wmail13.mailplug.com has no CNAME records.
Question and response
Question
dig wmail13.mailplug.com. NS
Answer
The response has no answer records.
Authority
mailplug.com. 328 SOA ns.mailplug.com. root.mailplug.com. 2026022501 21600 1800 1209600 400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0