mx2.ns1.bg has no NS records.
NS records for this zone are defined at ns1.bg.
NS records for this zone are defined at ns1.bg.
Question and response
Question
dig mx2.ns1.bg. NS
Answer
The response has no answer records.
Authority
ns1.bg. 1800 SOA coco.ns.cloudflare.com. dns.cloudflare.com. 2396111442 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0