mxbackup2.sbservers.cz has no CNAME records.
Question and response
Question
dig mxbackup2.sbservers.cz. NS
Answer
The response has no answer records.
Authority
sbservers.cz. 1800 SOA darwin.ns.cloudflare.com. dns.cloudflare.com. 2397466684 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0