mail.ddns.net has no NS records.
NS records for this zone are defined at ddns.net.
NS records for this zone are defined at ddns.net.
Question and response
Question
dig mail.ddns.net. NS
Answer
The response has no answer records.
Authority
ddns.net. 1800 SOA nf1.no-ip.com. hostmaster.no-ip.com. 2679608165 10800 1800 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0