mails.tsinghua.edu.cn has no NS records.
NS records for this zone are defined at tsinghua.edu.cn.
NS records for this zone are defined at tsinghua.edu.cn.
Question and response
Question
dig mails.tsinghua.edu.cn. NS
Answer
The response has no answer records.
Authority
tsinghua.edu.cn. 600 SOA dns2.edu.cn. szhu.net.tsinghua.edu.cn. 2099126318 3600 900 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0