mx51.codetel.com.do has no MX records.
Question and response
Question
dig mx51.codetel.com.do. MX
Answer
The response has no answer records.
Authority
codetel.com.do. 86400 SOA ns1.claro.net.do. hostmaster.claro.net.do. 2017030953 21600 900 604800 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0