Messages delivery to a particular domain are failing with the Message Audit Logs showing a delivery failing with the SMTP response
554 5.4.0 < #4.7.0 smtp; 421 4.7.0 [TSS04] Messages from 192.0.2.100 temporarily deferred due to unexpected volume or user complaints - 188.8.131.52; see https://postmaster.yahooinc.com/error-codes>'
Release : 10.8.0
Messaging Gateway is attempting to deliver messages to a destination server which will not accept them due to either rate limits or end user customer complaints.
Messaging Gateway cannot deliver mail to a server that refuses to accept it. Usually these email delivery rejections contain some information regarding the cause and a link to the destination mail server's policy. To resume sending email to the destination domain, you will need to comply with the receiver's email policy.