mail.ioi.com.ua has no NS records.
NS records for this zone are defined at ioi.com.ua.
NS records for this zone are defined at ioi.com.ua.
Question and response
Question
dig mail.ioi.com.ua. NS
Answer
The response has no answer records.
Authority
ioi.com.ua. 3600 SOA nsa1.srv53.net. support.dnshosting.org. 1773092967 1800 600 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0