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What is the issue you’re encountering
Yesterday I transferred the name servers to you, since then I have not been able to send emails through the website. It gets the error message I mentioned. I use the Smtp Fluent plugin so far there has been no such problem. I need your support please what can be done. Thank you!
According to the report at the link above, your MX records point to a host named mail in your domain. That hostname resolves to a non-routable IP of 192.0.2.0 from the reserved TEST-NET-1 allocation. If your SMTP plugin is attempting to use that server by its hostname, that would certainly lead to failure. You should replace that IP with the IP that your mailserver actually uses.