mxbackend.netaddress.usa.net has no MX records.
Question and response
Question
dig mxbackend.netaddress.usa.net. MX
Answer
The response has no answer records.
Authority
netaddress.usa.net. 3600 SOA cndvg001.usa.net. hostmaster.usa.net. 2008120800 1800 600 604800 3600
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0