mail.oo155.com has no CNAME records.
Question and response
Question
dig mail.oo155.com. NS
Answer
The response has no answer records.
Authority
oo155.com. 119 SOA ns-389.awsdns-48.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0