_spf.telstra.net has no MX records.
Question and response
Question
dig _spf.telstra.net. MX
Answer
The response has no answer records.
Authority
telstra.net. 6400 SOA dns0.telstra.net. hostmaster.telstra.net. 2021177231 10800 3600 604800 6400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0