mx.sendgrid.net has no NS records.
NS records for this zone are defined at sendgrid.net.
NS records for this zone are defined at sendgrid.net.
Question and response
Question
dig mx.sendgrid.net. NS
Answer
The response has no answer records.
Authority
sendgrid.net. 180 SOA ns10.dnsmadeeasy.com. dns.dnsmadeeasy.com. 2011293935 14400 600 1728000 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0