_emailing.heureka.cz has no MX records.
Question and response
Question
dig _emailing.heureka.cz. MX
Answer
The response has no answer records.
Authority
heureka.cz. 1800 SOA marjory.ns.cloudflare.com. dns.cloudflare.com. 2397911372 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0