mx.zctracker.com has no CNAME records.
Question and response
Question
dig mx.zctracker.com. NS
Answer
The response has no answer records.
Authority
zctracker.com. 551 SOA ns1.zohocorp.com. dnsadmin.vtitan.com. 2026022201 7200 1800 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0