www.rackspace.com has no MX records.
Question and response
Question
dig e13746.dscb.akamaiedge.net. MX
Answer
The response has no answer records.
Authority
dscb.akamaiedge.net. 1000 SOA n0dscb.akamaiedge.net. hostmaster.akamai.com. 1772014817 1000 1000 1000 1800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0