a.sec-ns.net has no MX records.
Question and response
Question
dig a.sec-ns.net. MX
Answer
The response has no answer records.
Authority
sec-ns.net. 1200 SOA a.prim-ns.de. hostmaster.nexinto.net. 2018012902 28800 7200 604800 1200
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0