sg1.rmailgateway.com has no NS records.
NS records for this zone are defined at rmailgateway.com.
NS records for this zone are defined at rmailgateway.com.
Question and response
Question
dig sg1.rmailgateway.com. NS
Answer
The response has no answer records.
Authority
rmailgateway.com. 900 SOA ns-1490.awsdns-58.org. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0