mail.successinet.com has no CNAME records.
Question and response
Question
dig mail.successinet.com. NS
Answer
The response has no answer records.
Authority
successinet.com. 2179 SOA ns1.successinet.com. hostmaster.successinet.com. 2018042516 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0