routing.wtnet.de has no MX records.
Question and response
Question
dig routing.wtnet.de. MX
Answer
The response has no answer records.
Authority
routing.wtnet.de. 86400 SOA dns1.wtnet.de. hostmaster.wtnet.de. 2016090101 86400 7200 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0