pfsense.kirub2.de has no MX records.
Question and response
Question
dig pfsense.kirub2.de. MX
Answer
The response has no answer records.
Authority
kirub2.de. 3600 SOA hydrogen.ns.hetzner.com. dns.hetzner.com. 2025062401 86400 10800 3600000 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0