cluster-b.mailcontrol.com has no CNAME records.
Question and response
Question
dig cluster-b.mailcontrol.com. NS
Answer
The response has no answer records.
Authority
mailcontrol.com. 822 SOA ns-a.blackspider.com. hostmaster.blackspider.com. 2026021202 3600 600 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0