mx1.dfw1.rackspace.com has no MX records.
Question and response
Question
dig mx1.dfw1.rackspace.com. MX
Answer
The response has no answer records.
Authority
mx1.dfw1.rackspace.com. 300 SOA ns.rackspace.com. hostmaster.rackspace.com. 1524235745 3600 300 1814400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0