smtp.clientpay.com has no NS records.
NS records for this zone are defined at clientpay.com.
NS records for this zone are defined at clientpay.com.
Question and response
Question
dig smtp.clientpay.com. NS
Answer
The response has no answer records.
Authority
clientpay.com. 900 SOA ns-596.awsdns-10.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0