_spf.solarwinds.com has no MX records.
Question and response
Question
dig _spf.solarwinds.com. MX
Answer
The response has no answer records.
Authority
solarwinds.com. 600 SOA a1-139.akam.net. hostmaster.solarwinds.com. 2013093108 1200 600 1209600 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0