mail.dc3.cz has no MX records.
Question and response
Question
dig mail.dc3.cz. MX
Answer
The response has no answer records.
Authority
dc3.cz. 600 SOA ns1.tele3.cz. root.tele3.cz. 1771828833 28800 7200 1209600 43200
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0