spf.usa.net has no MX records.
Question and response
Question
dig spf.usa.net. MX
Answer
The response has no answer records.
Authority
spf.usa.net. 360 SOA cndvg001.usa.net. hostmaster.usa.net. 2025090701 1800 600 604800 3600
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0