I have been reading that unless I use a yahoo smtp account I will no longer be able to send email from my yahoo account. My ISP (Optus) prevents an SMTP being used that is not an optus one. So until now my account has been using yahoo pop and optus smtp. It appears, from reading, that DMARC has broken this. It this is the case I will have to shut my yahoo accounts down and move to Google because it will not work this way on Optus.