mailus.hikvision.com has no CNAME records.
Question and response
Question
dig mailus.hikvision.com. NS
Answer
The response has no answer records.
Authority
hikvision.com. 180 SOA ns3.dnsv5.com. enterprise3dnsadmin.dnspod.com. 1771985203 3600 180 1209600 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0