mx2.sg.aliyun.com has no NS records.
NS records for this zone are defined at aliyun.com.
NS records for this zone are defined at aliyun.com.
Question and response
Question
dig mx2.sg.aliyun.com. NS
Answer
The response has no answer records.
Authority
aliyun.com. 600 SOA hidden-master.aliyun.com. hostmaster.alibaba-inc.com. 2014073071 3600 900 1209600 600
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0