ns4.aliyun.com has no MX records.
Question and response
Question
dig ns4.aliyun.com. MX
Answer
The response has no answer records.
Authority
aliyun.com. 600 SOA hidden-master.aliyun.com. hostmaster.alibaba-inc.com. 2014073064 3600 900 1209600 600
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0