mx01.rexx-systems.de has no MX records.
Question and response
Question
dig mx01.rexx-systems.de. MX
Answer
The response has no answer records.
Authority
rexx-systems.de. 300 SOA ns1.routing.net. hostmaster.routing.net. 2025092200 3600 3600 86400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0