spf_b.rackspace.com has no MX records.
Question and response
Question
dig spf_b.rackspace.com. MX
Answer
The response has no answer records.
Authority
rackspace.com. 300 SOA ns.rackspace.com. hostmaster.rackspace.com. 1770412730 300 300 1814400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0