o2.mail.sansan.com has no CNAME records.
Question and response
Question
dig o2.mail.sansan.com. NS
Answer
The response has no answer records.
Authority
sansan.com. 300 SOA ns-241.awsdns-30.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0