mx-20.pc5.atmailcloud.com has no NS records.
NS records for this zone are defined at atmailcloud.com.
NS records for this zone are defined at atmailcloud.com.
Question and response
Question
dig mx-20.pc5.atmailcloud.com. NS
Answer
The response has no answer records.
Authority
pc5.atmailcloud.com. 830 SOA ns-644.awsdns-16.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0