gatekeeper.singlehop.net has no MX records.
Question and response
Question
dig gatekeeper.singlehop.net. MX
Answer
The response has no answer records.
Authority
singlehop.net. 3600 SOA ns1.singlehop.com. netops.singlehop.com. 2012060503 86400 7200 3600000 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0