mx-20.pc4.atmailcloud.com has no NS records.
NS records for this zone are defined at pc4.atmailcloud.com.
NS records for this zone are defined at pc4.atmailcloud.com.
Question and response
Question
dig mx-20.pc4.atmailcloud.com. NS
Answer
The response has no answer records.
Authority
pc4.atmailcloud.com. 900 SOA ns-1015.awsdns-62.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0