mx51.codetel.com.do has no CNAME records.
Question and response
Question
dig mx51.codetel.com.do. NS
Answer
The response has no answer records.
Authority
codetel.com.do. 5438 SOA ns1.claro.net.do. hostmaster.claro.net.do. 2017030953 21600 900 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0