mail.sbb.rs has no CNAME records.
Question and response
Question
dig mail.sbb.rs. NS
Answer
The response has no answer records.
Authority
sbb.rs. 285 SOA dns1.sbb.rs. hostmaster.sbb.rs. 2026022700 86400 3600 1814400 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0