smtps21.o2.cz has no MX records.
Question and response
Question
dig smtps21.o2.cz. MX
Answer
The response has no answer records.
Authority
o2.cz. 10800 SOA dns.iol.cz. hostmaster.iol.cz. 2026030405 10800 3600 2592000 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0