mx.grhosting.cz has no CNAME records.
Question and response
Question
dig mx.grhosting.cz. NS
Answer
The response has no answer records.
Authority
grhosting.cz. 3571 SOA ns.grdns.cz. admin.grdns.cz. 2026010601 3600 1800 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0