secondary-nameserver.de has no MX records.
Question and response
Question
dig secondary-nameserver.de. MX
Answer
The response has no answer records.
Authority
secondary-nameserver.de. 86400 SOA secondary-nameserver.de. root.securenetwork.de. 2016022304 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0