mail.redrool.com has no NS records.
NS records for this zone are defined at redrool.com.
NS records for this zone are defined at redrool.com.
Question and response
Question
dig mail.redrool.com. NS
Answer
The response has no answer records.
Authority
redrool.com. 1800 SOA ara.ns.cloudflare.com. dns.cloudflare.com. 2396878480 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0