i1.smtp.net has no NS records.
NS records for this zone are defined at smtp.net.
NS records for this zone are defined at smtp.net.
Question and response
Question
dig i1.smtp.net. NS
Answer
The response has no answer records.
Authority
smtp.net. 180 SOA ns10.digicertdns.com. dns.digicertdns.com. 2009010303 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0