Is the fqdn of your server mydomain.com or ns1.mydomain.com? That makes a big difference, as postfix threats mydomain.com and ns1.mydomain.com as different domains, so no problems should occur.
Please post the output of:
grep mydomain.com /etc/postfix/main.cf
to see if mydomain.com is used anywhere in that file instead of ns1.mydomain.com
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