sid.silverpop.com has no NS records.
NS records for this zone are defined at silverpop.com.
NS records for this zone are defined at silverpop.com.
Question and response
Question
dig sid.silverpop.com. NS
Answer
The response has no answer records.
Authority
silverpop.com. 900 SOA ns1.acoustic-adm.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0