smail-srv2.dts.de has no MX records.
Question and response
Question
dig smail-srv2.dts.de. MX
Answer
The response has no answer records.
Authority
dts.de. 3600 SOA ns1.dts-online.net. hostmaster.dts-online.net. 2026021100 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0