SMTP.GOOGLE.com has no CNAME records.
Question and response
Question
dig SMTP.GOOGLE.com. NS
Answer
The response has no answer records.
Authority
GOOGLE.com. 53 SOA ns1.GOOGLE.com. dns-admin.GOOGLE.com. 873609184 900 900 1800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0