mx4.mail.bg has no CNAME records.
Question and response
Question
dig mx4.mail.bg. NS
Answer
The response has no answer records.
Authority
mail.bg. 5948 SOA ns.mail.bg. webmaster.mail.bg. 2023072677 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0