_spf.strato.com has no MX records.
Question and response
Question
dig _spf.strato.com. MX
Answer
The response has no answer records.
Authority
strato.com. 3600 SOA karlsruhe.nic.xlink.net. hostmaster.cronon-isp.net. 2026022200 3600 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0