smtp2-ext.ord1.corp.rackspace.com has no MX records.
Question and response
Question
dig smtp2-ext.ord1.corp.rackspace.com. MX
Answer
The response has no answer records.
Authority
smtp2-ext.ord1.corp.rackspace.com. 300 SOA ns.rackspace.com. hostmaster.rackspace.com. 1528380565 3600 300 1814400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0