imap.gmail.com has no NS records.
NS records for this zone are defined at gmail.com.
NS records for this zone are defined at gmail.com.
Question and response
Question
dig imap.gmail.com. NS
Answer
The response has no answer records.
Authority
gmail.com. 60 SOA ns1.google.com. dns-admin.google.com. 878435500 900 900 1800 60
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0