mx.xcastlabs.com has no CNAME records.
Question and response
Question
dig mx.xcastlabs.com. NS
Answer
The response has no answer records.
Authority
xcastlabs.com. 1800 SOA dan.ns.cloudflare.com. dns.cloudflare.com. 2396867623 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0