mta81.r3.rpost.net has no NS records.
NS records for this zone are defined at r3.rpost.net.
NS records for this zone are defined at r3.rpost.net.
Question and response
Question
dig mta81.r3.rpost.net. NS
Answer
The response has no answer records.
Authority
r3.rpost.net. 900 SOA ns-143.awsdns-17.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0