mx13.blackberry.com has no NS records.
NS records for this zone are defined at blackberry.com.
NS records for this zone are defined at blackberry.com.
Question and response
Question
dig mx13.blackberry.com. NS
Answer
The response has no answer records.
Authority
blackberry.com. 86400 SOA ns-770.awsdns-32.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0