co has no MX records.
Question and response
Question
dig co. MX
Answer
The response has no answer records.
Authority
co. 900 SOA ns0.centralnic.net. hostmaster.centralnic.net. 1759592683 900 1800 6048000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0