You will need to see what MX record is pointing to your domain. For example try network-tools.com using a DNS record search like this. It may be that their server has an old cached record. In that case, you will need to inform the sending postmaster of the issue and ask him to clear the DNS cache and/or see if he can get the correct MX record from a different DNS provider. Take a look at any non-deliverable messages and see if you can tell what server is sending these messages and that is likely going to be the one that needs “fixed”.
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