mailserver.purelymail.com has no NS records.
NS records for this zone are defined at purelymail.com.
NS records for this zone are defined at purelymail.com.
Question and response
Question
dig mailserver.purelymail.com. NS
Answer
The response has no answer records.
Authority
purelymail.com. 900 SOA ns-373.awsdns-46.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0