ns4.superdnsserver.net has no MX records.
Question and response
Question
dig ns4.superdnsserver.net. MX
Answer
The response has no answer records.
Authority
superdnsserver.net. 900 SOA ns1.superdnsserver.net. hostmaster.superhosting.bg. 2013076243 3600 600 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0