radius0.uklinux.net has no MX records.
Question and response
Question
dig radius0.uklinux.net. MX
Answer
The response has no answer records.
Authority
uklinux.net. 1800 SOA guy.ns.cloudflare.com. dns.cloudflare.com. 2394818963 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0