sgmail100.mail.163.com has no NS records.
NS records for this zone are defined at 163.com.
NS records for this zone are defined at 163.com.
Question and response
Question
dig sgmail100.mail.163.com. NS
Answer
The response has no answer records.
Authority
163.com. 60 SOA ns4.nease.net. admin.nease.net. 20292265 7200 1800 1209600 60
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0