a.zdnscloud.com has no MX records.
Question and response
Question
dig a.zdnscloud.com. MX
Answer
The response has no answer records.
Authority
zdnscloud.com. 3600 SOA dns1.zdnscloud.info. mail.knet.cn. 1 3600 3600 3600 900
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0