mail.tionnet.com has no NS records.
NS records for this zone are defined at tionnet.com.
NS records for this zone are defined at tionnet.com.
Question and response
Question
dig mail.tionnet.com. NS
Answer
The response has no answer records.
Authority
tionnet.com. 300 SOA ns1.p201.dns.oraclecloud.net. hostmaster.tionnet.com. 11 3600 600 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0