Every time a specific partner, whos using King Host as email provider, sends me a message, he gets that error. I could not find anything on Yahoo configs to whitelist that IP or his domain.
<***@***>: host mta5.am0.yahoodns.net[188.8.131.52] said:
553 5.7.2 [TSS09] All messages from 184.108.40.206 will be permanently
deferred; Retrying will NOT succeed. See
https://postmaster.yahooinc.com/error-codes (in reply to MAIL FROM command)