mail-a.linkedin.com has no NS records.
NS records for this zone are defined at linkedin.com.
NS records for this zone are defined at linkedin.com.
Question and response
Question
dig mail-a.linkedin.com. NS
Answer
The response has no answer records.
Authority
linkedin.com. 300 SOA dns1.p09.nsone.net. hostmaster.linkedin.com. 1660282913 3600 600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0