_spf.tierra.net has no MX records.
Question and response
Question
dig _spf.tierra.net. MX
Answer
The response has no answer records.
Authority
tierra.net. 3600 SOA ns1.domaindiscover.com. hostmaster.tierra.net. 2026022303 7200 1800 604800 28800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0