dns1.nts-online.net has no MX records.
Question and response
Question
dig dns1.nts-online.net. MX
Answer
The response has no answer records.
Authority
nts-online.net. 300 SOA dns1.nts-online.net. admin.nts-online.net. 638182470 7200 1200 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0