mx1.sg.aliyun.com has no CNAME records.
Question and response
Question
dig mx1.sg.aliyun.com. NS
Answer
The response has no answer records.
Authority
aliyun.com. 554 SOA hidden-master.aliyun.com. hostmaster.alibaba-inc.com. 2014073074 3600 900 1209600 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0