spf.active24.com has no MX records.
Question and response
Question
dig spf.smtp.cz. MX
Answer
The response has no answer records.
Authority
smtp.cz. 3600 SOA ns.dns4.cz. hostmaster.active24.cz. 2026022301 10800 1800 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0