mx.rtn.net.do has no CNAME records.
Question and response
Question
dig mx.rtn.net.do. NS
Answer
The response has no answer records.
Authority
rtn.net.do. 3508 SOA ns1.accountsupport.com. dnsadmin.accountsupport.com. 2017111657 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0