_spf.billtrust.com has no MX records.
Question and response
Question
dig _spf.billtrust.com. MX
Answer
The response has no answer records.
Authority
billtrust.com. 180 SOA ns20.digicertdns.com. dns.digicertdns.com. 2009014643 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0