dc.xo.com has no MX records.
Question and response
Question
dig dc.xo.com. MX
Answer
The response has no answer records.
Authority
dc.xo.com. 3600 SOA ns-twbg.verizonwireless.com. hostmaster.xo.com. 2016040997 86400 86400 86400 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0