ipv6.cloud.do has no MX records.
Question and response
Question
dig ipv6.cloud.do. MX
Answer
The response has no answer records.
Authority
cloud.do. 1800 SOA iris.ns.cloudflare.com. dns.cloudflare.com. 2396769180 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0