b.ns.maxcluster.net has no MX records.
Question and response
Question
dig b.ns.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