ns.rackspace.com has no MX records.
Question and response
Question
dig ns.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