mail.sharepointonline.com has no NS records.
NS records for this zone are defined at sharepointonline.com.
NS records for this zone are defined at sharepointonline.com.
Question and response
Question
dig mail.sharepointonline.com. NS
Answer
The response has no answer records.
Authority
sharepointonline.com. 300 SOA ns1-221.azure-dns.com. azuredns-hostmaster.microsoft.com. 1 3600 300 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0