Christian F Buser via Mailman-Users writes:

"from what I can see from the text files you sent me, it's a matter of the mailing list configuration. The sender ( From: ) needs to be set as the mailing list address and not as the original sender (eg, ekxxxxxka@yahoo.com)

Your provider, and Yahoo!, are advocating violating the standards that *define* the Internet. Putting the list's name in From means that the list is claiming responsibility for the content of the message.

• but all mailing lists I know usually have the original sender's name in the From-field...

That is correct usage according to the definition of the From field.

So what am I doing wrong here?

<rant> According to the standards that define the Internet, *you* are doing nothing wrong (except maybe you shouldn't be accepting posts from Yahoo! email addresses[1]). Yahoo! screwed up by leaking millions of users' address books to spammers and phishers. DMARC is the only effective way to protect the people *in* those address books from spam and phishing messages apparently sent by their friends who use Yahoo! for email, but Yahoo! is completely unapologetic about the collateral damage to both mailing lists and to many small businesses. I tell my friends that friends don't let friends use Yahoo! accounts -- both for reasons of social conscience and because there are a lot of ways that Yahoo!-originated mail can end up in the bitbucket.[2] </rant>

So bow in the direction of the nearest root nameserver, apologize to the Higher Powers of the Internet, and set dmarc_moderation_action to "Munge From". This will put the mailing list's address in From, along with the user's address in the "display name", for those posters whose email providers abuse DMARC the same way that Yahoo! does, and only those posters.

Footnotes: [1] I'm not entirely joking. For example, that is the official policy of my employer, the Japanese Ministry of Education, because of the mess AOL and Yahoo! created in April 2014 in this way. Ironically enough, the Japanese affiliate is one of the few in the Yahoo! family that (still) does NOT cause this problem.

[2] Of the popular freemail providers, GMail and Hotmail do not have this problem (yet, they say they won't, but nobody can be 100% sure they'll never leak a few million address books to the black hats :-( ). You can can check your local providers by looking up the TXT record for the _dmarc subdomain of the domain, and checking if it contains "p=reject". For example:

$ host -t TXT _dmarc.yahoo.de
v=DMARC1\; p=reject\; pct=100\; rua=mailto:dmarc_y_rua@yahoo.com\;

Your posts will bounce or be silently discarded at many subscribers to many mailing lists.