smtp02.sudumail.com has no NS records.
NS records for this zone are defined at sudumail.com.
NS records for this zone are defined at sudumail.com.
Question and response
Question
dig smtp02.sudumail.com. NS
Answer
The response has no answer records.
Authority
sudumail.com. 600 SOA ens11.bigwww.com. hostmaster.hichina.com. 2026011209 3600 1200 86400 600
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0