smtp-01.servidoresdns.net has no NS records.
NS records for this zone are defined at servidoresdns.net.
NS records for this zone are defined at servidoresdns.net.
Question and response
Question
dig smtp-01.servidoresdns.net. NS
Answer
The response has no answer records.
Authority
servidoresdns.net. 3600 SOA atlante.servidoresdns.net. hostmaster.arsys.es. 2005152371 86400 7200 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0