ispf1.mlbox.email has no CNAME records.
Question and response
Question
dig ispf1.mlbox.email. NS
Answer
The response has no answer records.
Authority
mlbox.email. 1142 SOA carla.ns.cloudflare.com. dns.cloudflare.com. 2397192880 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0