mx3.atcomp.cz has no CNAME records.
Question and response
Question
dig mx3.atcomp.cz. NS
Answer
The response has no answer records.
Authority
atcomp.cz. 191 SOA ns6.atcomp.cz. hostmaster.atcomp.cz. 2026010601 14400 3600 3600000 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0