cmail01.mailplug.com has no CNAME records.
Question and response
Question
dig cmail01.mailplug.com. NS
Answer
The response has no answer records.
Authority
mailplug.com. 175 SOA ns.mailplug.com. root.mailplug.com. 2026020400 21600 1800 1209600 400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0