mitigation.phx01.singlehop.net has no MX records.
Question and response
Question
dig mitigation.phx01.singlehop.net. MX
Answer
The response has no answer records.
Authority
phx01.singlehop.net. 3600 SOA ns1.singlehop.net.phx01.singlehop.net. netops.singlehop.com.phx01.singlehop.net. 2009070100 172800 900 1209600 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0