wm13.mailplug.com has no CNAME records.
Question and response
Question
dig wm13.mailplug.com. NS
Answer
The response has no answer records.
Authority
mailplug.com. 396 SOA ns.mailplug.com. root.mailplug.com. 2026022501 21600 1800 1209600 400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0