mail.yeonov.com has no NS records.
NS records for this zone are defined at yeonov.com.
NS records for this zone are defined at yeonov.com.
Question and response
Question
dig mail.yeonov.com. NS
Answer
The response has no answer records.
Authority
yeonov.com. 3601 SOA dns1.registrar-servers.com. hostmaster.registrar-servers.com. 1772811133 43200 3600 604800 3601
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0