mx1b20.thinline.cz has no CNAME records.
Question and response
Question
dig mx1b20.thinline.cz. NS
Answer
The response has no answer records.
Authority
thinline.cz. 1800 SOA ns1.thinline.cz. hostmaster.thinline.cz. 1533063932 86400 300 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0