outset.
And some email receivers, such as Gmail, don't strictly obey the sender's DMARC policy. Gmail takes into account more than that before deciding how to handle an inbound message. And in particular, Gmail appears to reliably ignore senders' DMARC when a message passes through a mail list like Groups.io.