smtpmx.axtel.com.mx has no CNAME records.
Question and response
Question
dig smtpmx.axtel.com.mx. NS
Answer
The response has no answer records.
Authority
axtel.com.mx. 2546 SOA ns-mty.axtel.net. soporteip.axtel.com.mx. 762215079 7200 1200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0