smtp.sip.is has no NS records.
NS records for this zone are defined at sip.is.
NS records for this zone are defined at sip.is.
Question and response
Question
dig smtp.sip.is. NS
Answer
The response has no answer records.
Authority
sip.is. 2560 SOA ns01.ns.sip.is. hostmaster.sip.is. 1773267217 16384 2048 1048576 2560
Additional
The response has no additional records.