mail.online.rs has no MX records.
Question and response
Question
dig mail.online.rs. MX
Answer
The response has no answer records.
Authority
online.rs. 86400 SOA dns1.eunethosting.com. hostmaster.eunet.co.rs. 2026022310 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0