Emails sent from cPanel to yahoo users are being rejected with the following error:
Messages from X temporarily deferred due to unexpected volume or user complaints – 184.108.40.206; see https://postmaster.yahooinc.com/error-codes
This happens because the IP was blacklisted by Yahoo due to user complaints.
To check if the IP is blocked or the domain name, add another domain name to cPanel and test sending emails to yahoo.
If those are also rejected, then the IP is blacklisted in Yahoo’s internal blacklist and you need to submit a check&delist request here: https://vzmmemberservices.secure.force.com/?fc=SSR
NOTE: The domain name that is used for sending has to have proper DKIM, PTR, and SPF records set.
Yahoo will answer via email within a few hours:
and notify you if the IP can be removed (no user complaints were received recently) or not:
To prevent this from happening again, follow the instructions here to set up a Yahoo FBL (FeedBack Loop) that will notify you if a Yahoo user marks your email as spam.