mailing.masterdc.com has no CNAME records.
Question and response
Question
dig mailing.masterdc.com. NS
Answer
The response has no answer records.
Authority
masterdc.com. 3204 SOA ns1.master.cz. noc.master.cz. 2025120266 7200 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0