The mail domain name being non-existent, removed or broken DKIM-related DNS resource records or incorrect configuration of DNS service are among the causes that can raise this error.
As the first step, check and confirm whether the email is sent with the correct domain name. The domain needs to resolve to a valid IP globally, and you must verify that all DKIM-related DNS resources are within the domain zone. Then via the DKIM in cPanel, disable and enable mail signing again.
The DNS propagation period can take up to 48 hours, and wait patiently until the changes are loaded, updated, and synchronized.
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