smtp243.em5service.cz has no MX records.
Question and response
Question
dig smtp243.em5service.cz. MX
Answer
The response has no answer records.
Authority
em5service.cz. 1800 SOA ns1.zcom.cz. hostmaster.zcom.cz. 1772821305 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0