_v4nets.nic.cz has no MX records.
Question and response
Question
dig _v4nets.nic.cz. MX
Answer
The response has no answer records.
Authority
nic.cz. 1800 SOA a.ns.nic.cz. hostmaster.nic.cz. 1771859059 14400 3600 1209600 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0