alt3.aspmx.l.google.com has no CNAME records.
Question and response
Question
dig alt3.aspmx.l.google.com. NS
Answer
The response has no answer records.
Authority
l.google.com. 6 SOA ns1.google.com. dns-admin.google.com. 873285235 900 900 1800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0