mail.ddns.net has no MX records.
Question and response
Question
dig mail.ddns.net. MX
Answer
The response has no answer records.
Authority
ddns.net. 1800 SOA nf1.no-ip.com. hostmaster.no-ip.com. 2678090273 10800 1800 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0