mail.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.linkedin.com. NS
Answer
The response has no answer records.
Authority
linkedin.com. 60 SOA ns1-42.azure-dns.com. hostmaster.linkedin.com. 1 3600 300 604800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0