alt1.aspmx.l.google.com has no NS records.
NS records for this zone are defined at l.google.com.
NS records for this zone are defined at l.google.com.
Question and response
Question
dig alt1.aspmx.l.google.com. NS
Answer
The response has no answer records.
Authority
l.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