mx-in.g.apple.com has no NS records.
NS records for this zone are defined at g.apple.com.
NS records for this zone are defined at g.apple.com.
Question and response
Question
dig mx-in.g.apple.com. NS
Answer
The response has no answer records.
Authority
g.apple.com. 300 SOA a.gslb.aaplimg.com. hostmaster.apple.com. 1715723413 1800 300 60480 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0