mx1.active24.com has no CNAME records.
Question and response
Question
dig mx1.active24.com. NS
Answer
The response has no answer records.
Authority
active24.com. 3288 SOA alfa.ns.active24.cz. hostmaster.active24.cz. 2025110406 10800 1800 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0