sender2.servercore.mx has no NS records.
NS records for this zone are defined at servercore.mx.
NS records for this zone are defined at servercore.mx.
Question and response
Question
dig sender2.servercore.mx. NS
Answer
The response has no answer records.
Authority
servercore.mx. 1800 SOA a.ns.srv.cr. dns.cloudflare.com. 2397072318 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0