ns8.knipp.net has no MX records.
Question and response
Question
dig ns8.knipp.net. MX
Answer
The response has no answer records.
Authority
knipp.net. 86400 SOA ns2.knipp.de. dnsmaster.irondns.net. 2021042160 10800 3000 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0