smtp.bancotui.com has no CNAME records.
Question and response
Question
dig smtp.bancotui.com. NS
Answer
The response has no answer records.
Authority
bancotui.com. 278 SOA dns1.cloud.claro.com.do. bancotui.claro.net.do. 3110620 14400 7200 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0