sip.cooolbox.bg has no MX records.
Question and response
Question
dig sip.cooolbox.bg. MX
Answer
The response has no answer records.
Authority
cooolbox.bg. 1800 SOA bradley.ns.cloudflare.com. dns.cloudflare.com. 2397213675 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0