mail.fortimail.com has no NS records.
NS records for this zone are defined at fortimail.com.
NS records for this zone are defined at fortimail.com.
Question and response
Question
dig mail.fortimail.com. NS
Answer
The response has no answer records.
Authority
fortimail.com. 300 SOA ns3.fortinet.com. mis.fortinet.com. 1 604800 7200 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0