spf.yettel.rs has no MX records.
Question and response
Question
dig spf.yettel.rs. MX
Answer
The response has no answer records.
Authority
yettel.rs. 3600 SOA dinosaur.cetin.rs. Administrator. 255 600 300 86400 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0