pop.gmail.com has no MX records.
Question and response
Question
dig pop.gmail.com. MX
Answer
The response has no answer records.
Authority
gmail.com. 60 SOA ns1.google.com. dns-admin.google.com. 878435500 900 900 1800 60
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0