Apex domains cannot have CNAME records.
Question and response
Question
dig corpmail.netsolir.com. NS
Answer
corpmail.netsolir.com. 1 NS ns2.netsolir.com.
corpmail.netsolir.com. 1 NS ns1.netsolir.com.
Authority
The response has no authority records.
Additional
ns1.netsolir.com. 1 A 203.170.64.140
ns2.netsolir.com. 1 A 203.170.64.141
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0