b.nic.ruhr has no MX records.
Question and response
Question
dig b.nic.ruhr. MX
Answer
The response has no answer records.
Authority
nic.ruhr. 3600 SOA ns0.centralnic-dns.com. hostmaster.centralnic-dns.com. 1574704049 86400 7200 3600000 172800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0