6.168.belnet.sk has no MX records.
Question and response
Question
dig 6.168.belnet.sk. MX
Answer
The response has no answer records.
Authority
belnet.sk. 900 SOA ns.inetadmin.eu. root.inetadmin.eu. 1772266785 86400 900 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0