external.smtprelay.rackspace.net has no MX records.
Question and response
Question
dig external.smtprelay.rackspace.net. MX
Answer
The response has no answer records.
Authority
external.smtprelay.rackspace.net. 300 SOA ns.rackspace.com. hostmaster.rackspace.com. 1645030770 3600 300 1814400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0