mixer.unisender.com has no NS records.
NS records for this zone are defined at unisender.com.
NS records for this zone are defined at unisender.com.
Question and response
Question
dig mixer.unisender.com. NS
Answer
The response has no answer records.
Authority
unisender.com. 3600 SOA ns1.yandexcloud.net. mx.cloud.yandex.net. 1 10800 900 604800 900
Additional
. 0 OPT ; payload 1024, xrcode 0, version 0, flags 0