mx-net28.ext.oss.as9143.net has no MX records.
Question and response
Question
dig mx-net28.ext.oss.as9143.net. MX
Answer
The response has no answer records.
Authority
ext.oss.as9143.net. 900 SOA ns1.as9143.net. dnsadmin.as9143.net. 113 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1400, xrcode 0, version 0, flags 0