gmail-smtp-in.l.google.COM has no MX records.
Question and response
Question
dig gmail-smtp-in.l.google.COM. MX
Answer
The response has no answer records.
Authority
l.google.COM. 60 SOA ns1.google.COM. dns-admin.google.COM. 873978624 900 900 1800 60
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0