sender2.servercore.mx has no CNAME records.
Question and response
Question
dig sender2.servercore.mx. NS
Answer
The response has no answer records.
Authority
servercore.mx. 749 SOA a.ns.srv.cr. dns.cloudflare.com. 2397072318 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0