smtp.google.com has no NS records.
NS records for this zone are defined at google.com.
NS records for this zone are defined at google.com.
Question and response
Question
dig smtp.google.com. NS
Answer
The response has no answer records.
Authority
google.com. 60 SOA ns1.google.com. dns-admin.google.com. 873285235 900 900 1800 60
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0