Apex domains cannot have CNAME records.
Question and response
Question
dig mailer.airtel.com. NS
Answer
mailer.airtel.com. 2 NS aaadel.mantraonline.com.
mailer.airtel.com. 2 NS dnsdel.mantraonline.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0