relay.outbox.de has no MX records.
Question and response
Question
dig relay.outbox.de. MX
Answer
The response has no answer records.
Authority
outbox.de. 86400 SOA a.ns14.net. internetx.outbox.de. 2026011385 43200 7200 1209600 86400
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0