mack.ns.cloudflare.com has no MX records.
Question and response
Question
dig mack.ns.cloudflare.com. MX
Answer
The response has no answer records.
Authority
cloudflare.com. 300 SOA ns3.cloudflare.com. dns.cloudflare.com. 2398129632 10000 2400 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0