inbound.postmarkapp.com has no NS records.
NS records for this zone are defined at postmarkapp.com.
NS records for this zone are defined at postmarkapp.com.
Question and response
Question
dig inbound.postmarkapp.com. NS
Answer
The response has no answer records.
Authority
postmarkapp.com. 300 SOA ns-1262.awsdns-29.org. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0