mx.bbox.fr has no NS records.
NS records for this zone are defined at bbox.fr.
NS records for this zone are defined at bbox.fr.
Question and response
Question
dig mx.bbox.fr. NS
Answer
The response has no answer records.
Authority
bbox.fr. 900 SOA primary.localdomain. dnsextmaster.bouyguestelecom.fr. 2012091938 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0