Sending to Yahoo e mail accounts from my Outlook e mail

Last year Google and Yahoo made a joint announcement that will effectively change how commercial email will be sent and received on a global scale forever. Both Yahoo and Google email platforms will begin to block and aggressively filter incoming email traffic that doesn’t meet new message authentication and procedural requirements. Additionally, they also included some infrastructure and performance thresholds associated with commercial email best practices.
Announcement from Yahoo - https://blog.postmaster.yahooinc.com/post/730172167494483968/more-secure-less-spam

Announcement from Google - https://blog.google/products/gmail/gmail-security-authentication-spam-protection/