Yahoo changed thier DMARC policy in 2014 to proactively protect users from increasing email forgery spam. Here's information about why your messages will be rejected due to failed authentication.
Why are my emails bouncing?
In 2014 Yahoo updated the DMARC record with "p=reject" for the "yahoo.com" domain.
This means all DMARC compliant mail receivers including us are now bouncing emails sent as "@yahoo.com" addresses that aren't sent through Yahoo servers. Any messages without a proper Domain Keys Identified Mail (DKIM) signature or Sender Policy Framework (SPF) alignment will be rejected by our servers.
Senders who use their "@yahoo.com" address as the "From" address to send messages are impacted by this change. More information can be found at https://help.yahoo.com/kb/SLN24016.html
Yahoo's help central suggests that if your emails from "@yahoo.com" addresses are being blocked, the sender should should make sure they are using Yahoo's outgoing mail servers, and have their username, password and reply-to address set correctly. We recommend that you do not use Yahoo's IOS apps, Yahoo's Android apps, and email clients with an "easy setup" to set up your Yahoo account. If you set up your account manually using your email client, and Yahoo's recommended settings, you shouldn't encounter any issues.