mx.z.com has no CNAME records.
Question and response
Question
dig mx.z.com. NS
Answer
The response has no answer records.
Authority
z.com. 1508 SOA ns1.cf.gmointernet.jp. dns.cloudflare.com. 2396445585 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0