mx2.mail.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.mail.aliyun.com.gds.alibabadns.com. NS
Answer
The response has no answer records.
Authority
gds.alibabadns.com. 600 SOA gdsns1.alibabadns.com. none. 2018122017 1800 600 3600 360
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0