_spf.frontier.com has no MX records.
Question and response
Question
dig _spf.frontier.com. MX
Answer
The response has no answer records.
Authority
frontier.com. 3600 SOA auth.roch.ny.frontiernet.net. hostmaster.frontiernet.net. 2602200001 900 300 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0