_spf.onemail.vn has no NS records.
NS records for this zone are defined at onemail.vn.
NS records for this zone are defined at onemail.vn.
Question and response
Question
dig _spf.onemail.vn. NS
Answer
The response has no answer records.
Authority
onemail.vn. 3600 SOA ns-master.inet.vn. postmaster.inet.vn. 2015072800 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0