ns4.cdnsun.net has no MX records.
Question and response
Question
dig ns4.cdnsun.net. MX
Answer
The response has no answer records.
Authority
cdnsun.net. 180 SOA cdnsun.net. hostmaster.worldcdn.com. 1771925444 1200 180 1209600 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0