mail.execulink.com has no CNAME records.
Question and response
Question
dig mail.execulink.com. NS
Answer
The response has no answer records.
Authority
execulink.com. 6435 SOA mallet.execulink.net. sysadmin.execulink.com. 2026010801 158914 300 604800 345600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0