_spf.m-net.de has no MX records.
Question and response
Question
dig _spf.m-net.de. MX
Answer
The response has no answer records.
Authority
m-net.de. 600 SOA ns1.m-online.net. hostmaster.m-net.de. 2026021000 3600 1800 3600000 600
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0