mx.inbound.elasticemail.com has no NS records.
NS records for this zone are defined at elasticemail.com.
NS records for this zone are defined at elasticemail.com.
Question and response
Question
dig mx.inbound.elasticemail.com. NS
Answer
The response has no answer records.
Authority
elasticemail.com. 900 SOA ns-270.awsdns-33.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0