mailhk.hikvision.com has no MX records.
Question and response
Question
dig mailhk.hikvision.com. MX
Answer
The response has no answer records.
Authority
hikvision.com. 180 SOA ns3.dnsv5.com. enterprise3dnsadmin.dnspod.com. 1771209643 3600 180 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0