customer.hkn.cloud has no MX records.
Question and response
Question
dig customer.hkn.cloud. MX
Answer
The response has no answer records.
Authority
hkn.cloud. 300 SOA neos.hkn.de. hostmaster.hkn.de. 1772004224 28800 7200 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0