This issue is caused by Yahoo’s policy change
DMARC.org was formed to allow email senders to influence unauthenticated mail by publishing their preferences in a flexible policy. It also enables participating email providers to provide reports so that senders can improve and monitor their authentication infrastructure.
Google is participating in DMARC along with other email domains like AOL, Comcast, Hotmail, and Yahoo! Mail. In addition, senders like Bank of America, Facebook, Fidelity, LinkedIn, and Paypal have already published policies for Google and other receivers to follow.
It affects all mail providers including Google. Google so far have not provided a full solution
Here’s what they’ve provided us with: https://support.google.com/mail/answer/6010282