sshmail.smtp25.com has no NS records.
NS records for this zone are defined at smtp25.com.
NS records for this zone are defined at smtp25.com.
Question and response
Question
dig sshmail.smtp25.com. NS
Answer
The response has no answer records.
Authority
smtp25.com. 900 SOA ns-1553.awsdns-02.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0