smtp-usa2.onexmail.com has no NS records.
NS records for this zone are defined at com.
NS records for this zone are defined at com.
Question and response
Question
dig smtp-usa2.onexmail.com. NS
Answer
The response has no answer records.
Authority
onexmail.com. 179 SOA ns-tel1.qq.com. webmaster.qq.com. 1773374093 300 600 86400 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0