The emails are rejected by destination mail server and accumulate in the Messaging Gateway delivery queue with the following status:
421 4.7.0 [TSS04] Messages from IPADDRESS temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes
The destination mail server is rate limiting or otherwise deferring email delivery from the Messaging Gateway due to some email acceptance policy
A 421 or 451 SMTP error indicates a temporary problem blocking the delivery of the message. Message delivery can be delayed for multiple reasons including:
It is recommended to raise a case with 3rd party to obtain the support.