mail.eserver.rs has no CNAME records.
Question and response
Question
dig mail.eserver.rs. NS
Answer
The response has no answer records.
Authority
eserver.rs. 10525 SOA ns1.sk.eserver.net. maxim.azarov.com. 2026022701 10800 3600 1209600 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0