spf.protection.inumbo.com has no MX records.
Question and response
Question
dig spf.protection.inumbo.com. MX
Answer
The response has no answer records.
Authority
inumbo.com. 300 SOA ns.rackspace.com. info.halon.se. 1770635208 21600 3600 1814400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0