dcv.cloudflare.com has no MX records.
Question and response
Question
dig dcv.cloudflare.com. MX
Answer
The response has no answer records.
Authority
dcv.cloudflare.com. 1800 SOA graham.ns.cloudflare.com. dns.cloudflare.com. 2397394654 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0