monica.mailspec.com has no CNAME records.
Question and response
Question
dig monica.mailspec.com. NS
Answer
The response has no answer records.
Authority
mailspec.com. 574 SOA duke.ns.cloudflare.com. dns.cloudflare.com. 2396499705 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0