mx3.ddos-guard.net has no MX records.
Question and response
Question
dig mx3.ddos-guard.net. MX
Answer
The response has no answer records.
Authority
ddos-guard.net. 3600 SOA ns1.ddos-guard.net. support.ddos-guard.net. 1771617362 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0