mail.pc021.co.rs has no CNAME records.
Question and response
Question
dig mail.pc021.co.rs. NS
Answer
The response has no answer records.
Authority
pc021.co.rs. 1336 SOA dave.ns.cloudflare.com. dns.cloudflare.com. 2395654791 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0