smtp.si21.com has no MX records.
Question and response
Question
dig smtp.si21.com. MX
Answer
The response has no answer records.
Authority
si21.com. 3600 SOA ns1.kabi.si. postmaster.kabi.si. 202207680 3600 1800 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0