mx21.mailer.airtel.com has no NS records.
NS records for this zone are defined at mailer.airtel.com.
NS records for this zone are defined at mailer.airtel.com.
Question and response
Question
dig mx21.mailer.airtel.com. NS
Answer
The response has no answer records.
Authority
mailer.airtel.com. 900 SOA infoblox.localdomain. please_set_email.absolutely.nowhere. 184 10800 3600 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0