_spf.dfn-cert.de has no MX records.
Question and response
Question
dig _spf.dfn-cert.de. MX
Answer
The response has no answer records.
Authority
dfn-cert.de. 86400 SOA robin.dfn-cert.de. hostmaster.dfn-cert.de. 2026021801 10800 1800 604800 86400
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0