dfw1.rackspace.net has no MX records.
Question and response
Question
dig dfw1.rackspace.net. MX
Answer
The response has no answer records.
Authority
dfw1.rackspace.net. 300 SOA ns.rackspace.com. hostmaster.rackspace.com. 1770734274 3600 300 1814400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0