https://senders.yahooinc.com/best-practices/

Delivery has failed to these recipients or groups:
Your message couldn't be delivered. Despite repeated attempts to contact the recipient's email system it didn't respond.
Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their email system isn't accepting connection requests from your email system. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem.
For more information and tips to fix this issue see this article: https://go.microsoft.com/fwlink/?LinkId=389361.

Solution: Enable SPF and/or DKIM on the domain.