mail02.bigcenter.com has no NS records.
NS records for this zone are defined at bigcenter.com.
NS records for this zone are defined at bigcenter.com.
Question and response
Question
dig mail02.bigcenter.com. NS
Answer
The response has no answer records.
Authority
bigcenter.com. 1800 SOA fred.ns.cloudflare.com. dns.cloudflare.com. 2396057798 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0