https://help.yahoo.com/kb/postmaster/SLN7253.html
Error: "554 5.7.9: Message not accepted for policy reasons" when sending email to Yahoo
If your emails aren't being received by Yahoo accounts, it's a good idea to review our Yahoo Mail deliverability FAQ to make sure you're following industry standards for bulk email senders.
Why you're seeing this error
Your message wasn't delivered because Yahoo was unable to verify that it came from a legitimate email sender.
Your email failed one or more authentication checks that Yahoo uses to verify emails are truly sent from the domains they claim to originate from.
