mail05.vimn.com has no NS records.
NS records for this zone are defined at vimn.com.
NS records for this zone are defined at vimn.com.
Question and response
Question
dig mail05.vimn.com. NS
Answer
The response has no answer records.
Authority
vimn.com. 86400 SOA dns1.p09.nsone.net. hostmaster.nsone.net. 1652930137 600 600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0