_spf.telstra.com.au has no MX records.
Question and response
Question
dig _spf.telstra.com.au. MX
Answer
The response has no answer records.
Authority
telstra.com.au. 900 SOA adns05.bigpond.com. hostmaster.bigpond.com. 2026022007 7200 3600 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0