spmx.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 spmx.l.google.com. NS
Answer
The response has no answer records.
Authority
l.google.com. 60 SOA ns1.google.com. dns-admin.google.com. 879543655 900 900 1800 60
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0