mx143.emailfiltering.com has no CNAME records.
Question and response
Question
dig mx143.emailfiltering.com. NS
Answer
The response has no answer records.
Authority
emailfiltering.com. 1166 SOA dell.ns.cloudflare.com. dns.cloudflare.com. 2395336644 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0