sky.ns.cloudflare.com has no MX records.
Question and response
Question
dig sky.ns.cloudflare.com. MX
Answer
The response has no answer records.
Authority
cloudflare.com. 300 SOA ns3.cloudflare.com. dns.cloudflare.com. 2397006400 10000 2400 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0