mx01.mailplug.com has no CNAME records.
Question and response
Question
dig mx01.mailplug.com. NS
Answer
The response has no answer records.
Authority
mailplug.com. 8 SOA ns.mailplug.com. root.mailplug.com. 2026020400 21600 1800 1209600 400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0