dns01.providerbox.de has no MX records.
Question and response
Question
dig dns01.providerbox.de. MX
Answer
The response has no answer records.
Authority
providerbox.de. 900 SOA dns01.providerbox.de. dnsadmin\@providerbox.de. 1736513396 900 1800 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0