On April 5, 2014, Yahoo broke every discussion mailing list on the internet: http://www.ietf.org/mail-archive/web/ietf/current/msg87153.html AOL followed soon after, and in June of 2016, Gmail also published a similar DMARC policy.

This action means that any yahoo, aol or gmail user who posts a message to a discussion list would see their message bounced by most of the other big ISPs who honor the DMARC policy, e.g. Outlook/Hotmail, Comcast, and others.

To workaround this issue, DataBack/CCtoMany now rewrites the From: header on all discussion list posts. See this page for more details.