mail.eri.cz has no CNAME records.
Question and response
Question
dig mail.eri.cz. NS
Answer
The response has no answer records.
Authority
eri.cz. 2560 SOA ns1.websupport.cz. admin.websupport.cz. 1756122190 7200 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0