nameserver-1-de.maxcluster.net has no MX records.
Question and response
Question
dig nameserver-1-de.maxcluster.net. MX
Answer
The response has no answer records.
Authority
maxcluster.net. 600 SOA primary.maxnameserver.net. info.maxcluster.net. 2026021906 3600 1000 1209600 600
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0